Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 93

The incomplete enzymatic grafting from polyalcohols opens opportunities for the
synthesis of unique structures, as shown in several examples by Hans and coworkers.
For example, heterografted molecular bottle brushes were synthesized starting from
PCL-grafted polyglicydol [ 54 ] . The grafting yield in this step was about 50%.
Selective acetylation of the hydroxyl groups at the PCL graft ends was achieved
via enzymatic reaction with vinyl acetate without acetylation of the remaining hy-
droxyl groups at the polyglycidol backbone. The latter were quantitatively converted
to ATRP initiator groups, and initiation of MMA n -butyl acrylate yielded well-
defined heterografted polymers. Alternatively, the remaining hydroxyl groups on
the polyglycidol backbone were used to initiate chemically catalyzed ROP of lac-
tide to produce a heterografted polymer comprising PCL and PLA grafts [ 55 ] . When
the endgroups of the PCL were capped with acrylates, the resulting materials could
be formulated into UV-crosslinked microspheres (Fig. 8 ) .
The first synthesis of graft copolymers in scCO 2 was reported by Villarroya et al.
from a polyHEMA-PMMA random copolymer by the combination of ATRP and
enzymatic ROP of CL [ 56 ]. The authors first synthesized the random copolymer
containing 13 and 30% HEMA by ATRP in scCO 2 . Although PCL grafting was con-
firmed, not all hydroxyl groups participated in the grafting reaction (33%). Similar
to the situation in conventional solvents, steric effects were thought to reduce the
grafting efficiency. The results were similar when both the ATRP reaction and en-
zymatic grafting were carried out simultaneously in one-pot, i.e., all components
for the radical and the enzymatic reaction were present at the same time. This ap-
proach resulted in about 40% functionalization of the hydroxyl groups. In another
study, the authors confirmed that the limitation in grafting efficiency was due to
two reasons: (1) the poly(MMA- co -HEMA) probably has a blocky structure due to
Fig. 8 Chemoenzymatic synthesis of heterografted polymers comprising PCL and PLA grafts
on a polyglycidol backbone. After acrylation, these polymers can be formulated into crosslinked
microspheres [ 55 ]