Chemistry Reference
In-Depth Information
generally very similar to those obtained in anionic polymerizations, al-
though the mechanism is very different. The reaction is thought to pro-
ceed through a tertiary oxonium ion formed by addition of a proton to one
of the O atoms of the cyclic siloxane. The mechanism may involve step
growth, as well as the expected chain growth. Examples of other catalysts
used include metal sulfonate/acid chloride combinations.
46
In the case of
step-growth polymerizations, tin catalysts are very effective.47
47
Polymerization of nonsymmetrical cyclic siloxanes gives stereochemi-
cally variable polymers [-SiRR'O-] that are analogous to the totally or-
ganic vinyl and vinylidene polymers [-CRR'CH
2
-]. In principle, it should
be possible to prepare siloxanes in the same stereoregular forms
(isotactic and syndiotactic) that have been achieved for some of their or-
ganic counterparts,
8, 48-49
as mentioned in chapter 1. This goal has been
accomplished to some extent (see chapter 5). The major advantage is the
crystallizability generally observed for isotactic and syndiotactic stereo-
regular forms of the polymer, whereas the stereoirregular (atactic) modi-
fication is inherently noncrystallizable. Strictly alternating copolymers
have recently been reported.
50
In some cases, an end blocker such as YR'SiR
2
OSiR
2
R'Y is used to form
reactive -OSiR
2
R'Y chain ends.
51-52
Interesting examples include triaryl-
amines,
53
nitrobenzoxadiazole fluorescent groups via thiol-ene coupling.
54
Homopolymerizations of this type are discussed in detail elsewhere.
8, 10
Some studies have focused on the preparation of porous polysiloxane
materials
55-60
such as low-density aerogels.
61
Mesoporous and ultra-large
pore siloxane structures can be prepared by condensation of tetraethy-
lorthosilicate (TEOS) and other silica precursors. These materials show
porosity, sometimes ordered, with pore sizes up to 30 nm.
62-66
Hollow
nano/microstructures have also been prepared, by ionic polymerization.
67
In a reversal of roles, siloxane chains have been substituted into
poly(
p
-xylylene).
68
It has become important to develop environmentally-friendly methods
for preparing any of these materials.
69
2.3 OTHER APPROACHES AND COPOLYMERIZATIONS
Atom transfer radical polymerization (ATRP) has been used in the area of
polysiloxane
70
and there have been a number of studies on controlling the
stereochemical structures of polysiloxanes.
71-74
Templated syntheses of
ladder-like siloxane structures
75
and the placement of silsesquioxane
units in siloxane backbones
76
have also been explored. Quintana et al.
