Polymer Chemistry-

Dendrimers and HyperbranchedPolymers

Aims of this part:

History of Dendrimers and Hyperbranched Polymes

Constitution of Dendrimers

Constitution of Hyperbranched Polymers

Terminology of Branching and Generations

Convergent and Divergent Synthesis

Applications of Dendrimers and Hyperbranched Polymers

Dendron Modifications

Braun, Chedron, Rehann, Ritter, Voit “Polymer Synthesis: Theory and

Practice”, Springer, 5th Edition, 2013. Section 4.1

Lechner, Gehkre, Nordmeier, „Makromolekulare Chemie“, Springer, 5th

Edition, 2014. Section 3.2

Branching in Polymers

Dendrimers

Globular polymers of ongoing ideal and perfect branching in all directions

• globular, dense shape

• high functionality

• excellent solubility

• low solution viscosity

• no entanglements

• properties strongly dependent on end groups

(Tg, melt h, solub.)

• step wise synthesis – convergent (core last) or divergent (core first)

• perfectly branched, nearly defect free

• full control over mass and size

• „monomodal“ (at least a lot more than normal polymers)

• high symmetry, regular globular shape

History of DendrimersInitial concepts:

• „cascade“ or „repeating“ synthesis, Vögtle, Synthesis

1978

• „Cascade and Nonskid-Chain-like Syntheses of

Molecular Cavity Topologies“

• iterative concepts can also be found by Lehn (1973)

and Cram (1975)

• Denkewalter et al., patent 1983: lysine dendrimers

(studied by Aharoni, Crosby and Walsh, 1982) lysine

tree‘s used in peptide synthesis

Modern concepts:

• Tomalia et al., 1985, PAMAMs up to G7 possessing trigonal branching centers

• Fréchet and Hawker, 1990, convergent synthesis (polyether)

• Miller and Neenan, 1990, convergent polyarylates

• Moore and Xu, 1991, poly(arylacetylene)s

Theoretical Aspects

Key properties:

Core functionality (Nc), functionality of the branching site (Nb), Generation (G)

Number of end groups z: z = NcNbG

Number of repeating units: Nr = Nc [(NbG+1 - 1)/(Nb-1)]

Building blocks: Core unit, branching dendrons, surface function/ligand

Number of generations: Number of branching sites excluding core

Nc= 3

Nb = 2

G = 3

z = 24

Nr = 45

Generation guessing

Dendron modifications

Monodendron G2-4Fullerene or CNT

modificationaW - functionalised polymer

Janus dendrimers

(amphiphilic structure)

Giant soapsDendronised polymers

(macromonomer, grafting

onto, graftig from)

PAMAM

Most researched dendrimer discovered in 1985

Commercialised – Tradename is „starburst“

Applications (copied from Dendritech)

Imaging: PAMAM dendrimer conjugates with paramagnetic ions are being

studied for use as magnetic resonance imaging (MRI) contrast agents.

Sensors: Due to their organized structure, ease of modification, and strong

adsorption behavior to a variety of substrates, PAMAM dendrimers can be used

to produce monolayers or stacked film layers, which can be used as sensors to

detect hazardous chemical vapors.

In vitro Diagnostics: Dendrimer-antibody conjugates are used in an

immunoassay for rapid and sensitive detection of markers

indicative of heart attacks.

Janus Dendrimers

Percec et al: Science 21 May 2010: Vol. 328, Issue 5981, pp. 1009-1014 DOI: 10.1126/science.1185547

Usually 4 dendrons

Convergent approach only

Highly organised amphiphilic block-

copolymers!

Self-assembly is possible + occurs

Different dendrons can be

inhomogenous in branches!

Hyperbranched Polymers

Flory (1952)One-pot polycondensation reaction

• Not perfectly branched

• Low control over mass and size

• Broad molar mass distribution

• Irregular shape (globular, amorphous

structure, low viscosity)

• No gelation (high solubility)

Types of units present:

• Dendritic unit (both B reacted)

• Linear unit (one B reacted)

• Terminating unit (only A reacted)

• Focal unit (A did not react, but

both B units) present only once

History of Hyperbranched Polymers

Branched structures studied since about 1860s

• Hunter, Woollett, 1921, branched (but intractable) aromatic

polyethers

• Early 1940s: Flory‘s and Stockmayer‘s theories on gelation

• Branched polysaccharides (glycogen, amylopectin a.o.) and

fractal natural structures

• P.J. Flory, J. Am. Chem. Soc. 1952, 74, 2718: branched

polymers

• without „insolube gel formation“ from ABx monomers

• Baker et al, patent on highly branched polyesters, 1972

• Kricheldorf, Zang, Schwarz, 1982, branched polyesters using

AB2 monomers

• Kim, Webster, 1988, hyperbranched polyarylenes

• Fréchet, Hawker, Lee, 1991, hyperbranched polybenzylethers

Jean M. J.

Fréchet

Paul J. Flory

Flory’s Conditions for Hyperbranched Polymers

Prerequisite for „ideal“ hyperbranched polymers based on ABx monomers:

• A reacts only with B

• all B have the same reactivity (independent of molar mass)

• no cyclics

• no side reactions degree of branching = 0.5 (50% linear units, 25 %

dendritic units, 25% terminal units)

(purely statistical process)

no gelation (since critical conversion is exactly at 100%)

one unreacted A function = Focal unit

T = D+1, (n+1) unreacted B functions (for DP = n and for AB2 monomer)

Advantage over Dendrimers:

Much cheaper large scale applications possible (blends, coatings, additives)

Degree of Branching

DB influenced by:

• Conversion

• Side reactions

• Core molecules

• Slow monomer addition

• Post modification

• Steric and inductive effects

• Intermediate formation

• Reactivity of functional groups (SCVP?, ROMBP?)

𝐷𝐵 =𝐷 + 𝑇

𝐷 + 𝑇 + 𝐿, 𝑇 = 𝐷

𝐷𝐵 =2𝐷

2𝐷 + 𝑇

DB can be calculated from NMR or IR or similar

– T/D/L must be differentiated

Synthesis of Hyperbranched Polymers

Adding B3 allows for some

control over molar mass

(„endcapping“ of focal unit)

A2 + B3 - Approach

Used when AB2-Monomer is difficult to synthesise (e.g. reactivity too high)

Danger of

crosslinking

(sol and gel

formation)

Very little control

over molar mass

and topology!

No focal unit is present

Monomers are often

commercially available!

Applications of Hyperbranched Polymers

see also: B. Voit:J. Polym. Sci. Part A: Polym. Chem: 38, 2506-2525 (2000)

Highlight “New Developments in Hyperbranched Polymers”

Classical Applications (in bulk)

• Blends (increase of modulus, heat stability)

• Additives (rheology, dying, adesion, compatibilization)

• Coatings and resins

New Fields (in solution or as thin films

• Sensor materials

• Surface sensitive or reactive materials

• Catalysis, micelles

• Molecular imprinting

• Globular templates

• Photosensitive material

• Pharmaceutical and medical application?First commercialised product

(BOLTORN by Perstop)

Hybrane – by DSM

Potential applications of HYBRANE:

• Cross-linker

• Surfactants

• Diesel additive

• Controlled release

• Viscosity modifier (e.g. inks)

• Detergent

• Anchor for catalysts, proteins etc.

• Physical encapsulation (dyes,

• latent catalysts)

• Core for star polymers

• Adhesive

• Toner resin

• Multifunctional carrier

base resin

Mn = 2016

10 end groups

Aromatic-Aliphatic Hyperbranched Polyesters

P1-OH, Polyol with phenolic end-groups

• Random growth

• Equal reactivity of both B groups

• DB = 50% (via NMR)

• Molar mass Mn up to 60,000 g/mol

• Soluble in THF and others

• Amorphous, Tg = 113 oC

• Less brittle than fully aromatic hb

polyesters

• Monomer commercially available

• Stable in modification reactions

ideal for kinetic studies

ideal for coating application

Both, dendrimers and hyperbranched polymers have several

commercialised products and are often used as additives

Hyperbranched polymers from the AB2 approach usually have a DB of 50%

and bear one A unit in the final polymer (focal unit)

Dendrimers can be made in a bottom-up (divergent) and a top-down

(convergent) approach, the latter one allowing also for polymer

modifications with dendrons and amphiphilic dendrimers

Hyperbranched Polymers are very broadly distributed, vary in their degree

of branching and are diverse in their constitution

Dendrimers are Polymers of ideal constitution and branching with a defined

molecular weight and chemical structure (Core, Generations, Shell)

Summary

Other synthetic approaches towards hyperbranched polymers bear the risk

of cross-linking and thus sol/gel formation