Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 82

Fig. 2 Synthesis of acrylated chiral PMCL ( left ) and microparticles ( right ) obtained from acry-
lated polymers by oil-in-water emulsion photopolymerization [ 5 ]

HEA) according to route B is not straightforward. The lipase does not distinguish
between the ester group of a monomer, a polyester chain, and the ester group
present in HEA or HEMA. Consequently, the esters groups in HEA and HEMA
will be activated as well and take part in transesterification processes, which results
in a complex mixture of polymer endgroups when monomers like

ε -caprolactone

(CL) and -pentadecalactone (PDL) are used [ 6 , 7 ]. The transesterification pro-

cess occurs at moderate frequency at low monomer conversion but it becomes
dominant at longer reaction times. Although this presents a restriction for the enzy-
matic synthesis of strictly mono-acrylated polymers from HEA and HEMA, fully
dimethacrylated PCL and PPDL were obtained by combination with end-capping
with vinyl methacrylate [ 6 ] . Taking advantage of the inevitable transesterification
reactions, Takwa et al. reported an elegant single-step solvent-free enzymatic route
to telechelic PPDL by conducting the enzymatic ROP in the presence of ethylene
glycol di(meth)acrylate (route C in Fig. 1 ) . Polymers with

ω

96% diacrylate and
dimethacrylate endgroups were obtained (Scheme 1 ) [ 8 ] . In the same paper, the
authors introduced a synthetic route to telechelic dithioPPDL and thiol-acrylate
functional PPDL. The thiol functional macromonomers were used together with
other ene-monomers to give crosslinked PPDL thin films using a UV-induced pho-
topolymerization [ 9 ]. In this first example of chemoenzymatic synthesis of coatings,
semicrystalline networks were obtained owing to the high crystallinity of PPDL.
Various

>

-methylenemacrolides were enzymatically polymerized to polyesters
having polymerizable methacrylic methylene groups in the main chain (Fig. 3 , left).
The free-radical polymerization of these materials produced crosslinked polymer
gels [ 10 , 12 ]. A different chemoenzymatic approach to crosslinked polymers was
recently introduced by van der Meulen et al. for novel biomedical materials [ 11 ] .
Unsaturated macrolactones like globalide and ambrettolide were polymerized by en-
zymatic ROP. The clear advantage of the enzymatic process is that polymerizations
of macrolactones occur very fast as compared to the chemically catalyzed reactions
[ 13 ]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble
and fully amorphous materials (Fig. 3 , right).

α