Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 101

Novozym 435
Benzyl alcohol
bulk /45°C

a

O

R

O

H O

n

O

n

O

R

poly( (S) -4-MeCL): E = 16.9
poly( (S) -4-EtCL): E = 7.1
poly(

4-MeCL: R = CH 3
4-EtCL: R = CH 2 CH 3
4-PrCL: R = CH 2 CH 2 CH 3

(R) -4-PrCL): E = 2.1

b

Novozym 435
Benzyl alcohol
tetramethylbenzene/45°C

O

CH 3

O

H O

n

O

n

O

CH 3

poly( = 13

poly( (S) -4-MeCL): E = 93
poly( (R) -5-MeCL): E = 60

(S) -3-MeCL): E

3-MeCL
4-MeCL
5-MeCL
6-MeCL

Scheme 8 Enzymatic polymerization of various substituted ε -caprolactones

and 4-MeCL showed moderate polymerization kinetics with initial rate constants
of 1.3 and 1.1 h − 1 , respectively. On the other hand, 6-MeCL did not polymer-
ize 1h − 1 . Although the initiation (i.e., ring opening of 6-MeCL with the

initiator benzyl alcohol) clearly occurred, the absence of further consumption of 6-
MeCL was indicative of the virtual absence of propagation. This was explained by
the configuration of the secondary alcohol formed after the ring opening of

( k i < 0 . )

( S ) -6-

MeCL. The formed -alcohol is unreactive in CALB-catalyzed esterifications of

secondary alcohols (see above), hampering polymerization of this monomer on a
practical time scale.
van Buijtenen et al. investigated the ROP of lactones with different ring
sizes. In contrast to the ring-opening of the lactones with ring sizes of seven
or less, ring-opening of the larger lactones with ring sizes between 8 and 13
was

( S )

( R ) -selective [ 85 ] . The formation of a reactive ( R ) -secondary alcohol for

ω -methylated lactones with ring sizes of eight or more enabled straightforward

kinetic resolution polymerization of these substrates. 7-Methylhepanolactone
(7-MeHL), 8-methyloctanolactone (8-MeOL) and 12-methyldodecanolactone (12-
MeDDL) (Fig. 10 d ) were successfully polymerized. The highest activity was
observed for the polymerization of 7-MeHL 270 s − 1

( k cat = ) , followed closely

223 s − 1

by that of 12-MeDDL ( k cat = ) . The polymerization of 8-MeOL was

44 s − 1 around five times slower than the polymerization of 7-MeHL and

12-MeDDL. In all cases, good molecular weights were obtained for the chiral
polymers (14-16 kDa), with polydispersity indexes (PDIs) varying between 1.23
and 2.25.

( k cat = )