Hydrolases Part I: Enzyme Mechanism, Selectivity and Control in the Synthesis of Well-Defined Polymers - Enzymatic Polymerisation - page 67

tuned by copolymerization with CL and the degree of branching remained close to
zero, showing that these secondary alcohols are unreactive. Subsequent reaction of
the secondary alcohols with hexylisocyanate showed that these polyesters can be
readily post-modified by suitable reagents.

4 Polycarbonates

Cyclic carbonates are an interesting class of monomers because polycarbonates
such as poly(trimethylene carbonate) are often amorphous and show well-defined
degradation properties, making them suitable for a variety of biomedical applica-
tions. ROP of cyclic carbonates is typically conducted with metal-based catalysts
such as Sn(Oct) 2 but lipases were also found to be active in the enzymatic ROP
of a variety of substituted and unsubstituted cyclic carbonates (Fig. 7 ) . Trimethy-
lene carbonate (TMC) was the first monomer to be evaluated in enzymatic ROP
[ 83 - 85 ]. Novozym 435 gave a rapid polymerization reaction, and an M n of 15 kDa
was reached with complete monomer conversion within 120 h at 70 ◦ C[ 85 ] . Higher
molecular weights have been reached by using lipase PPL [ 83 ]( M n 169 kDa,
PDI 3.5) and PPL immobilized on silica nanoparticles ( M n 48 kDa, PDI 2.0) [ 86 ]
although this required a temperature of 100 ◦ C. Other cyclic carbonates such as hex-
amethylene carbonate (HMC) [ 87 ] or substituted carbonates 1-MeTMC [ 90 ]and
2,2-diMeTMC [ 158 ](Fig. 8 ) have also been successfully polymerized into poly-
mers of good molecular weight. The use of protected functional groups at the TMC
ring, such as the 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (MBC) [ 82 , 88 ] ,
5-benzyloxy-trimethylene carbonate (BTMC) [ 89 ] , or tartaric-acid-derived cyclic
carbonates (TAC) (Fig. 8 ) , allows for easy access to the hydroxy-functional or
carboxy-functional polycarbonates, which may be used to tune the degradation be-
havior of these polymers.
Lipases not only catalyze ROP but also catalyze transacylation reactions. This
was exploited by preparing random copolymers of TMC with CL or PDL [ 91 , 92 ,
159 ] . The PDL-TMC poly(carbonate-esters) show co-crystallization behavior of

Fig. 8 Chemical structures of cyclic carbonates explored in lipase-catalyzed ROP