Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 85

Fig. 5 Synthesis of poly(butadiene- b -caprolactone) from hydroxy-functional polybutadiene [ 16 ]

time, according to incorporation by initiation and transesterification. The latter is
due to the fact that the enzyme activates both the carbonyl bond of the monomer
(CL) and the PCL esters. Consequently, the nucleophile (initiator) will be incorpo-
rated by both mechanisms, i.e., initiation and transesterification. It was found that all
polybutadiene macroinitiators formed block copolymers irrespective of the molec-
ular weight. Initiation efficiency was > 80%, and the presence of water-initiated

chains was less than 30%.
Several authors reported the use of hydroxy-functional poly(ethylene glycol)
(PEG) as macroinitiator for enzymatic ROP. Panova and Kaplan conducted a de-
tailed mechanistic and kinetic study of the interplay between monomer conversion,
chain initiation, and chain propagation of the enzymatic ROP of CL in the pres-
ence of monohydroxy-functional methoxy-PEG [ 17 ] . The results confirmed the
dynamic character of the reaction and the presence of a product mixture containing
unreacted macroinitiator, water-initiated PCL, cyclic PCL, and the corresponding
block copolymer. At otherwise identical reaction conditions, the amount of reacted
monohydroxy-functional methoxy-PEG was concentration-dependent, being 35%
at higher concentrations and up to 84% at lower concentrations for identical reac-
tion times. Moreover, block copolymers from mono- and difunctionalized PEG were
obtained by Feng He et al. by the enzymatic ROP of CL [ 18 ] , and by Srivastava and
Albertsson with CL and 1,5-dioxepane-2-one (DXO) [ 19 ]. Kaihara et al. reported
the enzymatic ROP of trimethylene carbonate from a copolymer of PEG and a cyclic
acetale with two terminal hydroxyl groups [ 20 ] . The resulting amphiphilic triblock
copolymer was used to form micellar structures, and pH-dependent drug release was
successfully shown.
While all previous examples employ enzymatic ROP, there are two reports
on block copolymer synthesis employing enzymatic polycondensation. The first
one was published by Sharma et al. and describes the synthesis and solid-state
properties of polyesteramides with poly(dimethylsiloxane) (PDMS) blocks [ 21 ] .
The polycondensation was carried out with various ratios of dimethyl adipate,