side-chain modification and grafting onto via olefin cross-metathesis

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Side-Chain Modifi cation and Grafting Onto via Olefi n Cross-Metathesis

Lucas Montero de Espinosa , Kristian Kempe , Ulrich S. Schubert , Richard Hoogenboom , * Michael A. R. Meier *

Olefi n cross-metathesis is introduced as a versatile polymer side-chain modifi cation tech-nique. The reaction of a poly(2-oxazoline) featuring terminal double bonds in the side chains with a variety of functional acrylates has been successfully performed in the presence of Hoveyda–Grubbs second-generation catalyst. Self-metathesis, which would lead to polymer–polymer coupling, can be avoided by using an excess of the cross-metathesis partner and a catalyst loading of 5 mol%. The results suggest that bulky acrylates reduce chain–chain coupling due to self-metathesis. Moreover, different func-tional groups such as alkyl chains, hydroxyl, and allyl acetate groups, as well as an oligomeric poly(ethylene glycol) and a perfl uorinated alkyl chain have been grafted with quantita-tive conversions.

1. Introduction

The synthesis of designed complex polymeric architec-tures, such as side-group functional polymers and graft polymers, is very often limited by the incompatibility of monomer structure and polymerization technique. [ 1 , 2 ] Poly mer post-polymerization modifi cation provides an

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L. M. de Espinosa, M. A. R. MeierLaboratory of Applied Chemistry, Karlsruhe Institute of Technology (KIT), Institute of Organic Chemistry, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany E-mail: [email protected] K. Kempe, [+] U. S. SchubertLaboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-Schiller-University Jena, Philosophenweg 7, 07743 Jena, Germany K. Kempe, U. S. SchubertJena Center of Soft Matter (JCSM), Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena, Germany R. HoogenboomSupramolecular Chemistry Group, Department of Organic Chemistry, Ghent University, Krijgslaan 281 S4, 9000 Ghent, Belgium E-mail: [email protected] [+] Nanostructured Interfaces and Materials group, Department of Chemical & Biomolecular Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia

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effi cient approach for the introduction of functional groups along the poly mer backbone. In this manner, various mole-cules as well as end-functionalized macromolecules can be attached following a “grafting-onto” approach. [ 1 ] Further-more, initiating sites can be introduced enabling the growth of polymeric chains in a “grafting-from” approach. [ 3 ] The literature dealing with the functionalization of polymers with small molecules is vast, and some recent studies include the use of azide–alkyne cycloaddition, [ 4 , 5 ] thiol-ene addition, [ 6 , 7 ] amidation, [ 8 ] thia-michael addition, [ 9 ] epoxide ring-opening with thiols, [ 10 ] and amines [ 11 ] among many others. [ 12 ] On the other hand, the “grafting-onto” method has been extensively applied in the synthesis of graft (co-)polymers using esterifi cations, [ 13 ] azide–alkyne cycloaddi-tions, [ 14 , 15 ] Diels–Alder cycloadditions, [ 16 , 17 ] thiol-ene addi-tions, [ 18 ] and others.

Olefi n metathesis, although widely used as polymeriza-tion technique in two main different approaches, namely acyclic diene metathesis [ 19 ] and ROMP, [ 20 ] has not yet been used and studied in detail as “grafting-onto” tech-nique. Moreover, there is a single example by Mecking and co-workers [ 21 ] on the use of olefi n metathesis as side-chain modifi cation technique of a methacrylate-substituted polyethylene. The cross-metathesis between polymer-bound and dissolved olefi ns was reported by Blechert and co-workers, [ 22 ] but only cross-linked resins were used. Double bonds are generally unreactive toward

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Scheme 1 . Side-chain functionalization of 10-undecenoic acid derived poly(2-oxazoline) via cross-metathesis with acrylates in the presence of the Hoveyda–Grubbs second-generation catalyst.

common polymerization methods such as polyesterifi -cations, polyamidations, or ring-opening polymeriza-tion of lactones among others. Therefore, the synthesis of polymers with unsaturated pendant chains should be straightforward from a variety of monomers containing a double bond along with the polymerizable group. In this respect, terminal double bonds are preferred since ethylene will be released (and easily removed) as side-product if another terminal olefi n is used as cross-metathesis (CM) partner. [ 23 ] Taking into account, the wide variety of commercially available olefi n compounds, the functionalization of olefi n-containing polymers via cross-metathesis would offer a fast access, ideally in one synthetic step, to a large number of functionalities. The main potential drawback of olefi n metathesis as grafting-onto method is the occurrence of self-metathesis (SM), which can take place either between two olefi ns of the same polymer chain (leading to cycles), or between two olefi ns of different chains resulting in polymer–polymer coupling and, fi nally, a cross-linked network. In this aspect, Coates and Grubbs [ 24 ] reported that the intramolecular SM of polymers bearing terminal alkenes as side chains can be quantitative if the resulting cycles are stable (fi ve-membered rings). However, these problems can be overcome using an excess of the CM partner and/or by its careful selection. As reported by Grubbs and co-workers, [ 25 ] electron-rich olefi ns such as terminal double bonds tend to homodimerize while, on the other hand, electron-defi cient olefi ns such as acrylates homodimerize very slowly and give preferentially the CM product. In this way, if a terminal double bond and an acrylate are chosen as metathesis partners, the CM selectivity is maximized, and the resulting internal olefi n product does not (or does only very slowly) undergo secondary metathesis reac-tions. [ 25 ] Taking this into account, it should be possible to graft acrylates onto a polymer bearing pendant terminal double bonds by reaction with an acrylate excess. Fur-thermore, the high functional-group tolerance of com-mercially available olefi n metathesis catalysts allows the usage of a wide variety of functionalized acrylates, which will, in turn, result in a wide spectrum of struc-tures and, thus, accessible properties. Complementary to cross-metathesis, thiol-ene addition reactions represent a versatile approach for the conversion of alkene func-tionalities and have recently been used with the same goal; [ 7 , 26 ] however, it must be noted that the double bond functionality is lost upon addition of the thiol. The pro-posed cross-metathesis side-chain functionalization is a complementary approach in which the double bond is retained, or “upgraded” to an α , β -unsaturated ester (if acrylates are used as CM partner) available for manifold further modifi cations.

The cationic ring-opening polymerization (CROP) of 2-oxazolines provides a versatile toolbox for the synthesis

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of highly functional polymers. [ 27 ] The ease of incorpo-rating functional sites using 2-substituted-2-oxazoline monomers, functional initiators, and termination agents enables the application of this polymer class for various post-polymerization modifi cations. Thus, poly(2-oxazo-line)s have already been used for a wide variety of effi cient reactions, including azide–alkyne chemistry, [ 28 ] thiol-ene chemistry, [ 7 , 26 , 28d ] and Diels–Alder chemistry. [ 28d , 29 ] Poly(2-(dec-9-enyl)-2-oxazoline) (PDecEnOx) derived from 10-undecenoic acid was chosen for this study ( P1 , Scheme 1 ). [ 7 ] This polymer possesses terminal double bonds in the side chain well separated from the polymer backbone by a C8 spacer. This fact might reduce the steric congestion at the reacting points, a common drawback of the "grafting-onto" technique, [ 1 ] enabling a high function-alization degree. Moreover, the low polydispersity index of P1 should allow for an easy identifi cation of undesired cross-linking during the olefi n metathesis reactions by size-exclusion chromatography (SEC).

2. Results and Discussion

The success of olefi n metathesis as side-chain modifi ca-tion technique mainly depends on the ability of metath-esis catalysts to maximize CM over SM. However, the correct election of the reaction conditions, namely solvent, concentration, temperature, and reaction time will also signifi cantly affect the fi nal result. The terminal double bonds present in every repeating unit of PDecEnOx ( P1 ) are well spaced from the backbone, which in theory should decrease the probability of intermolecular SM, but the short spacing between repeating units might have the opposite effect. Thus, initial work was focused on studying different catalysts and reaction conditions taking methyl acrylate as cross-metathesis partner (Scheme 1 ).

From previous experience, we know that the Hoveyda–Grubbs second-generation catalyst ( C1 ) performs gen-erally well in the CM of acrylates and terminal double

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bonds. [ 30 ] It is also known that an excess of acrylate and low reaction temperatures favor the CM product, [ 31 ] and that dichloromethane is usually the best option among other solvents when working at low temperatures. In order to fi nd the optimum reaction conditions, we varied different reaction parameters and studied the conver-sion and the extent of SM using 1 H NMR spectroscopy and SEC. It should be noted that, although it is also pos-sible to identify and quantify SM by 1 H NMR spectros-copy (multiplet centered at 5.3 ppm), it is impossible to differentiate intermolecular SM from intramolecular SM, and for this reason, SEC analysis is used instead in this regard throughout the discussion of the results. As to SEC analysis of the products, it must be pointed out that the starting poly(2-oxazoline) P1 used in this work already displays a (very) small shoulder in the high molar mass region due some minor side reactions during its synthesis. Therefore, it is impossible to quantify the intermolecular SM after the olefi n metathesis reactions by integration of the appearing high molar mass shoulders. For this reason, the polydispersity index (PDI) is used instead as parameter to obtain information about the extent of intermolecular SM. Although the prevention of this side-reaction is of major importance for this study, the occurrence of a small amount of intramolecular SM is less problematic, since it will not compromise the polydispersity of the product. In order to ensure a high CM/SM ratio, the reactions need to be performed under dilute conditions keeping the double bonds apart (well dissolved), and both the solvent and


Table 1. Reaction conditions and analytic data of the cross-metathes

Polymer Acrylate/olefi n

Acrylate eq. a) C1 [mol%] a)

P1 – – –

P2

7 4

P3

7 4

P4

7 4

P5

7 4

P6

12 4

P7

7 5

P8

10 5

P9

7 4

P10 10 5

a) Related to polymer repeating unit; b) Calculated from 1 H NMR spec(system A unless specifi ed, see Supporting Information); e) SEC system

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the excess of acrylate play an essential role in this regard. However, at lower concentration of reactants, the catalyst activity might also be reduced. In summary, it is neces-sary to identify a suitable solvent/acrylate/catalyst pro-portion that provides the highest catalyst activity with the highest CM/SM ratio.

Taking these considerations into account, we reacted P1 (PDI 1.13) with 5 equivalents of methyl acrylate in dichloromethane (0.6 M ) at 40 ° C and in the presence of 2 mol% (per repeating unit) of C1 . The analysis of the reac-tion mixture by 1 H NMR spectroscopy after 2 h showed a high terminal double bond conversion of 98%, but SEC analysis revealed a clear shoulder in the high molar mass region (PDI = 1.40), indicating intermolecular self-metath-esis. Starting from these initial reaction conditions, single parameters were modifi ed one-at-a-time to investigate their effect on the cross-metathesis functionalization. A decrease in the temperature to 30 ° C, which was expected to increase the CM selectivity, drastically dropped the con-version to 56%. An increase in the acrylate excess from 5 to 10 equivalents also resulted in a lower conversion of 74%, most likely due to a dilution effect. Moreover, a decrease in the catalyst loading from 2 to 0.5% gave a very low con-version of only 15%. Finally, three other catalysts, namely Grubbs second generation, Zhan and M5 1 (Umicore) were tested. These catalysts usually display similar activities to C1 in the cross-metathesis of acrylates and terminal ole-fi ns; [ 31 ] however, the conversion dropped to 58%, 72%, and 63%, respectively, under the same reaction conditions. As

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is of P1 with different acrylates.

Conv. [%] b) Yield [%] c)

M n [kDa] d)

PDI d)

– – 12.0 1.13

99.5 92 15.7 1.33

99.5 89 16.4 1.31

99.5 84 17.3 1.27

99.5 72 18.1 1.15

95.0 80 16.6 1.21

99.5 87 23.6 e) 1.19 e)

99.5 60 20.7 e) 1.14 e)

99.5 87 18.3 1.23

99.5 88 14.6 1.23

troscopy; c) Isolated yield after precipitation; d) SEC analytical data B was used (see Supporting Information).

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Figure 1 . SEC traces (eluent: THF) of a) the starting poly(2-oxazo-line) P1 and the products of cross-metathesis with alkyl acrylates P2 - P5 (SEC system A), b) P1 and the products of cross-metathesis P6 , P9, and P10 (SEC system A), c) P1 and the products of cross-metathesis P7 and P8 (SEC system B).

conclusion of this fi rst set of experiments, it was dem-onstrated that even though a dilution of the reaction mixture is required to avoid intermolecular SM, high dilutions have a negative effect on the catalyst activity. Moreover, Hoveyda–Grubbs second-generation catalyst is the best suited among the olefi n metathesis catalysts tested. C1 was subsequently applied for a screening of the other reaction conditions searching for the best balance between conversion and CM selectivity. It was found that it is possible to obtain a conversion over 99.5% (limit of NMR sensitivity) maintaining an acceptable PDI value of 1.33 using 7 equivalents of methyl acrylate and 4 mol% of C1 in dichloromethane (0.6 M ) at 40 ° C ( P2 , Table 1 ).

One of the main conclusions of these initial experi-ments is that both the solvent and the acrylate effec-tively avoid SM by hindering the approach of polymer chains. However, while the solvent molecules move freely throughout the reaction, once the acrylate reacts with the pendant chains it might play a more effi cient role in pre-venting SM simply due to its own size and the fact that it is attached. If this is true, increasing the size of the acr-ylate should lead to decreased SM and thus, a decrease in the PDI value. In order to prove this hypothesis, we applied the optimized reaction conditions with varying acrylate sizes (methyl, ethyl, butyl, and hexyl acrylates) and analyzed the products by 1 H NMR and SEC after 2 h of reaction. The results (Table 1 ) were in line with our predictions and the cross-metathesis of P1 with hexyl acrylate gave the product ( P5 ) with the lowest PDI (1.15) with a quasi-quantitative conversion of terminal double bonds ( > 99.5%). The polydispersity of the functionalized PDecEnOx increased as the alkyl chain of the acrylates decreased with a maximum of 1.33 for the methyl acr-ylate derivative. This can also be seen qualitatively in the SEC traces of P2 – P5 (Figure 1 ) with the decrease of the high molar mass shoulder from P2 to P5 . Therefore, we can conclude that as the size of the acrylates increases, the intermolecular SM decreases. An explanation might be that once the acrylate is attached, its alkyl residue produces signifi cant steric hindrance proportional to its size to prevent intermolecular SM. Finally, it is important to mention that SM was not observed by 1 H NMR (below detection limit) and the degree of functionalization with the respective acrylates is close to quantitative ( > 99% con-sidering the detection limits). The analysis of the reaction mixtures by 1 H NMR revealed the occurrence of acrylate SM in a very low extent. Although the characteristic signal of the acrylate SM product partially overlapped with the double bond signal of the polymer, it could be determined that only a 5% (or less) of the excess of acrylate under-went SM under the reaction conditions. Moreover, the CM polymer modifi cation reaction with acrylates showed a very high selectivity toward the E-confi gured product ( > 98%), as is well known for CM with acrylates performed

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on small molecules. Only in the case of P6 (see below), the E -selectivity was somewhat lower (96%).

In order to demonstrate the versatility of olefi n cross-metathesis as side-chain functionalization technique, we then studied the reaction of P1 with various acrylates containing different functional groups. Therefore, we tested 2-hydroxyethyl acrylate (HE-Ac), polyethyleng-lycol acrylate (PEG-Ac, M w = 480 Da), an acrylate bearing


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Figure 2 . 1 H NMR spectra (in CDCl 3 unless specifi ed) of a) the starting poly(2-oxazoline) P1 , b) P6 (in MeOD), c) P7 , d) P8 and e) P10 .

a perfl uorinated alkyl chain (F-Ac), and tert -butyl acr-ylate ( t Bu-Ac) as cleavable group precursor of carboxylic acid functionalities. C1 was also used for these reactions because of its well-known tolerance towards many func-tional groups, and the reaction conditions were optimized taking those used with the alkyl acrylates as starting point. The optimized reaction conditions and analytical data of the obtained polymers are collected in Table 1 , SEC traces are shown in Figure 1 , and selected 1 H NMR spectra are shown in Figure 2 .

Of special interest was the effect of the solvent choice for the cross-metathesis reaction of P1 with F-Ac. Dichlo-romethane was initially used; however, a cross-linked polymer was immediately obtained due to selective disso-lution of P1 over F-Ac. When chloroform was used instead, the situation was reversed and F-Ac was selectively solubi-lized over P1 producing a virtual extra-excess of acrylate. This effect resulted in quantitative conversion of terminal double bonds together with a low PDI value of 1.21. Indeed, we were able to synthesize the functionalized derivatives reaching close to quantitative conversions and low PDI values between 1.19 and 1.23 (Table 1 ). Due to different sol-ubility of the obtained products, two different SEC systems had to be used for their analysis (see Supporting Informa-tion for details). The results obtained with PEG-Ac, F-Ac, and t Bu-Ac are in good agreement with the already men-tioned steric effect caused by bulky acrylates; once again, low PDIs were obtained without compromising the conver-sion of terminal double bonds, which remained very high.


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Furthermore, very little or no high molar mass shoulder can be observed in their SEC traces (Figure 1 ). Furthermore, we also tested the more challenging (because less selec-tive) cross-metathesis of P1 with cis -2-butene-1,4-diace-tate, a precursor of terminal allyl acetate groups useful for palladium-catalyzed cross-coupling reactions. [ 32 ] This olefi n is also electron rich and thus has a similar reactivity as the terminal double bonds of P1 , with high tendency to undergo SM. Moreover, their CM product has a similar reactivity to the reactants, which means that secondary metathesis reactions are likely to take place reversing the reaction. However, an excess of 10 equivalents of cis -2-butene-1,4-diacetate in the presence of 5 mol% of C1 at 40 ° C was suffi cient to provide a high conversion of 99.5% with a PDI of 1.23 ( P10 , Table 1 ). These conditions are only somewhat more demanding than those required by the small acrylates studied, and similar to those required by the bigger PEG-Ac and F-Ac. Furthermore, as already men-tioned, the cross-metathesis product between P1 and t Bu-Ac ( P9 ) was reacted with trifl uoroacetic acid to cleave the ester group and obtain free carboxylic acids at the end of the alkyl pendant chains ( P11 ). The reaction proceeded with high conversion as observed by disappearance of the singlet of the tert -butyl group in the 1 H NMR spectrum at 1.44 ppm, and the appearance of a broad signal centered at 7.90 ppm corresponding to the new carboxylic acid groups (see Supporting Information). Moreover, the backbone signals (3.84–3.06 ppm) remained unaltered. The analysis of P11 by SEC was impossible in any of the systems used;

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however, the cleavage was also monitored by FTIR–ATR (see Supporting Information), which clearly showed a shift of the C�O stretching band from 1710 cm − 1 (conjugated ester) to 1691 cm − 1 (conjugated carboxylic acid) and the broad COO–H stretching band (3500–2500 cm − 1 ).

In conclusion, the cross-metathesis of PDecEnOx (P1) , a polymer exhibiting terminal alkene functionalities in the side chain with a number of functional acrylates and cis -2-butene-1,4-diacetate has been demonstrated to be a straightforward way to synthesize side-chain function-alized poly(2-oxazoline)s. Undesired cross-linking pro-duced via interchain self-metathesis can be avoided by working with an excess of CM partner, and high cross-metathesis selectivities can be achieved using Hoveyda–Grubbs second-generation catalyst in dichloromethane at 40 ° C. Furthermore, this strategy should be valid for any polymeric structure as long as a metathesis catalyst com-patible with the functional groups of the polymer can be found. Moreover, a large variety of functional groups can be introduced via this method and also used for further transformations. Compared to other functionalization protocols, such as thiol-ene chemistry, this approach has the advantage of not consuming the alkene moiety in the poly mer, thus leaving it for further modifi cation reactions.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : U.S.S. and K.K. thank the Landesgradutiertenförderung Thüringen and the Carl-Zeiss-Foundation for fi nancial support.

Received: July 17, 2012; Revised: August 10, 2012; Published online: ; DOI: 10.1.002/marc.201200487

Keywords: cross-metathesis; functionalization of polymers; graft copolymers; poly(2-oxazoline)

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side-chain modification and grafting onto via olefin cross-metathesis

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