Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 94

the different reactivity ratios of the monomers in the radical polymerization; and
(2) the hydroxyl groups are too close to the polymer backbone. Consequently, the
grafting efficiency was significantly improved when highly randomized poly(MMA-
co -HEMA) from starved-feed polymerization was used (80%), and when a PEG
spacer was introduced between the polymer backbone and the hydroxyl group
(100%) [ 57 ] .

2.5 Branched Structures

The only report on chemoenzymatic synthesis of branched polymers is from Peeters
et al. [ 58 ] . Heterotelechelic PCL macroinimer was synthesized in a one-pot en-
zymatic procedure by using 2-hydroxyethyl -bromoisobutyrate as a bifunctional

initiator. A polymerizable endgroup was introduced by subsequent in situ enzymatic
acrylation with vinyl acrylate. Synthesis of branched polymers by self-condensing
ATRP of the macroinimers was successfully conducted with and without the addi-
tion of MMA as a comonomer.

α

3 Chemoenzymatic Approaches to Chiral (Co)polymers

3.1 Enantioselectivity Issues in Enzyme-Catalyzed Reactions

Introducing chirality into polymers has distinctive advantages over the use of
nonchiral or atactic polymers because it adds a higher level of complexity, allowing
for the formation of hierarchically organized materials. This may have benefits in
high-end applications such as nanostructured materials, biomaterials, and electronic
materials. Synthetically, chiral polymers are typically accessed by two methods.
Firstly, optically active monomers - often obtained from natural sources - are poly-
merized to afford chiral polymers. Secondly, chiral catalysts are applied that induce
a preferred helicity or tacticity into the polymer backbone or activate preferably one
of the enantiomers [ 59 - 64 ] .
Polymers derived from natural sources such as proteins, DNA, and polyhy-
droxyalkanoates are optically pure, making the biocatalysts responsible for their
synthesis highly appealing for the preparation of chiral synthetic polymers. In recent
years, enzymes have been explored successfully as catalysts for the preparation of
polymers from natural or synthetic monomers. Moreover, the extraordinary enan-
tioselectivity of lipases is exploited on an industrial scale for kinetic resolutions
of secondary alcohols and amines, affording chiral intermediates for the pharma-
ceutical and agrochemical industry. It is therefore not surprising that more recent
research has focused on the use of lipases for synthesis of chiral polymers from
racemic monomers.