polymer brushes via surface-initiated controlled radical polymerization

Polymer Brushes via Surface-Initiated Controlled Radical Polymerization:Synthesis, Characterization, Properties, and Applications

Raphael Barbey, Laurent Lavanant, Dusko Paripovic, Nicolas Schuwer, Caroline Sugnaux, Stefano Tugulu, andHarm-Anton Klok*

Ecole Polytechnique Federale de Lausanne (EPFL), Institut des Materiaux, Laboratoire des Polyme`res, Batiment MXD, Station 12,CH-1015 Lausanne, Switzerland

Received February 5, 2009

Contents

1. Introduction 54372. Synthesis 5439

2.1. Polymerization Strategies 54392.1.1. Surface-Initiated Atom Transfer Radical

Polymerization (SI-ATRP)5439

2.1.2. Surface-Initiated Reversible-AdditionFragmentation Chain Transfer (SI-RAFT)Polymerization

5450

2.1.3. Surface-Initated Nitroxide-MediatedPolymerization (SI-NMP)

5452

2.1.4. Surface-Initiated Photoiniferter-MediatedPolymerization (SI-PIMP)

5453

2.2. Control of Architecture 54532.2.1. Block Copolymer Brushes 54542.2.2. Random Copolymer Brushes 54572.2.3. Binary Brushes 54572.2.4. Hyperbranched, Comb-Shaped, and Highly

Branched Brushes5457

2.2.5. Cross-linked Brushes 54592.2.6. Free-Standing Brushes 54592.2.7. Gradient Brushes 54602.2.8. Variation of Brush Density 5460

2.3. Variation of Substrate 54612.3.1. Polymer Brushes Grafted from Silicon

Oxide5461

2.3.2. Polymer Brushes Grafted from Silicon 54672.3.3. Polymer Brushes Grafted from Metal Oxide

Surfaces5468

2.3.4. Polymer Brushes Grafted from ClayMineral Surfaces

5470

2.3.5. Polymer Brushes Grafted from GoldSurfaces

5472

2.3.6. Polymer Brushes Grafted from Metal andSemiconductor Surfaces

5473

2.3.7. Polymer Brushes Grafted from CarbonSurfaces

5474

2.3.8. Polymer Brushes Grafted from PolymerSurfaces

5474

2.4. Patterning Strategies 54832.4.1. Microcontact Printing 54832.4.2. Electron Beam-Assisted Methods 54852.4.3. UV Irradiation-Assisted Methods 5485

2.4.4. SPM-Assisted Methods 54862.4.5. Nanoimprint and Contact Lithography 54872.4.6. Other Patterning Techniques 5487

2.5. Postmodification of Polymer Brushes 54872.5.1. Postmodification of Hydroxyl-Functionalized

Polymer Brushes5488

2.5.2. Postmodification of CarboxylicAcid-Functionalized Polymer Brushes

5490

2.5.3. Postmodification of CarboxylicEster-Functionalized Polymer Brushes

5491

2.5.4. Postmodification of Epoxide-FunctionalizedPolymer Brushes

5492

2.5.5. Postmodification of Other Side-ChainFunctional Polymer Brushes

5494

2.5.6. (Selective) Chain End Postmodification 54943. Characterization 54964. Properties and Applications of Polymer Brushes 5498

4.1. Responsive Surfaces 54984.1.1. Solvent Responsive Polymer Brushes 54984.1.2. Thermoresponsive Polymer Brushes 55004.1.3. pH- and Ion-Sensitive Polymer Brushes 5502

4.2. Nonbiofouling Surfaces 55044.2.1. Neutral Nonbiofouling Polymer Brushes 55044.2.2. Zwitterionic Nonbiofouling Polymer Brushes 5507

4.3. Cell Adhesive Surfaces 55074.3.1. Peptide/Protein-Functionalized Polymer

Brushes5507

4.3.2. Patterned Polymer Brushes 55094.3.3. Thermoresponsive Polymer Brushes 5509

4.4. Protein Binding and Immobilization 55104.4.1. Noncovalent Protein Binding 55104.4.2. Covalent Protein Immobilization 5512

4.5. Chromatography Supports 55134.6. Membrane Applications 55134.7. Antibacterial Coatings 55154.8. Low Friction Surfaces 5515

5. Conclusions and Outlook 55176. Acknowledgments 55177. References 5517

1. Introduction

Polymer brushes are ultrathin polymer coatings consistingof polymer chains that are tethered with one chain end to aninterface, which generally is a solid substrate. At highgrafting densities, i.e. when the distance between neighboring

* To whom correspondence should be addressed. E-mail: [email protected]. Fax: +41 21 693 5650. Telephone: +41 21 693 4866.

Chem. Rev. 2009, 109, 54375527 5437

10.1021/cr900045a CCC: $71.50 2009 American Chemical SocietyPublished on Web 10/21/2009

grafting points is small, steric repulsion leads to chainstretching and a brush-type conformation of the surface-tethered chains. At lower grafting densities, surface-tetheredpolymer chains can adopt various other conformations, whichare referred to as mushroom or pancake.14

Polymer brushes can be prepared following two mainstrategies: (i) the grafting to and (ii) the grafting fromstrategies.3 The grafting to strategy involves the attachmentof prefabricated polymers via either physisorption512 (Figure1A) or covalent bond formation (chemisorption) (Figure1B).1320 Although experimentally very straightforward, thegrafting to strategy suffers from several limitations, whichmake it difficult to produce thick and very dense polymerbrushes. Steric repulsions between polymer chains hamperthe formation of dense polymer brushes.21,22 Furthermore,with increasing polymer molecular weight, the reactionbetween the polymer end-group and the complementarygroup on the substrate surface becomes less efficient.

In the grafting from approach (Figure 1C), the polymerizationis directly initiated from initiator-functionalized surfaces.3,2225Controlled/living polymerization techniques26,27 are particu-larly attractive for the preparation of polymer brushes

Raphael Barbey was born in Saint-Imier (Bern, Switzerland) in 1980. Hereceived his M.Sc. degree in chemical and biochemical engineering fromthe Ecole Polytechnique Federale de Lausanne (EPFL, Switzerland) in2004. After a one year academic internship working on the developmentof reaction calorimetry applied to polymerization reactions in supercriticalfluids, he joined the group of Prof. H.-A. Klok. He is currently pursuinghis doctoral studies in the field of polymer brushes and their use forbiomedical and bioanalytical applications.

Laurent Lavanant was born in 1979 in Rennes (France) and graduatedfrom the Ecole Nationale Superieure de Chimie de Paris (France) andthe University Pierre et Marie Curie (Paris, France) with a master inchemical engineering and molecular chemistry, respectively. He receivedhis Ph.D. degree in 2005 from the University of Rennes (France) afterworking with Jean-Francois Carpentier. After postdoctoral research withH.-A. Klok from 2005 to 2007, he is now a research scientist at SensileMedical AG (Hgendorf, Switzerland) and works on the development ofmedical devices.

Dusko Paripovic was born in 1981 and studied at the Faculty of Technologyand Metallurgy in Belgrade (Serbia) to obtain his degree in organicchemical technology and polymer engineering. He completed his diplomathesis in the group of Prof. Morbidelli at ETH Zurich in 2006. Currentlyhe works with Prof. H.-A. Klok at EPF Lausanne pursuing his Ph.D. onthe topic of biofunctionalized polymer brushes.

Nicolas Schuwer was born in Besancon (France) in 1984. He obtainedhis bachelor degree in chemistry from the University of Franche-Comte(France) and his master degree in physical chemistry from bo Akademi(Turku, Finland). He is currently working as a Ph.D. student with Prof.H.-A. Klok at the Ecole Polytechnique Federale de Lausanne (Switzerland).

Caroline Sugnaux was born in Morges (Vaud, Switzerland) in 1984. Afteraccomplishing her master thesis in the group of Prof. S. Mecking(Konstanz, Germany), she obtained her master degree in molecular andbiological chemistry in 2008 from the Ecole Polytechnique Federale deLausanne (EPFL, Switzerland). She is currently carrying out a Ph.D. thesisunder the supervision of Prof. H.-A. Klok.

5438 Chemical Reviews, 2009, Vol. 109, No. 11 Barbey et al.

following the grafting from strategy, as they allow accuratecontrol over brush thickness, composition, and architecture.2831Examples include anionic polymerization,3234 cationicpolymerization,3437 ring-opening polymerization,3846 andring-opening metathesis polymerization.4751 Conventionalfree radical polymerization has also found widespread usefor the synthesis of polymer brushes.22,5264 Most of thepolymer brushes produced by the grafting from approach,however, are prepared using surface-initiated controlledradical polymerization techniques.

This article concentrates exclusively on polymer brushesprepared via surface-initiated controlled radical polymeri-zation and is an attempt to summarize the state-of-the-art inthis field. The following sections will successively discussthe synthesis of polymer brushes via surface-initiated con-trolled radical polymerization, the characterization of thesesurface-tethered polymers, as well as their properties andapplications.

2. Synthesis

2.1. Polymerization StrategiesAmong the different controlled/living polymerization

techniques, radical-based strategies are most frequently used.Compared to other controlled/living polymerization meth-ods, radical-based polymerization reactions have severaladvantages, notably in terms of compatibility with bothaqueous and organic media as well as a high tolerance towarda wide range of functional groups. In the following sections,the four major surface-initiated controlled radical polymer-ization (SI-CRP) techniques will be discussed in detail. Table1 provides an overview of the different polymer brushes thathave been prepared using SI-CRP. The brushes in Table 1are classified according to the nature of the polymerbackbone. For each polymer, Table 1 indicates both the CRPtechniques that have been used and the different brusharchitectures that have been produced.

2.1.1. Surface-Initiated Atom Transfer RadicalPolymerization (SI-ATRP)

Among the different controlled radical polymerizationtechniques that are available, atom transfer radical polym-erization (ATRP) has been most extensively used to producepolymer brushes. Compared to other controlled radicalpolymerization techniques, ATRP is chemically extremelyversatile and robust. ATRP was first reported in 19956567and has been extensively reviewed.6872 ATRP relies on thereversible redox activation of a dormant alkyl halide-terminated polymer chain end by a halogen transfer to atransition metal complex. The formal homolytic cleavage ofthe carbon-halogen bond, which results from this process,generates a free and active carbon-centered radical speciesat the polymer chain end. This activation step is based on asingle electron transfer from the transition metal complexto the halogen atom, which leads to the oxidation of thetransition metal complex. Then, in a fast, reversible reaction,the oxidized form of the catalyst reconverts the propagatingradical chain end to the corresponding halogen-cappeddormant species. Many parameters, such as ligand totransition metal ratio, CuII to CuI ratio, type of ligand,counterion, solvent, or initiator, influence the performanceof (SI)-ATRP and thus offer the possibility to fine-tune thereaction.7381

SI-ATRP was first reported in 1997 by Huang and Wirth,who successfully grafted poly(acrylamide) (PAM) brushesfrom benzylchloride-derivatized silica particles.82 Shortlythereafter, Ejaz et al. described the preparation of poly(methylmethacrylate) (PMMA) brushes that were grown from 2-(4-chlorosulfonylphenyl)ethyl silane self-assembled monolayers(SAMs) obtained using the Langmuir-Blodgett technique.83These authors found that addition of free, sacrificial initiator(p-toluenesulfonyl chloride) was necessary to achieve acontrolled polymerization. In the absence of sacrificialinitiator, the initiator concentration and, related to this, theconcentration of the deactivating CuII species was too lowto allow a controlled polymerization. Instead of adding asacrificial initiator, another strategy to overcome the insuf-ficient deactivator concentration that results from surface-confined ATRP is to add the deactivating CuII species directlyto the polymerization solution. This was successfully dem-onstrated by Matyjaszewski et al. for the synthesis of

Stefano Tugulu was born in Ahlen (Germany) in 1974. He received hisdiploma degree in chemistry from the University of Bielefeld (Germany)in 2001. In 2007 he obtained his Ph.D. working with Prof. H.-A. Klok onfunctional polymer brushes prepared via surface-initiated controlled radicalpolymerizations at the Max Planck Institute for Polymer Research (Mainz,Germany) and at the Ecole Polytechnique Federale de Lausanne(Switzerland). He is currently working as a R&D project manager in thefield of medical device technology at Thommen Medical (Waldenburg,Switzerland).

Harm-Anton Klok was born in 1971 and studied chemical technology atthe University of Twente (Enschede, The Netherlands) from 1989 to 1993.He received his Ph.D. in 1997 from the University of Ulm (Germany)after working with M. Moller. After postdoctoral research with D. N.Reinhoudt (University of Twente) and S. I. Stupp (University of Illinois atUrbana-Champaign), he joined the Max Planck Institute for PolymerResearch (Mainz, Germany) in early 1999 as a project leader in the groupof K. Mullen. In November 2002, he was appointed to the faculty of theEcole Polytechnique Federale de Lausanne (EPFL, Switzerland).

Polymer Brushes via Surface-Initiated Polymerization Chemical Reviews, 2009, Vol. 109, No. 11 5439

polystyrene (PS) brushes from bromoisobutyrate-function-alized silicon wafers.84

A significant increase in the rate of SI-ATRP was observedfor polymerizations carried out in polar and, in particular,aqueous media.75,78,85,86 Jones et al. synthesized 50-nm-thickPMMA brushes in a controlled fashion within 4 h ofpolymerization time using a CuIBr/2,2-bipyridine (bpy)catalyst system in a water/methanol mixture as solvent.87 Apurely aqueous-based system was used by Huang et al. forthe preparation of 700-nm-thick poly(2-hydroxyethyl meth-acrylate) (PHEMA) brushes via water-accelerated SI-ATRP using a mixed halide CuICl/CuIIBr2/bpy catalystsystem (Scheme 1).88 As described by Matyjaszewski et al.,the use of such mixed halide systems represents, because ofthe higher free energy of dissociation of the C-Cl bondcompared to the C-Br bond, a valuable tool to shift theequilibrium between dormant and propagating radical specieson the side of dormant species, which leads to an increaseover the control of the polymerization.89

In SI-ATRP, chain growth starts from an ATRP initiatorthat is immobilized on a substrate. The same transition metalcomplexes that mediate SI-ATRP, however, can also be usedto grow polymer brushes in a controlled fashion fromsurfaces modified with a conventional radical initiator. Thisprocess is referred to as surface-initiated reverse ATRP (SI-RATRP). SI-RATRP has been successfully used by Sedjoet al. to prepare PS and PS-b-PMMA brushes from aconventional radical azo-functionalized silica substrate usingCuIIBr2/bpy complex as deactivating agent.90 Later, Wanget al. described the synthesis of PMMA brushes fromperoxide-derivatized substrates in the presence of CuIICl2/bpy complex.91,92

The (possible) presence of residual amounts of the metalcatalyst in polymers prepared via (SI)-ATRP often raisesconcerns, in particular with the use of these materials in(bio)medical applications. Matyjaszewski and co-workershave developed an ATRP variant that allows to overcomethese concerns and which makes it possible to reduce theconcentration of the copper catalyst to a few ppm andincreases the tolerance toward oxygen or other radical trapsin the polymerization system. This ATRP variant is referredto as activators (re)generated by electron transfer ATRP orA(R)GET ATRP.9397 A(R)GET ATRP involves the use ofreducing agents, such as ascorbic acid, SnII 2-ethylhexanoate,or Cu0, to continuously restore CuI from CuII and has alsobeen successfully applied to surface-initiated polymeriza-tion.98104

Summarizing, SI-ATRP has been proven to be an excellenttechnique to prepare polymer brushes. ATRP is chemicallyversatile, compatible with a large assortment of monomersand functional groups, and tolerates a relatively high degreeof impurities. In particular, ATRP is relatively insensitivetoward small residual traces of oxygen, which are readilyremoved by oxidation of the ATRP catalyst. The fact thatmost of the standard ATRP catalyst systems, as well assurface immobilizable initiators, are commercially availablein ready-to-use quality or can be synthesized relatively easilyalso makes ATRP an attractive technique from an experi-mental point of view. SI-ATRP, however, also has limita-tions. In particular, the controlled polymerization of mono-mers that can complex or react with the metal catalyst, suchas pyridine-containing or acidic monomers, can be challeng-ing. For pyridinic monomers, this problem can be partiallyovercome by using highly coordinative tri- or tetradentateligands to form the catalytic transition metal complex.105,106

Figure 1. Synthetic strategies for the preparation of polymer brushes: (A) physisorption of diblock copolymers via preferential adsorptionof the red blocks to the surface (grafting to approach); (B) chemisorption via reaction of appropriately end-functionalized polymers withcomplementary functional groups at the substrate surface (grafting to approach); (C) polymer brushes grown via surface-initiated polymerizationtechniques (grafting from approach).


The preparation of acidic polymer brushes has been ac-complished via ATRP of the corresponding sodium salts.107113An interesting exception has been reported in a recentpublication by Jain et al., who reported the first example of

successful direct SI-ATRP of a protonated acidic monomer,2-(methacryloyloxy)ethyl succinate (MES).113 Another limi-tation of (SI)-ATRP is related to the transition metal catalyst,which can be difficult to remove. Residual traces of

Table 1. Overview of Polymer Brushes Prepared via Surface-Initiated Controlled Radical Polymerization


Table 1. Continued


catalysts in the final polymer brushes might have undesir-able consequences for applications, such as in the bio-medical or electronic industry. However, some methods,in particular A(R)GET ATRP, have been developed thatallow to reduce the amount of copper to the level of afew ppm.72

2.1.2. Surface-Initiated Reversible-Addition FragmentationChain Transfer (SI-RAFT) Polymerization

In contrast to ATRP, where the equilibrium between thedormant and active, propagating chains is based on reversibletermination, reversible-addition fragmentation chain transfer(RAFT) polymerization is based on reversible chaintransfer.114116 A distinct advantage of RAFT polymerization

is its relative simplicity and versatility, since conventionalfree radical polymerizations can be readily converted into aRAFT process by adding an appropriate RAFT agent, suchas a dithioester, dithiocarbamate, or trithiocarbonate com-pound, while other reaction parameters, such as monomer,initiator, solvent, and temperature, can be kept constant.RAFT polymerization has also been successfully used toprepare polymer brushes via surface-initiated polymerization.SI-RAFT can be performed using two different strategies,which use either surface-immobilized conventional freeradical initiators or surface-immobilized RAFT agents (Scheme2). These two different strategies will be discussed in moredetail in the following paragraphs.

Table 1. Continued

a H, B, and R refer to homopolymer, block copolymer, and random copolymer brushes. The superscripts are references to the relevant publications.

Scheme 1. Preparation of PHEMA-b-PDMAEMA Diblock Copolymer Brushes via SI-ATRP from 2-Bromoisobutyrate-Functional Thiol SAMs on Gold Surfaces88


An early example of SI-RAFT polymerization was re-ported by Baum and Brittain, who prepared 30-nm-thickPMMA brushes as well as 11-nm-thick PS and poly(N,N-dimethylacrylamide) (PDMAM) brushes from azo-function-alized silicon wafers in the presence of the chain transferagent (CTA) 2-phenylprop-2-yl dithiobenzoate and freeinitiator (2,2-azoisobutyronitrile (AIBN)) (Scheme 2A).117Addition of free initiator (e.g., AIBN) was shown to facilitatepolymer brush growth not only because it acts as a scavengerfor possible trace amounts of impurities in the polymerizationmixture but also since it increases the amount of radicals inthe system, which are necessary to avoid early terminationby CTA capping, as the concentration of the surface initiatorsis particularly low. Several other groups used the samestrategy to grow poly(chloromethylstyrene) (PCMS),118 poly-(pentafluorostyrene) (PPFS),118 poly(sulfobetaine methacry-late) (PSBMA),119 poly(sodium 4-styrenesulfonate) (PSS-

(Na)),119 PMMA,120 poly(poly(ethylene glycol) methyl ethermethacrylate) (PPEGMEMA),120 and poly(2-(dimethylami-no)ethyl methacrylate) (PDMAEMA)120 brushes from azo-functionalized substrates or to graft PHEMA brushes fromsurfaces bearing peroxide groups.121

In addition to the use of free radical initiator-modifiedsubstrates, as was described in the previous paragraph, SI-RAFT can also be carried out using surface-immobilizedRAFT agents. The RAFT agent can be immobilized in twodifferent ways, which are referred to as the R-group and Z-groupapproaches (parts B and C, respectively, of Scheme 2). In theR-group approach, the RAFT agent is attached to the surfacevia the leaving and reinitiating R group. This strategy has beenused to prepare a wide variety of polymer brushes fromdithiobenzoate- or trithiocarbonate-derivatized silicon wafers,122125silica (nano)particles,126132 titania133 or CdSe134 nanoparticles,cotton,135 gold nanoparticles,136 cellulose,137 or multiwalled

Scheme 2. SI-RAFT Polymerization: (A) Bimolecular Process as Reported by Baum and Brittain117 for the Preparation ofPMMA Brushes from Azo-Functionalized Silicon Wafers; (B) R-Group Approach To Grow PBA Brushes from DithiobenzoateModified Silica Nanoparticles as Described by Li and Benicewicz;127 (C) Z-Group Approach for the Grafting of PMA Brushesfrom Silica Particles Supported Trithiocarbonate Derivative146


carbon nanotubes (carbon MWNTs).138142 The Z-groupapproach is based on the immobilization of the RAFT agentvia the stabilizing Z group and has been successfully usedto prepare a variety of methacrylic, acrylic, styrenic, andacrylamide-based brushes.143149

Compared to other CRP techniques, RAFT polymerizationis extremely versatile and tolerates a wide range of (sensitive)functional groups. A drawback of RAFT polymerization isthat it involves the use of chain transfer agents that areusually not commercially available and which need to beprepared via multistep synthesis. SI-RAFT polymerizationmethods that involve the use of surface-immobilized CTAshave specific limitations. The R-group approach, comparableto a grafting from process, always involves the surfacedetachment of the RAFT agent during the polymerization,which might broaden the molecular weight distribution viabimolecular termination at an unusually high rate, whereasthe Z-group approach, which can be compared with a graftingto approach, might suffer from a decrease of brush graftingdensities with increasing brush length, since the RAFT agentanchored to the surface will be less and less accessible.126,144,146

2.1.3. Surface-Initated Nitroxide-Mediated Polymerization(SI-NMP)

Nitroxide-mediated polymerization is based upon revers-ible activation/deactivation of growing polymer chains by anitroxide radical.150155 Husseman et al. reported the firstexample of surface-initiated nitroxide-mediated polymeri-zation (SI-NMP) and successfully produced up to 120-nm-thick PS brushes from 2,2,6,6-tetramethylpiperidinyloxy(TEMPO) functionalized chlorosilane SAMs supported onsilicon substrates (Scheme 3).156 In SI-NMP the maximumnumber of persistent radicals157 is limited by the total numberof initiator moieties on the substrate surface, which is,especially for planar substrates with low specific surfaceareas, relatively low. Consequently, the reversible cappingbecomes ineffective due to the quasi infinite dilution ofpersistent radicals in the reaction medium. In their contribu-tion, Husseman et al. managed to overcome this issue byadding a predetermined amount of free alkoxyamine tothe reaction mixture. The addition of free (sacrificial)initiator, however, leads to the formation of free, non-surface-attached polymer and requires an additional, final washingstep to remove physisorbed polymer from the resultingpolymer brushes. Husseman et al. also found that the numberaverage molecular weight (Mn) and the polydispersity index(Mw/Mn) of the grafted PS are almost equal to the values offree polymer in solution. Nitroxide-mediated polymerizationwas also used by several other groups for the formation of

PS brushes from TEMPO-functionalized silicon wafers orglass slides,158163 magnetite164166 or titanium166,167 nanopar-ticles, steel,168 Merrifield resins,169 carbon,168 and carbonMWNTs.170,171 In addition, several other polymer brushes,such as poly(3-vinylpyridine) (P3VP),165,166 poly(4-vinylpy-ridine) (P4VP),170,171 PSS(Na),170 and poly(4-(poly(ethyleneglycol) methyl ether)styrene) (PSPEG) brushes,162 have beensuccessfully prepared via SI-NMP from TEMPO-modifiedsubstrates.

A drawback of TEMPO-mediated polymerization is thatits utility is essentially limited to styrenic monomers. NMPhas been found to yield acrylic polymers with low Mn andrelatively high polydispersities compared to those of poly-mers prepared from styrenic monomers.153,172 Especially withacrylic and methacrylic monomers, chain growth and revers-ible deactivation compete with -H elimination of thegrowing polymer chain.173 Studies were conducted in orderto find a more universal alkoxyamine initiator as an alterna-tive to TEMPO-based systems.174,175 First, an acyclic -phos-phonylated nitroxide, N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (DEPN), was identified as agood candidate for NMP of acrylic and styrenic monomers.However, a slightly higher percentage of termination reac-tions was observed for DEPN-mediated polymerizations ofstyrenic monomers compared to TEMPO-mediated poly-merizations.176,177 Parvole et al. developed a strategy to growpoly(n-butyl acrylate) (PBA) and poly(ethyl acrylate) (PEA)brushes from azo-grafted surfaces by adding DEPN, whichacts as a chain growth moderator (so-called bimolecularpolymerization system).178180 Instead of using a conventionalfree radical initiator-modified substrate, DEPN-mediatedpolymerizations can also be carried out using surface-immobilized DEPN. This strategy has been used for thepreparation of PS,177,181186 PBA,181,185,187189 and poly(2-(dimethylamino)ethyl acrylate) (PDMAEA) brushes.185 An-other alternative to prepare styrenic, acrylic, or acrylamide-or acrylonitrile-based polymer brushes involves the use ofR-hydrido nitroxide, which was identified to yield wellcontrolled bulk polymerizations.175 These R-hydrido nitroxidecompounds were successfully used as a free initiator to moderatethe SI-NMP from TEMPO-functionalized surfaces190,191 or fromR-hydrido nitroxide-functionalized surfaces.192195

In conclusion, SI-NMP represents a valuable method forthe controlled fabrication of polymer brushes. An advantageof SI-NMP is that no further catalysts are required. Thisobviates the need for additional purification steps and reducesthe chance to introduce impurities, which is advantageous,especially for sensitive applications, e.g. in the electronicand biomedical sector. The relatively high polymerization

Scheme 3. SI-NMP of Styrene from a TEMPO-Functionalized Silicon Wafer, as Reported by Husseman et al156


temperatures, however, may cause problems when thermallysensible monomers are used. Another drawback of NMP isthat controlled polymerization requires judicious choice ofthe mediating nitroxide for a particular monomer. This isfurther hampered by the fact that many mediating radicalsare not commercially available and need to be prepared,which requires an additional synthetic effort.

2.1.4. Surface-Initiated Photoiniferter-MediatedPolymerization (SI-PIMP)

The concept of iniferter-mediated polymerization, whichis based on the use of a special class of nonconventionalinitiators (named iniferters), was proposed by Otsu et al. in1982.196,197 Iniferters are molecules that can simultaneouslyact as initiators, transfer agents, and terminators. Thecontrolled nature of the polymerization relies on the pho-tolytic dissociation of the photoiniferter molecule into areactive carbon-centered radical and a relatively stabledithiocarbamyl radical. While the carbon-centered radicalreadily undergoes addition of monomer units to initiate chainpropagation, the persistent dithiocarbamyl radical does notparticipate in initiation but acts as a transfer agent andinduces reversible termination of the growing polymer chain(iniferter).198 In the absence of termination or transferreactions, the polymerization proceeds only during irradiationof light and via a predominantly controlled radical polym-erization mechanism, which is based on reversible termina-tion. Since the concentration of radicals, and therefore therate of polymerization, is directly related to the intensity ofirradiating light, surface-initiated photoiniferter-mediatedpolymerization (SI-PIMP) is spatially and temporally coupledto the location, intensity, and duration of UV irradiation.199,200

Otsu et al. reported the first example of SI-PIMP, whichinvolved the use of a photoiniferter (dithiocarbamate deriva-tive)-functionalized PS gel for the preparation of varioussurface-attached di- and triblock copolymers.201 Matsuda andco-workers extensively used SI-PIMP to prepare a widevariety of polymer brushes from benzyl-N,N-diethyldi-thiocarbamate-functionalized substrates. Among others,PDMAM, poly(acrylic acid) (PAA), poly(N-isopropyl acry-lamide) (PNIPAM), PCMS, poly(poly(ethylene glycol) meth-acrylate) (PPEGMA), poly(sodium methacrylate) (PMAA-(Na)), and poly(methacrylic acid) (PMAA) brushes wereprepared via this strategy.202206 De Boer et al. used atrimethoxysilane-modified benzyl-N,N-diethyldithiocarbam-ate derivative to modify the surface of silicon substrates andgrow PS brushes (Scheme 4). These authors reported thesuccessful preparation of up to 100-nm-thick PS brusheswithin 15 h of irradiation with 365 nm UV light.207

The fact that SI-PIMP is usually carried out withoutadditional free deactivating species has led to a contro-versial discussion about the controlled nature of this tech-nique, since the concentration of the deactivating dithiocar-bamyl radicals is considered not to be sufficient to effectivelyconvert propagating polymer chains to the correspondingdormant species. To study the living nature of this poly-merization technique, several groups have performed kineticstudies.200,207209 Studies of the SI-PIMP of methyl meth-acrylate performed by Rahane et al. indicated a pseudolivingbehavior due to irreversible termination reactions, leadingto the loss of surface free radicals, with increasing exposuretime. The nonlinear growth of the PMMA brushes as afunction of irradiation time was mainly attributed to bimo-lecular termination reactions, rather than chain transfer tomonomer.209 To circumvent irreversible termination reac-tions, Luo et al. as well as Rahane et al. reported a strategyto increase the amount of deactivating species, which aremandatory to provide a controlled radical polymerizationbehavior, by adding tetraethylthiuram disulfide to the po-lymerization mixture as a source of deactivating dithiocar-bamyl radicals.210,211

The limitations of SI-PIMP are related to the fact that onlyphotostabile surfaces and monomers can be used. Gold is avery challenging substrate, since exposure to UV light leadsto the deterioration of the initiator (here the iniferter)SAMs.207 However, recently, Vancso and co-workers suc-cessfully prepared PNIPAM212 and PMAA213,214 brushes frombenzyl-N,N-diethyldithiocarbamate-modified gold substratesby means of a UV lamp emitting at 300 nm coupled with a280 nm cutoff filter. Furthermore, SI-PIMP requires surfacesthat are readily accessible for UV exposure. For instance,microchannels, tubes, or small cavities are difficult to modify,since these substrates are difficult to irradiate and aninhomogeneous distribution of light intensity might causeinhomogeneous brush growth. On the other hand, SI-PIMPprovides a versatile route to 2D- and 3D-microstructuredpolymer brushes without being particularly limited to specialtypes of monomers. Furthermore, SI-PIMP does not requirethe removal of polymerization catalyst and is thereforeespecially suitable for the preparation of material surfacesfor biomedical or electronic applications.

2.2. Control of ArchitectureIn addition to allowing relatively accurate control over

brush thickness, the use of surface-initiated controlled radicalpolymerization (SI-CRP) also enables control and variationof the architecture of polymer brushes. SI-CRP has been

Scheme 4. Preparation of PS-b-PMMA Brushes by SI-PIMP from a Benzyl-N,N-diethyldithiocarbamate-Derivatized SiliconSubstrate207


successfully used to prepare block and random copolymerbrushes as well as gradient brushes. Furthermore, binarybrushes, various branched polymer brush architectures, aswell as cross-linked and free-standing brushes have also beenproduced using SI-CRP. All of these different architectureswill be discussed in this section. Finally, this section willalso discuss the different strategies that have been developedto vary and control the initiator surface concentration andconsequently the grafting density of the tethered polymerchains.

2.2.1. Block Copolymer Brushes

After homopolymer brushes, SI-CRP techniques have beenmostly used to prepare block copolymer brushes (Figure 2A).These are usually synthesized either to confirm the livingnessof the SI-CRP or to prepare nanostructured phase-separatedthin films (see section 4.1). Table 2 gives an overview ofthe different types of diblock copolymer brushes that havebeen prepared so far. In addition to the nature of the first,surface-tethered, and second blocks, Table 2 also indicatesfor each diblock copolymer brush the SI-CRP technique(s)that have been used.

The first diblock copolymer brush synthesized via SI-CRPwas reported by Otsu et al. in 1986.201 In their work, theauthors used a cleavable photoiniferter immobilized on a PSgel to prepare surface-attached PS-b-PMMA diblock co-polymers. Nakayama and Matsuda later reported the suc-cessful formation of PDMAM-b-PS, PDMAM-b-PAA, PD-MAPAM-b-PDMAM, and PDMAM-b-PBMA diblockcopolymer brushes from dithiocarbamated PS films viasequential SI-PIMP of the corresponding monomers.215 In

1999, Husseman et al. and Matyjaszewski et al. preparedblock copolymer brushes using other SI-CRP strategies.84,156Husseman et al. reported a PS-b-(PS-co-PMMA) blockcopolymer brush grown from TEMPO-functionalized siliconsubstrates via nitroxide-mediated polymerization,156 whereasMatyjaszewski et al. reported the successful synthesis of PS-b-poly(methyl acrylate) and PS-b-PtBA diblock copolymerbrushes from 2-bromoisobutyrate-derivatized silicon wafersvia SI-ATRP.84 Later, Baum and Brittain used SI-RAFT toprepare PS-b-PDMAM and PDMAM-b-PMMA diblockcopolymer brushes from azo-functionalized silicon wafersin the presence of 2-phenylprop-2-yl dithiobenzoate as chaintransfer agent.117

In addition to diblock copolymer brushes, SI-CRP has alsobeen used to prepare triblock copolymer brushes. In theirseminal paper, Otsu et al. already reported the synthesis ofsurface-attached PS-b-PMMA-b-PS, PS-b-poly(p-chlorosty-rene)-b-PMMA, and PS-b-PMMA-b-PMA triblock copoly-mers via SI-PIMP.201 Nakayama et al. used SI-PIMP toproduce PDMAPAM-b-PS-b-PDMAPAM triblock copoly-mer brushes.200 SI-ATRP has been used to grow PS-b-PMA-b-PS,216 PMA-b-PS-b-PMA,216 PMA-b-PMMA-b-PHEMA,217,218 or PMMA-b-PDMAEMA-b-PMMAbrushes.218 Triblock copolymer brushes comprising twooppositely charged polyelectrolyte blocks, PMETAC-b-PMMA-b-PMAA(Na), were successfully prepared via SI-ATRP by Osborne et al.110 Genzer and co-workers criticallyinvestigated the feasibility of SI-ATRP to produce multiblockcopolymer brushes and reported the successful prepara-tion of multiblock copolymer brushes composed of up tothree (PMMA-b-PHEMA) or (PMMA-b-PDMAEMA) se-

Figure 2. Overview of different polymer brush architectures that can be prepared via surface-initiated controlled radical polymerization.(A) block copolymer brushes (section 2.2.1); (B) random copolymer brushes (section 2.2.2); (C) cross-linked polymer brushes (section2.2.5); (D) free-standing polymer brushes (section 2.2.6); (E) hyperbranched polymer brushes (section 2.2.4); (F) highly branched polymerbrushes (section 2.2.4); (G) Y-shaped binary mixed polymer brushes (section 2.2.3); (H) standard binary mixed brushes (section 2.2.3); (I)molecular weight gradient polymer brushes (section 2.2.7); (J) grafting density gradient polymer brushes (section 2.2.7); (K, L) chemicalcomposition gradient polymer brushes (section 2.2.7).


Tabl

e2.

Ove

rvie

wofD

iblo

ckC

opol

ymer

Brus

hes

Prep

ared

bySu

rfac

e-In

itiat

edC

ontr

olle

dR

adic

alPo

lym

eriz

atio

n(s)

Seco

ndbl

ock

Firs

tblo

ckPo

ly(m

ethac

rylate

)Po

ly(ac

rylate

)Po

ly(ac

rylam

ide)

Poly

(styre

ne)

Poly

(viny

lpyrid

ine)

Poly

(meth

acryl

ate)

PAH

MA

PMM

AA

TRP6

26

PBM

APD

MA

EMA

ATR

P286

,287

,736

PBzM

APS

ATR

P672

PCD

MA

PMM

AA

TRP7

40

PDM

AEM

APG

MA

ATR

P338

PPFS

ATR

P439

PHEM

AA

TRP3

61,7

50PS

ATR

P494

PMM

AA

TRP3

60R

AFT

481

PPEG

MA

ATR

P388

PtB

MA

ATR

P608

PEM

OM

APM

MA

ATR

P264

PGM

APP

EGM

AR

AFT

125

PPFS

ATR

P257

PHEM

APD

MA

EMA

ATR

P88,

361

PMA

ATR

P769

PNIP

AM

ATR

P328

RA

FT14

3

PMM

AA

TRP2

84,2

88,2

89

PPEG

MA

ATR

P508

PTFE

MA

ATR

P516

PMA

APN

IPA

MPI

MP6

81

PMA

A(N

a)PH

EMA

ATR

P111

PMM

APB

zMA

-co

-PM

MA

ATR

P487

PBA

ATR

P785

PAM

ATR

P505

,784

PSR

AFT

122,

139,

144

P4V

PA

TRP7

98

PCD

MA

ATR

P740

PMA

RA

FT14

4PN

IPA

MR

ATR

P401

PIM

P211

,407

PDM

AEM

AR

AFT

120,

122,

143

ATR

P444

ATR

P218

,360

,397

,482

,507

,744

PGM

AA

TRP3

38,4

82,6

72,7

58

PHEM

AA

TRP8

7,74

4,76

1

PMM

AA

TRP7

58

PTFE

MA

ATR

P397

PMPC

PDM

AEM

AA

TRP7

51

PGM

AA

TRP6

21

PMPC

ATR

P751

PPEG

MA

PDM

AEM

AA

TRP3

88,4

95PN

IPA

MA

TRP2

34PS

S(Na

)A

TRP3

51

PGM

AR

AFT

125

PHEM

AA

TRP4

86,5

08

PMM

AA

TRP3

69

PTFE

MA

ATR

P760

PPEG

MEM

APD

MA

EMA

ATR

P507

PSA

TRP4

94

RA

FT12

0

POM

API

MP2

10

PTFE

MA

ATR

P543

PSBM

APS

S(Na

)R

AFT

119

PTFE

MA

PHEM

AA

TRP5

16

PMM

AA

TRP3

97,7

99

Poly

(acryl

ate)

PAA

PSN

MP6

47

PBA

PDM

AEM

AN

MP1

85PM

AR

AFT

144

PSR

AFT

127

PMM

AA

TRP7

85Pt

BA

ATR

P833

ARG

ET10

2

NM

P181

,341

PEA

PSA

TRP4

76


Tabl

e2.

Con

tinue

dSe

cond

bloc

kFi

rstb

lock

Poly

(meth

acryl

ate)

Poly

(acryl

ate)

Poly

(acryl

amide

)Po

ly(st

yrene

)Po

ly(vi

nylpy

ridine

)PM

APD

MA

EMA

ATR

P/RA

FT66

8PB

AR

AFT

144

PPFS

ATR

P667

PHEM

AA

TRP4

72PC

PPU

AA

TRP4

73PS

ATR

P216

,471

PHD

FDA

ATR

P667

PPFP

AA

TRP6

67,8

39

PtB

AA

TRP6

08,8

38

PTFE

AA

TRP6

67

PtB

APM

AN

MP1

95PS

RA

FT54

9P2

VP

ATR

P106

,882

P4V

PA

TRP1

06

Poly

(acryl

amide

)PA

MPH

EMA

ATR

P781

PMM

AA

TRP7

84

PDM

AM

PBM

API

MP2

15PA

API

MP2

15PM

EAM

ATR

P538

PGM

AR

AFT

763

PNIP

AM

ATR

P538

PHEM

AA

TRP4

86

PMM

AR

AFT

117

PDM

APA

MPM

AA

PIM

P200

PDM

AM

PIM

P215

PSPI

MP2

00

PMEA

MPD

MA

MA

TRP5

38

PNIP

AM

ATR

P538

,540

PNIP

AM

PGM

API

MP6

31PA

GA

RA

FT14

7P4

VP

ATR

P475

PHEM

AA

TRP4

86PD

MA

MA

TRP5

37,5

38

PPEG

MA

ATR

P234

Poly

(styre

ne)

PAS

PSN

MP1

69

PCM

SPP

FSR

AFT

118

PFM

SPS

NM

P161

PPFS

PDM

AEM

AA

TRP4

39PD

VB

ATR

P263

PHEM

AA

TRP5

16

PMM

AA

TRP2

48

PSPD

MA

EMA

NM

P185

PAA

NM

P647

PAM

ATR

P399

PFM

SN

MP1

61P4

VP

NM

P171

PGM

AA

TRP7

62A

TRP/

RAFT

668

PDM

AM

RA

FT11

7PM

SA

TRP5

14

PHEM

AN

MP3

54PB

AA

TRP1

34,4

10,7

85PN

IPA

MA

TRP/

RAFT

668

PPFS

ATR

P667

PMM

API

MP2

07PB

zAA

TRP8

35R

AFT

141

PSA

TRP/

RAFT

668

RA

TRP/

ATR

P90

PHD

FDA

ATR

P667

NM

P184

ATR

P262

,327

,396

,444

,800

PMA

ATR

P84,

216,

770

PSPE

GN

MP1

62

PMM

A-co

-PCD

MA

ATR

P635

RA

FT12

2,13

4,14

4PS

-co

-PM

MA

NM

P156

,169

PMM

A-co

-PS

NM

P156

,169

PPFP

AA

TRP6

67

PtB

AA

TRP8

4,60

8,71

9,72

7,77

0,83

8

PTFE

AA

TRP6

67

PSd8

PMA

ATR

P840

PSP-

co-P

MM

APS

ATR

P797

PSS(

Na)

PHEM

AA

TRP3

54

PPEG

MA

ATR

P351

,808

Oth

ers

P2V

PPt

BA

ATR

P634

,882

P4V

PPN

IPA

MA

TRP4

75PS

NM

P874

PNV

PPD

MA

EMA

RA

FT75

2


quences.219 The authors found that the nature of the surface-attached macroinitiator and the nature of the monomer usedfor the subsequent block are important parameters thatinfluence the success of the surface-initiated block copo-lymerization process. While multiblock copolymer brushescomposed of PMMA and PHEMA were readily prepared,the synthesis of PMMA-b-PDMAEMA brushes proved tobe much more difficult. The authors discovered that whilechains terminated with PDMAEMA did not reinitiate ap-preciably to MMA, the chain ends remained intact and wouldallow further polymerization of DMAEMA.

The importance of the efficiency of the reinitiation stepfor the synthesis of well-defined (multi)block copolymerbrushes was also underlined by Kim et al., who investigatedthe preparation of surface-tethered triblock copolymerscomposed of PMA, PMMA, and PHEMA.217 These authorsfound that quenching the polymerization after the synthesisof each block with a large excess of CuIIBr2 preserved >95%of the active chain ends. In contrast, when just a simplesolvent rinsing step was applied between the synthesis ofthe different blocks, only 85-90% of the active chains wereable to reinitiate polymerization.

2.2.2. Random Copolymer Brushes

Random copolymer brushes (Figure 2B) can be preparedby SI-CRP of a mixture of two or more monomers. Randomcopolymer brushes have been mainly prepared to tune theproperties such as hydrophilicity as well as stimuli-responsiveness (see section 4.1). Table 3 gives an overviewof the different binary copolymer brushes that have beenproduced using SI-CRP. While most random copolymerbrushes are composed of linear polymer chains, SI-CRP hasalso been used to produce branched202,220223 and cross-linked131,194,224232 polymer brushes.

Due to differences in monomer reactivity, the compositionof a copolymer brush is not necessarily identical to themonomer feed. Ignatova et al. have prepared variouscopolymer brushes either via SI-NMP from TEMPO-functionalized stainless steel substrates or via SI-ATRP fromchloropropionated surfaces and used the composition of thefree copolymer that is formed in solution as a measure forthe composition of copolymer brushes.191,233 Copolymeriza-tion of an equimolar mixture of 2-(dimethylamino)ethylacrylate (DMAEA) with styrene (S) or butyl acrylate (BA)as comonomers resulted in copolymers with molar composi-tions of 40:60 and 45:55, respectively, whereas copolymer-ization of equimolar amounts of 2-(tert-butylamino)ethylmethacrylate (tBAEMA) and acrylic acid (AA) or styrene(S) afforded copolymers with compositions of 47:53(tBAEMA:AA) and 40:60 (tBAEMA:S). Neoh and co-workers used XPS analysis to compare the monomer feedcomposition to the final surface composition.234 In their study,the authors showed that a PNIPAM-co-PPEGMA copolymerbrush containing 1.8 mol % PEGMA was formed from apolymerization solution composed of 1 mol % PEGMA.

2.2.3. Binary Brushes

Binary mixed brushes (Figure 2G and H) are composedof two distinct polymer chains immobilized on a solidsubstrate with high grafting density.3 Depending on thespecific arrangement of the polymer chains, random, alter-nating, and gradient binary brushes can be distinguished.Zhao was the first to grow binary mixed brushes via SI-

CRP and prepared PMMA/PS binary brushes from mixedself-assembled monolayers (SAMs) of ATRP and NMPinitiators.235 In addition to mixed SAMs, vapor depositionof an ATRP initiator followed by backfilling with an NMPinitiator has also been used to form gradient binary mixedPMMA/PS brushes.236 One possible complication in thesynthesis of binary mixed brushes from a surface modifiedwith a mixture of orthogonal initiators is phase separationof the initiators, which can prevent the formation of a trulymixed binary polymer brush. To overcome this problem,Zhao and He synthesized a difunctional ATRP/NMP initia-tor-functionalized silane (Y-silane), which was subsequentlyused to prepare mixed binary PMMA/PS brushes.237 Usingthis difunctional initiator, the effects of molecular weighton the solvent-induced self-assembly238 and the changes insurface morphology239,240 of mixed PMMA/PS brushes werestudied. The difunctional Y-silane initiator was also used tograft well-defined PtBA/PS mixed brushes from silicananoparticles.241,242 Wang and Bohn reported the preparationof gradient mixed PNIPAM/PHEMA brushes.243 Thesebrushes were prepared via SI-ATRP from SAMs on gold.In a first step, a PNIPAM brush was grown from a spatiallyuniform initiator SAM. Using electrochemical etching, thePNIPAM brushes were partially removed, following bybackfilling with the ATRP initiator and surface-initiatedpolymerization of PHEMA.

2.2.4. Hyperbranched, Comb-Shaped, and HighlyBranched Brushes

In addition to linear brushes, SI-CRP techniques have alsobeen used to prepare architecturally more complex brushesincluding hyperbranched, comb-shaped, and highly branchedpolymer brushes.

Hyperbranched polymer brushes (Figure 2E) can beprepared in a one step reaction by self-condensing vinylpolymerization (SCVP)244 of AB* inimers (initiator-monomer) from appropriately initiator-modified substrates.245Inimers contain both a polymerizable double bond (A) anda group capable of initiating the polymerization of vinylgroups (B*). Mori et al. described the preparation ofhyperbranched polymer brushes via atom transfer radical(co)polymerization of the AB* inimers, 2-(2-bromopropio-nyloxy)ethyl acrylate and 2-(2-bromoisobutyryloxy)ethylmethacrylate, respectively.220,221,245,246 Other groups havemodified halloysite nanotubes via self-condensing atomtransfer radical (co)polymerization of 2-(bromoacetyloxy)-ethyl acrylate (BAEA)223 or chloromethylstyrene (CMS).247Xu et al. have prepared hyperbranched PPFS/silicon hybridsby copolymerization of CMS and pentafluorostyrene fromATRP initiator-functionalized silicon substrates.248 Mu et al.have used a hyperbranched PCMS brush grafted from silicananoparticles to initiate the subsequent polymerization ofMMA.249

Comb-shaped polymer brushes can be prepared by firstgrowing a homopolymer brush that contains functionalgroups in the side chains, followed by modification of theside chains with ATRP initiator groups or photoinifertermoieties and a second polymerization step to graft thearms.202,222,250252 For example, ATRP initiating groupswere attached to PGMA brushes by reaction of the epoxidemoieties with halogenated propionic acid derivatives. TheseATRP initiator-modified PGMA brushes were subsequentlyused to prepare PGMA-cb-PNIPAM,250 PGMA-cb-PPEG-MEMA,252 and PGMA-cb-PSS(Na) brushes.252 Along the


Table 3. Overview of Random Copolymer Brushes Prepared by Surface-Initiated Controlled Radical PolymerizationMonomer 2

Monomer 1 Methacrylate Acrylate Acrylamide Styrenic OthersMethacrylate

BIEMA MMA ATRP221BMA DMAEMA ATRP272BzMA EGDMA ATRP226

MMA ATRP487CDMA MMA ATRP635DHPMA GMA ATRP619,719DMAEMA BMA ATRP272 NIPAM ATRP642 CMS PIMP222

HEMA ATRP750tBMA ATRP609

EGDMA BzMA ATRP226 4VP ATRP364MAA PIMP228,229 RAFT131MMA ATRP231

GMA DHPMA ATRP619,719MMA ATRP225MPC ATRP620

HEMA DMAEMA ATRP750 S NMP169MMA ATRP271

HosMA MMA ATRP770MAA EGDMA PIMP228,229METAC SPMA(K) ATRP712MMA BIEMA ATRP221 S ATRP444

BzMA ATRP487 NMP156,169,432CDMA ATRP635 SP ATRP797,801,802,814EGDMA ATRP231GMA ATRP225HEMA ATRP271HosMA ATRP770PeMMA ATRP655

MPC GMA ATRP620PEGDMA PEGMA PIMP230PEGMA PEGDMA PIMP230 NIPAM ATRP234

PEGMEMA ATRP307,552PEGMEMA PEGMA ATRP307,552 NIPAM ATRP422

tBAEMA ATRP233PeMMA MMA ATRP655SPMA(K) METAC ATRP712tBAEMA PEGMEMA ATRP233 AA ATRP233 S ATRP233tBMA DMAEMA ATRP609

AcrylateAA AM PIMP523 S RAFT134

NIPAM ATRP690PIMP490

AAb PEGA PIMP718AA(Na) NIPAM ATRP654BA BAEA ATRP223

DMAEA NMP191BAEA BA ATRP223BPEA tBA ATRP220,221DMAEA BA NMP191 S NMP191EGDA NIPAM ATRP224MA S RAFT134PEGA AAb PIMP718

PEGASF PIMP524PEGASF PEGA PIMP524tBA BPEA ATRP220,221

AcrylamideAM AA PIMP523 MBAM ATRP232

N-BocAHAM ATRP578DMAM CMS PIMP202,222MBAM AM ATRP232

NIPAM ATRP227,363,424N-BocAHAM AM ATRP578NIPAM DMAEMA ATRP642 AA ATRP690 MBAM ATRP227,363,424 NVI ATRP650

PEGMA ATRP234 PIMP490PEGMEMA ATRP422 AA(Na) ATRP654

EGDA ATRP224Styrenic

CMS DMAEMA PIMP222 DMAM PIMP202,222GL S ATRP467S HEMA NMP169 AA RAFT134 SLS ATRP627 AN ATRP443,871

MMA ATRP444 DMAEA NMP191 GL ATRP467 MAn RAFT450NMP156,169,432 MA RAFT134 NMP194

TMI RAFT547VBCB NMP194

SLS S ATRP627SP MMA ATRP797,801,802,814

Others4VP EGDMA ATRP364

RAFT131AN S ATRP443,871MAn S RAFT450

NMP194NVI NIPAM ATRP650TMI S RAFT547VBCB S NMP194


same lines, the hydroxyl groups of PHEMA and PPEGMAbrushes have been used to introduce ATRP initiating2-bromoisobutyrate moieties,251 and PCMS brushes havebeen modified with dithiocarbamate derivatives to allowSI-PIMP.202,222

The synthetic strategies developed for the preparation ofcomb-shaped brushes can be readily extended to highlybranched or arborescent brushes (Figure 2F) by repetitionof the (co)polymerization/postmodification sequence usingappropriate functional monomers to act as grafting points.Matsuda and co-workers, for example, prepared highlybranched polymer brushes via successive photopolymeriza-tion of a CMS-containing monomer(s) mixture, followed bydithiocarbamylation.202,222 Xu et al. reported the formationof highly branched PPFS brushes via surface-initiated atomtransfer radical copolymerization of CMS and pentafluo-rostyrene from a surface-immobilized difunctional ATRPinitiator.248

2.2.5. Cross-linked Brushes

Cross-linked polymer brushes (Figure 2C) can be preparedvia two main pathways, namely the surface-initiated homo-or copolymerization of bifunctional monomers and thepostmodification of polymer brushes with appropriate cross-linking agents. The homopolymerization of ethylene glycoldimethacrylate derivatives via either SI-ATRP253255 or SI-PIMP256 is probably the easiest way to prepare cross-linkedpolymer brushes. The addition of a cross-linkable comono-mer to the polymerization mixture is also widely used forthe preparation of cross-linked brushes. Already in 1998,Wirth and co-workers successfully prepared cross-linkedpolyacrylamide (PAM) brushes on the interior surface ofsilica capillaries by adding 2% of N,N-methylenebisacryl-amide (MBAM) to the ATRP polymerization solution.232The use of ethylene glycol di(meth)acrylates as comono-mers for the preparation of cross-linked brushes has beenextensively reported for SI-ATRP,224,231 SI-RAFT,131 or SI-PIMP.228230

In addition to the homo- or copolymerization of bifunc-tional monomers, cross-linked brushes can also be obtainedby postmodification of appropriately functional polymerbrushes. A widely used strategy is based on the postmodi-fication of linear PGMA brushes with (di)amines suchas ethylenediamine,257 1,4-phenylenediamine,258 or octyl-amine.225 Edmondson et al. reported the use of methanolicNaOH to induce internal cross-linking via the pendentepoxide groups along the side chain of linear PGMAbrushes.259,260 Loveless et al. showed that P4VP brushes canbe reversibly cross-linked by the addition of a bis(PdII-pincer)compound, which noncovalently coordinates to the vinylpy-ridine units of the polymer brush.261

The preceding paragraphs have outlined two strategies thatcan be used to deliberately prepare cross-linked polymerbrushes. Cross-linking, however, can also occur in a lesscontrolled manner as a result of side reactions during SI-CRP. Huang et al., for example, found that detachment ofPHEMA brushes from gold substrates afforded insolublepolymer films.88 The insolubility of the grafted PHEMA wasattributed to intermolecular cross-linking via transesterifi-cation. In another report, it was observed that PPEGMAbrushes detach in the form of continuous films from siliconsubstrates upon exposure to cell culture medium, which alsosuggests that these brushes cross-link during surface-initiatedpolymerization.254

2.2.6. Free-Standing Brushes

In the previous section, various approaches were discussedthat can be used to prepare cross-linked polymer brushes. Ifthese cross-linked brushes are prepared on substrates thatcan be dissolved or sacrificed, then this provides opportuni-ties to produce free-standing 2D polymer films (Figure 2D)as well as polymer hollow spheres or tubes. This sectionwill give an overview of different free-standing brushes,either in the form of 2D films or hollow spheres or tubes,that have been prepared following this rationale.

Huck and co-workers described the preparation of quasi-2D polymer films by delaminating cross-linked PGMAbrushes grown from ATRP initiator-functionalized goldsubstrates upon electrolysis.259 The same group studied thebuckling process in these patterned quasi-2D films by locallyapplying a short electrolysis pulse to cleave the gold-sulfurbond, which tethers the film to the gold substrate.260

Hollow polymeric nanospheres have been prepared by HFetching of silica nanoparticles coated with a cross-linkedpolymer shell. Mandal et al. grafted PBzMA-co-PEGDMAbrushes via SI-ATRP from SiO2 particles to obtain hollowpolymer particles after HF etching of the sacrificial silicacores.226 Other methods to create such architectures are basedon postmodification of linear polymer brushes to producecross-linked shells, either via internal ring-opening reactionof moieties along the polymer brush, addition of a cross-linker, or UV irradiation. Hawker and co-workers, forexample, prepared random PS-co-PVBCB and PS-co-PMAncopolymer brushes via NMP from silica nanoparticles. Intheir study, they used the cyclobutene groups of PVBCBchains as thermal cross-linking agent or a diamine cross-linker to react with PMAn groups.194 Fu et al. prepared cross-linked hollow nanospheres via SI-ATRP.262,263 PS-b-PMMAcoated silica nanoparticles were irradiated with UV to inducedecomposition of the PMMA outer shell and cross-linkingof the PS shell. After HF treatment of the core, well-definedhollow nanospheres were obtained.262 A related strategy wasused by the same authors to prepare thin films of agglomer-ated and cross-linked hollow polymer nanospheres.263 To thisend, PPFS-b-PDVB block copolymer brushes were grownfrom silica nanoparticles using SI-ATRP. UV irradiation ofthe block copolymer-modified nanoparticles leads to inter-and intramolecular cross-linking of the residual double bondsin the PDVB layer, which simultaneously covalently stabi-lizes the PDVB shell of the particles and connects theindividual particles to form a continuous film. Removal ofthe silica core with HF afforded a porous fluoropolymer film.Recently, Morinaga reported another strategy to preparehollow nanospheres. The PEMO layer of PEMO-b-PMMAgrafted silica particles was internally cross-linked by cationicring-opening reaction of the oxetane groups catalyzed withboron trifluoride diethyl etherate, followed by HF etchingto remove the silica core.264

The strategies discussed above can be easily extended tothe preparation of polymeric nanotubes when porous mem-branes or nanowires instead of nano- or microparticles areused as sacrificial substrates. Cui et al. reported the formationof PNIPAM-co-PMBAM copolymer nanotubes by growingthe corresponding polymer brushes within a anodic aluminumoxide (AAO) membrane by SI-ATRP followed by thedissolution of the AAO membrane in 1 M NaOH solution.227


2.2.7. Gradient Brushes

Gradient brushes are brushes wherein one or more phys-icochemicalpropertiesvarycontinuouslyalongthesubstrate.265270Gradient brushes prepared via SI-CRP can be subdividedinto four groups; chemical composition gradient brushes(Figure 2K and L), grafting density gradient brushes (Figure2J), molecular weight gradient brushes (Figure 2I), or brushesprepared by a combination of several gradient architec-tures.266

Xu et al. prepared PMMA/PHEMA gradient copolymerbrushes via SI-ATRP by gradually adding an ATRP solutionof HEMA to the MMA polymerization mixture.271 Insteadof creating chemical composition gradients parallel to thesubstrate, polymer brushes can also be prepared that have acomposition gradient perpendicular to the substrate. Suchcomposition gradient brushes have been reported by Beersand co-workers, who prepared PBMA/PDMAEMA compo-sition gradient brushes via SI-ATRP from silicon substratesby using a microchannel filled with a solution gradient ofboth monomers.272

Grafting density gradient polymer brushes can be obtainedby SI-CRP from substrates covered with a surface concentra-tion gradient of polymerization initiators, iniferters, or chaintransfer agents, which can be prepared using various strate-gies. Wu et al. prepared gradient initiating layers by meansof the methodology developed by Chaudhury and White-sides.273 Briefly, an initiator concentration gradient along thesubstrate was generated by vapor diffusion of a 1-trichlo-rosilyl-2-(m/p-chloromethylphenyl)ethane/paraffin oil mixturefollowed by backfilling with n-octyl trichlorosilane (OTS).274,275This initiator gradient layer was subsequently used to graftPAM brushes via SI-ATRP following the conditions reportedby Huang and Wirth.82 Instead of first generating an initiatinggradient layer followed by backfilling with an ATRP inactivesilane, grafting density gradient brushes can also be preparedby first forming a gradient of inactive silane followed bybackfilling with an ATRP initiator. This strategy has beenused to produce density gradient PtBA brushes.276 Based onthe strategy of Wu et al., Zhao developed a method to preparegrafting density gradients of two chemically differentpolymer brushes propagating in opposite directions by vapordeposition of an ATRP initiator onto the substrate and thenbackfilling with a NMP initiator. The resulting gradientmixed initiator SAM was subsequently used for the prepara-tion of density gradient binary mixed PMMA/PS brushes.236In addition to vapor deposition, there are various otherstrategies to generate substrates covered with an initiatordensity gradient layer. Liu et al. used a linear temperaturegradient stage to generate a gradual variation in the thicknessof a dip-coated PGMA layer, which was then proportionallyderivatized with 2-bromo-2-methylpropionic acid followedby the ATRP of styrene.277 Washburn and co-workersproduced initiator gradient films by means of gradual additionof ATRP initiator to a test tube containing an OTS-modifiedsilicon wafer and then grew PHEMA brushes.278 Wang etal. used an electrochemical gradient to selectively desorbhexadecanethiol from a gold substrate followed by backfillingof the free areas with an ATRP initiator from which PNIPAMbrushes were successively grown.279 Wang and Bohn furtherdescribed the preparation of density gradient binary mixedPNIPAM/PHEMA brushes where the grafting density of eachpolymer varies in opposite directions along the substrate.243Their strategy involved a gradual chemical etching ofPNIPAM brushes grown from uniform ATRP initiator SAM

on gold followed by backfilling of the etched surface siteswith fresh ATRP initiator for the subsequent surface-initiatedpolymerization of HEMA.

Molecular weight (i.e., thickness) gradient brushes havebeen prepared using both SI-PIMP and SI-ATRP. Harris etal. described the synthesis of PMMA280 and PMAA281thickness gradient brushes from photoiniferter-modifiedsilicon substrates using a movable photomask, which permitscreation of a UV exposure time gradient along the substrate.A similar strategy was followed by Matsuda and co-workers,who used a movable sample stage instead of moving thephotomask.199 The same group also reported on the use of agradient neutral-density filter to introduce a continuous andunidirectional change of the irradiation intensity during thephotopolymerization for the preparation of molecular weightgradient films.200 PMMA,282 PAM,265,283 and PHEMA-b-PMMA284 thickness gradient brushes were successfullyprepared via SI-ATRP by continuously and gradually remov-ing the polymerization solution from the chamber containingthe ATRP initiator-functionalized substrate (draining method)with a -pump. Tomlinson et al. used a dipping sampleholder that allowed control of the longitudinal position ofthe ATRP initiator-functionalized substrates in a discrete orsemicontinuous manner to prepare step height, respectively,molecular weight gradient polymer brushes.219 Beers and co-workers generated PHEMA molecular weight gradientbrushes using microchannel-confined SI-ATRP, which al-lowed them to control the lateral composition of thepolymerization mixture.285 The same group also preparedPDMAEMA-b-PBMA block copolymer brush gradientsconsisting of a uniform PBMA bottom block and a molecularweight gradient PDMAEMA top block by gradually fillingthe chamber containing the living PBMA brushes coatedsubstrate with the second ATRP solution.286,287

By combining some of the methods described above,Genzer and co-workers have prepared orthogonal gradientpolymer brushes where physicochemical properties, such asmolecular weight and density of a given polymer ormolecular weights of two polymers, vary independently inorthogonal directions.288291 Such orthogonal gradient brushesare ideal candidates for high-throughput structure-propertyinvestigations.

2.2.8. Variation of Brush Density

In the previous section, various strategies have beenpresented that can be used to cover substrates with a polymerbrush density gradient. The control and variation of brushdensity will be discussed in more detail in this section. Incontrast to the previous section, which concentrated ondensity gradients, the focus here will be on techniques thatcan be used to homogeneously cover surfaces with a polymerbrush coating of controlled density.

The most commonly used method to vary brush densityis based on the modification of the substrate from which thebrush is grown with a mixture of an initiator-functionalizedcompound and a dummy compound that is not able toinitiate the polymerization reaction. This approach has beenused for the preparation of PPEGMEMA,292294 PMMA,295PMETAC,296 and PGMA295 brushes from mixed thiol self-assembled monolayers on gold substrates, to growPNIPAM,297 PPEGMA,298 and PDMAEMA299 brushes frommixed disulfide monolayers on gold, to generate PPEG-MA,254 PMAA(Na),111 and PHEMA300 brushes from mixedtrimethoxysilane monolayers on silicon wafers, as well as


for the synthesis of PMPC,301 PNIPAM,302 PHEMA,303 andPMMA303 brushes from mixed trichloro- or monochlorosi-lane-functionalized silicon substrates. Usually, the initiatorfunctionalized and dummy molecules have similar chemi-cal structures and are assumed to have good affinity to thesubstrate such that the relative amount of both compoundsin solution is equal to that on the surface.304306 However,XPS studies on mixed trimethoxysilane111 as well as mixedthiol294 SAMs have revealed a nonideal behavior whencomparing surface and solution compositions.

A second strategy to vary the initiator surface concentra-tion and brush density involves postmodification of aprecursor amino-, hydroxyl-, or allyl-terminated SAM witha compound that is able to initiate ATRP. Brown et al., forexample, modified substrates with different surface concen-trations of ATRP initiating groups by postmodifying theamine groups of a 3-(aminopropyl)trimethoxysilane (APTS)layer with different molar ratios of 2-bromoisobutyrylbromide and propionyl bromide. These mixed-initiator layer-modified substrates were subsequently used to preparePPEGMA brushes.307 Bao et al. used mixtures of 2-bro-mopropionyl bromide and 2-methylpropionyl bromide toderivatize hydroxyl-terminated monolayers on gold, whichwere then used to grow PMMA and PHEMA brushes.303Along the same lines, hydroboration of mixed octadecyl-trichlorosilane/15-hexadecenyltrichlorosilane layers has beenused to introduce hydroxyl groups that were selectivelyreacted with 2-bromoisobutyryl bromide and subsequent-ly used for the preparation of PMEMA, PPEGMEMA,PHEMA, PAM, PMMA, and PS brushes with variablegrafting densities.308,309 Statistical UV photodecompositionof the surface-fixed 2-(4-chlorosulfonylphenyl)ethyl trichlo-rosilane ATRP initiator was used by Yamamoto et al. tocontrol the density of PMMA brushes.310

The Langmuir-Blodgett technique provides another toolto modify substrates with a controlled surface concentrationof initiator and generate polymer brushes with controlleddensities. This method was successfully used to transferdefined monolayers of 2-(4-chlorosulfonylphenyl)ethyl-83 ornitroxide-functionalized158,159 alkoxysilanes onto silicon wa-fers, which were subsequently used to graft PMMA and,respectively, PS brushes.

Kizhakkedathu et al. grafted PDMAM brushes from ATRPinitiator-functionalized PS latex particles, which were syn-thesized with different initiator concentrations by changingthe feed ratio of styrene to initiator (2-(methyl-2-chloro-propionato)ethyl acrylate) during the particle preparation.311Instead of varying the mole fraction of initiator-modified

monomer during particle synthesis, the same group demon-strated that the brush density can also be controlled by carefulbasic hydrolysis of grafted polymer chains from the latexparticles.312

Finally, Wu et al. reported an interesting strategy to preparehighly dense PAM brushes on PDMS substrates. Mechanicalstretching of the PDMS substrate during both the initiatorfunctionalization step and the following SI-ATRP resultedin a highly dense PAM brush upon relief of the strain.313

2.3. Variation of SubstrateSI-CRP techniques have been used to grow polymer

brushes from a wide variety of different substrates. In orderto graft polymer brushes, the substrate surface needs to bemodified with an appropriate initiator, iniferter, or RAFTagent, which can be introduced either in a single step or viaa multistep protocol (Figure 3). The one step protocolsrequire the use of molecules that contain the appropriateinitiator, iniferter, or RAFT agent as well as functional groupsthat can react with complementary functional groups on thesubstrate surface. Alternatively, the substrate surface can bemodified with molecules that introduce certain functionalgroups, which can then be modified with the desired initiator,iniferter, or RAFT agent in a subsequent (series of) reac-tion(s). The focus of this section will be on the modificationof the substrate surface with the initiator, iniferter, or RAFTagent needed for the SI-CRP process. This section consistsof eight parts, which will successively discuss the preparationof polymer brushes via SI-CRP from silicon oxide, silicon,metal oxide, clay mineral, gold, metal and semiconductor,carbon, and polymer surfaces. For each of these classes ofsubstrates, the discussion will concentrate on the surfacechemistry that is available to introduce functional groups thatallow SI-CRP.

2.3.1. Polymer Brushes Grafted from Silicon Oxide

Among the different substrates that have been used toproduce polymer brushes via SI-CRP, silicon oxide has beenmost extensively used. Table 4 provides an overview of thedifferent initiators, iniferters, and RAFT agents that havebeen used to graft polymer brushes from silicon oxidesurfaces. For each example, Table 4 specifies the nature ofthe anchoring group, the chemical structure of the initiator,iniferter, or RAFT agent, the polymerization technique, aswell as the nature and geometry of the substrate surface.Since the focus of this section is on the surface chemistries

Figure 3. Substrate surface modification with initiator, iniferter, or RAFT agent: (A) one step strategy; (B) multistep strategy.


Table 4. Overview of Initiators, Iniferters, and RAFT Agents That Have Been Used To Grow Polymer Brushes from Silicon OxideSurfaces


Table 4. Continued


that are available to introduce polymerization active groupsand to avoid unnecessary lengthening of the table, the natureof the linker that connects the anchoring group and theinitiator, iniferter, or RAFT agent is not specified.

Polymer brushes have been grafted from a wide range ofsilicon oxide substrates including wafers, glass or quartzslides, porous and nonporous particles, as well as capillariesand membranes. Polymer brushes are also frequently pro-duced from thin silicon oxide layers that have been depositedonto metallic substrates. For the modification of silicon oxidesurfaces with initiators, iniferters, or RAFT agents, twogeneral strategies are available, which will be discussed inthe following paragraphs. The first strategy, which is mostfrequently used, is based on the chemisorption (covalentattachment) of organosilane molecules. A second possibilityto modify silicon oxide surfaces with functional groups thatcan initiate SI-CRP is based on the physisorption ofpolyelectrolyte macroinitiators.

The use of organosilane reagents to introduce functionalgroups that can initiate or mediate SI-CRP is a directextension of the concept of organosilane self-assembledmonolayers (SAMs), which have been extensively investi-gated since the 1980s.314 Commonly, SiO2 surfaces areactivated prior to the grafting step to clean the surface andmaximize the number of silanol groups. Usually, H2SO4/H2O2 mixtures (piranha) or oxygen plasma are employed.These procedures render the surface hydrophilic and promotethe formation of a thin layer of water onto the SiO2 surface.There is a general consensus that trace amounts of waterare essential for the formation of a well-packed monolayerof organosilane molecules.315 The formation of organosilaneSAMs on silicon oxide surfaces is believed to proceed via asequence of surface adsorption, hydration, and silanizationsteps. In this process, silanol groups (Si-OH) on the SiO2surface react with organosilane molecules such asR-SiRxCl3-x or R-SiRx(O(CH2)nCH3)3-x through a conden-sation reaction to form Si-O-Si chemical bonds.316,317 Thisprocess is not necessarily limited to the surface and, undercertain conditions, the organosilane SAM may developin three dimensions because the dehydration may happenbetween organosilane monomers and SAM instead ofbetween organosilane monomers and surface functionalgroups.317 The chemisorption of organosilane molecules to

silicon oxide substrates is a reaction that is very sensitive tomany experimental parameters, such as reaction time, tem-perature, or water content.318324 In addition to the reactionconditions, also the structure of the organosilane reagent and,specifically, the number of hydrolyzable groups influencethe quality of the resulting organosilane layer. The chemi-sorption of both mono- (R3SiX), di- (R2SiX2), and tri-(RSiX3) functional organosilanes, where X is a hydrolyzablegroup (usually X ) Cl, OR, NMe2), has been investigatedextensively.325 Monofunctional organosilane molecules(R3SiX) are attractive in terms of the reproducibility of theorganosilane layer because only one type of grafting ispossible. Trifunctional organosilane molecules (RSiX3) aremore reactive compared to their monofunctional analoguesbut are capable of polymerizing in the presence of water. Inaddition to covalent attachment, 2D horizontal polymeriza-tion and 3D surface-induced polycondensation are possible.325Difunctional organosilane molecules (R2SiX2) are the leastfrequently used silanes to modify silicon oxide substrates.In addition to covalent attachment, chemisorption of difunc-tional organosilanes on silicon oxide can also lead to verticalpolymerization and the formation of a thicker (i.e., non-monolayer) organosilane film.325

For the modification of silicon oxide surfaces with functionalgroups that can initiate or mediate SI-CRP, many organosilanereagents that contain one polymerization active group and one(-SiMe2Cl, -SiMe2OEt) or three hydrolyzable groups (-Si-(OMe)3, -Si(OEt)3, -SiCl3) have been used. In addition,several examples of organosilane molecules functionalized withone polymerization active group and two hydrolyzable groupssuch as -SiMe(OEt)2326,327 or -SiMe(OMe)2328 have beenreported. Finally, a few examples of organosilane moleculesfunctionalized with two orthogonal polymerization activegroups and one or three hydrolyzable groups have beendescribed.237,238,240242,329 The use of these asymmetric di-functional initiator-terminated SAMs, which have beenreferred to as Y-SAMs, was presented in section 2.2.3.

As discussed above, the chemisorption of organosilanereagents on silicon oxide can be a very delicate process,which, among others, is sensitive to moisture. To overcomethe problem of moisture sensitivity, Ruhe and co-workersdeveloped an ATRP initiator functionalized with hydridosi-lane groups (-Si(Et)2H) to modify SiO2 surfaces.330 In this

Table 4. Continued

a LbL: layer-by-layer deposition.


case, a covalent bond is formed between the silicon atom ofthe hydridosilane and the oxygen atom of a hydroxyl groupon the surface, presumably upon the elimination of hydrogen.The distinct advantage of hydridosilanes is that they arestable even in moist environments. A different approach toovercome the moisture sensitivity of initiators or inifertersfunctionalized with organosilane moieties has been developedby Brittain and co-workers.331 These authors reported amultistep process that starts with the grafting of an allyldim-ethylsilane derivative onto the SiO2 surface, followed bypostfunctionalization of the organosilane layer with an ATRPinitiator. This strategy is based on earlier work by Shimadaand co-workers, who reported the modification of silica gelusing allylorganosilanes.332 In refluxing toluene, deallylationof allylsilanes takes place under the formation of anSi-OsSi bond with the silicon oxide substrate. This methodof surface functionalization has the merit that allylsilanesare stable toward hydrolysis and can be purified by silicagel chromatography.332

Although organosilane reagents have been very extensivelyused to modify silicon oxide substrates with functional groupsthat can initiate or mediate SI-CRP, the resulting polymerbrushes are tethered via Si-OsSi bonds, which are thermallylabile and susceptible to hydrolytic cleavage.333,334 Recently,it was shown that poly(poly(ethylene glycol) methacrylate)(PPEGMA) brushes, prepared by SI-ATRP from glass orsilicon oxide substrates modified with a trimethoxysilane-based ATRP initiator, detach rapidly from the substrate whenhigh density brushes were incubated in cell culture me-dium.254 The reason for the detachment is still controversial,but it was proposed that detachment of the brushes involvescleavage of Si-O bonds that are located at the interfacebetween the brush and the substrate.315,335 Possible explana-tions for the detachment of the PPEGMA brushes may beosmotic stresses that act on the brushes in the cell culturemedium as well as steric crowding. Both of these factorscould induce additional tension along the already stretchedpolymer brush backbones, which could promote hydrolysisof the Si-O bonds and detachment of the brush. In tworecent reports, it has been demonstrated that polymer/surfaceinteractions can generate tensions along polymer backbonesthat are sufficient to mechanically break covalent bonds.336,337One possibility to overcome this problem could be to graftpolymer brushes via more robust Si-C bonds instead ofSi-O bonds. It has been shown, for example, that chlorinatedSiO2 surfaces (SiO2-Cl) are effective initiators for surface-initiated ATRP from oxidized silicon wafers,338 glassslides,338 or porous silica microparticles.339 In these cases,the resulting polymer brushes are covalently attached to theSiO2 surfaces via stable Si-C bonds. Alternatively, SiO2-Clsurfaces can be postmodified to initiate RAFT,340 reverseATRP,91 or bimolecular NMP341 SI-CRP reactions.

In addition to the use of low molecular weight organosilanemolecules, a second approach to modify silicon oxidesubstrates with ATRP initiators is based on the physisorptionof ATRP initiator-modified polyelectrolytes. Armes and co-workers have designed a cationic trimethylammonium-basedATRP macroinitiator and an anionic sulfate-based ATRPmacroinitiator, which were electrostatically adsorbed ontoultrafine anionic sols342 and aminated (cationic) planar oxi-dized silicon wafers, respectively.343 Recently, the layer-by-layer (LbL) deposition of the two oppositely chargedpolyelectrolyte macroinitiators discussed above, or analogues,hasbeenusedtofunctionalizeplanarsiliconoxidesubstrates.344,345

As indicated in Table 4, SI-CRP has been used to graftpolymer brushes from silicon oxide surfaces of variousgeometries. This section concludes with a few remarks onthe effects of the substrate geometry on the SI-CRP process.Recently, Genzer, Gorman, and co-workers have investigatedthe effect of confinement on the molecular weight andpolydispersity of polymer brushes prepared by SI-ATRP.346To this end, porous silicon oxide (etched silicon wafer) andporous anodic aluminum oxide (AAO) membranes with anominal pore size of 50 and 200 nm were used astemplates for the grafting from polymerization of methylmethacrylate (MMA). It was found that, under identicalpolymerization conditions, PMMA grown from poroussubstrates had a much lower molecular weight and a broadermolecular weight distribution compared to PMMA preparedvia solution ATRP. These differences were attributed toconfinement effects, which were related to reduced growthrates and more polydisperse chains. Kruk, Matyjaszewski,and co-workers, shortly thereafter, reported an improved SI-ATRP protocol that allows grafting of polymer brushes fromthe surfaces of cylindrical and spherical mesopores withimproved control over film thickness and with polydisper-sities comparable to those obtained in well-controlled solutionpolymerization.347 This improved control was achieved bythe addition of appropriate amounts of deactivating CuIIspecies in the polymerization reaction.

2.3.2. Polymer Brushes Grafted from Silicon

In contrast to silicon oxide, only a relatively small numberof reports has been published that describe the preparationof polymer brushes from silicon surfaces. Table 5 presentsan overview of the different ATRP initiators and RAFTagents that have been used to allow SI-CRP from siliconsurfaces.

The grafting of polymer brushes from silicon surfaces isattractive, since the polymer chains are tethered via robustSi-C bonds. This process starts with the preparation of ahydrogen-terminated silicon surface (Si-H), which can beobtained by treating a pristine silicon oxide substrate withdilute hydrofluoric acid to remove the native oxide layer.348After that, functional groups that are able to initiate ormediate SI-CRP can be immobilized in either a one stepprocess or a multistep process.

Most frequently, the initiators or RAFT agents are im-mobilized on silicon substrates via UV-induced coupling ofp- or o-chloromethylstyrene to provide a stable initiatormonolayer attached via robust Si-C bonds.248 The Si-Hgroup on the silicon surface can be homolytically dissociatedby UV irradiation to form a radical site, which reacts readilywith an alkene to give rise to a surface-tethered alkyl radicalon the -carbon. The radical subsequently abstracts an Hatom from the adjacent Si-H bond. The abstraction createsa new reactive silicon radical to allow the above reaction topropagate as a chain reaction on the Si-H surface.348 Theresulting chloromethylbenzene-functionalized surfaces eithercan be used to directly initiate SI-ATRP250,251,349358 or canbe postmodified with a RAFT agent.143 Along the same lines,-unsaturated alkyl ester118,119,257,359,360 and 4-vinylaniline234have also been photoimmobilized on silicon and subsequentlypostmodified with ATRP initiating groups234,257,359,360 orRAFT agents118,119 to allow SI-CRP. Kang and co-workershave demonstrated that halogenated silicon surfaces (Si-X;X ) Cl, Br), obtained via chlorination or bromination of


the hydrogen-terminated silicon surfaces, are themselveseffective initiators for SI-ATRP.361

2.3.3. Polymer Brushes Grafted from Metal OxideSurfaces

An increasing number of publications describes the graft-ing of polymer brushes from metal oxide surfaces via SI-CRP. Table 6 presents an overview of the different initiatorsand RAFT agents that have been used to grow brushes frommetal oxide surfaces. The different substrates are listedalphabetically in this table. To date, most examples of CRPinitiated from metal oxide surfaces have employed aluminum,titanium, or iron oxide substrates. Only very few examplesof polymer brushes grafted from other metal oxide surfacessuch as indium tin oxide, copper oxide, nickel oxide, zincoxide, and magnesium oxide have been reported.

Porous alumina membranes have been modified withpolymer brushes via SI-ATRP. Both one step and two stepprotocols have been used to graft ATRP initiator-function-alized organosilanes. The Al-OsSi bond formed uponreaction of the organosilane moieties with the surfacehydroxyl groups of the substrate is the strongest andhydrolytically most stable in the metal-O-Si series, al-though its strength is inferior to that of the Si-OsSi bond.362The two step approach for the modification of aluminasubstrates with ATRP initiators starts with the immobilizationof 3-aminopropyltrimethoxysilane followed by postmodifi-cation with 2-bromo-2-methylpropionyl bromide.227,363,364Alternatively, trichlorosilane-functionalized ATRP initiatorssuch as [(11-(2-bromo-2-methyl)propionyloxy)undecyl]-

trichlorosilane346,365367 and 1-(trichlorosilyl)-2-[m/p-(chlo-romethyl)phenyl]ethane368 can be grafted in a one stepreaction to the alumina substrate.

Similar to alumina substrates, ATRP initiators function-alized with triethoxy- or trichlorosilane moieties have beenused to modify the surface of Fe3O4 nanoparticles.369377 Inthese cases, the polymer brushes are believed to be tetheredthrough a Fe-OsSi bond. Alternatively, ligand-

polymer brushes via surface-initiated controlled radical polymerization

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