Chemistry Reference
In-Depth Information
sold under the trade name Novozym-435, immobilized on a macroporous acrylic
resin has been the work horse for synthesizing polymeric materials [
7
]. The mild
reaction conditions typically associated with enzymatic catalysis makes it an ideal
approach when sensitive siloxane linkages are present. Furthermore, the regio- and
enantio- selectivity offered by enzymatic catalysis can be used to acquire the de-
sired product. For example, Novozym-435 selectively esterified the C6 hydroxyl
of an α,β-ethyl glucoside in the preparation of sugar-functionalized silicones [
8
].
Since the disclosure of the enzymatic synthesis of silicon-containing esters and
amides by Dow Corning, [
9
] much has been learned about the relationship between
siloxanes and lipases. Clarson and co-workers described the synthesis of aromatic
silicone polyesters and polyamides [
10
,
11
]. Block copolymers derived from a di-
acid terminated disiloxane (CPrTMDS) and polyethylene glycol, as well as triblock
copolymers featuring the ring opening polymerization of ε-caprolactone have also
been examined [
12
,
13
]. Poojari et al. studied the molecular weight build up of three
siloxane-polyester systems; each polyester was composed of a block derived from
either 1,4-butanediol, 1,6-hexanediol or 1,8-octanediol and a block from CPrT-
MDS [
14
]. Richard Gross, in collaboration with the Scandola group, provided an
enzymatic route to siloxane-containing polyester amides and compared the relative
rates of polymerization of 1,8-octanediol and an aminopropyl-functionalized sili-
cone with diethyl adipate as the acyl donor [
15
]. We have described the enzymatic
synthesis of siloxane-containing polyesters in which all polymerizable components
were siloxane-derived [
16
]. The number of siloxane units present in the diol mono-
mer appeared to have little effect on the elongation kinetics, or the activation energy
required for the polymerization [
16
].
This chapter describes our recent work, which has been focused on using en-
zymes to modifying siloxane-containing materials. We are primarily interested in
designing environmentally benign methods for the polyesterification of siloxane-
containing monomers using lipase catalysis, and to incorporate enzymatic reaction
processes for synthesizing starting materials. Most of the work to be described will
focus on the use of N435 (lipase B from
Candida antarctica
immobilized on a mac-
roporous acrylic resin) as the biocatalyst.
6.2
Enzyme-Mediated Catalysis of Siloxane-Containing
Materials
Siloxane-containing polyesters derived from 1,3-bis(3-carboxypropyl)-1,1,3,3-tetra-
methyldisiloxane dimethyl ester (CPr-TMDS-DME) and 1,3-bis(3-hydroxypropyl)-
1,1,3,3-tetramethyldisiloxane (3HP-TMDS) or α,ω-bis(hydroxyalkyl)polydimeth-
ylsiloxane (HA-PDMS, Mw = 2000 g/mol by
29
Si NMR) have been synthesized
over the temperature range of 35-160 °C using N435 as a catalyst under solvent-
free reaction conditions (Scheme
6.1
). A stoichiometric amount of 3HP-TMDS and
CPr-TMDS-DME were combined with 5 wt% N435. Because HA-PDMS is nearly
an order of magnitude more massive than 3HP-TMDS, when it was used as the
