Chemistry Reference
In-Depth Information
Chapter 6
Enzymatic Modification and Polymerization
of Siloxane-Containing Materials
Mark B. Frampton, Jacqueline P. Séguin and Paul M. Zelisko
6.1 Introduction
Silicones and other siloxane-derived materials are some of the most industrially
and economically important polymer materials [
1
]. Siloxanes are valued for their
thermal stability, low glass transition temperatures, resistance to oxidation, low
permittivity values, and general biocompatibility [
2
]. The basis for these physical
characteristics is due in large part to the nature of the
Si-O-Si
linkage. The
Si-O-Si
angle is rather large, approximately 145°, compared to 109.5° for typical tetrahe-
dral systems. Given a relatively low barrier for linearization, 0.21-0.50 kcal/mol,
[
3
-
5
] it has been postulated that the
Si-O-Si
link is flexible enough to range from
90-180
°
. In addition, the siloxane linkage is highly polarisable due to the high ionic
nature the
Si-O
bond which has been determined to be approximately 51 % ionic
[
6
]. The
Si-O
bond possesses very low rotational bond energy and is also one of the
strongest chemical bonds known with covalent bond energy of 108 kcal/mol and
ionic bond energy of 242 kcal/mol [
2
]. The increased length of
Si-O
(1.64 Å) and
Si-C
(1.87 Å) bonds compared to the
C-O
(1.41 Å) and
C-C
(1.53 Å) bonds allows
for greater rotational freedom [
2
].
Polyesters are commercially available as a variety of products such as fibres,
fillings, coatings and textiles and are typically produced under extremes of heat
and pressure. Methods have been developed for producing polyester materials that
include strong acids and dialkyl tin reagents. While strong acids are ideal for poly-
ester synthesis, when acid sensitive functional groups are present, such as siloxanes,
they may not be compatible due to random redistribution reactions involving the
siloxane backbone or cleavage of the siloxane network [
1
].
Enzymatic methods, particularly those involving lipases, are becoming increas-
ingly popular in polymer chemistry. In this regard, lipase B from
Candida antarctica
,
