Chemistry Reference
In-Depth Information
demonstrated robust, antifouling, hydrophilic polysulfobetaine-based
brushes with diblock architecture fabricated by ATRP using initiator-
modified surfaces.
77
Emulsions play a role in some aspects of polysiloxane technology. Po-
lymerization has been carried out in emulsions,
78-85
and room temperature-
cured PDMS elastomers have been obtained from aqueous dispersions.
86
Poly(vinyl pyrrolidone) dispersions have been prepared in reactive PDMS
media.
87
Microemulsions containing PDMS also find use in applications
such as cosmetics. One novel study involved a surfactant solution in a
polysiloxane, droplets of which bounced on a vertically vibrated liquid sur-
face.
88
When the amplitude of the vibration exceeded a threshold value, a
polysiloxane-water-polysiloxane
double
emulsion was formed!
Polymerizations have also been carried out in the vapor phase,
89
in
plasmas,
90
enzymatically,
24, 91-94
and in compressed carbon dioxide.
95
Poly-
siloxanes have also been obtained by acid leachate from chrysotile
asbestos.
96
Polymerization of mixtures of monomers, such as (SiR
2
O)
m
with
(SiR'
2
O)
m
, can be used to generate random copolymers. These copoly-
mers are generally highly irregular in structure in a chemical rather
than stereochemical sense. Correspondingly, they also show little if any
crystallizability. Copolymerizations of this type are discussed in detail
elsewhere.
8, 10
A higher level of structure is illustrated by the preparation of micropo-
rous hybrid polymers from functionalized cubic siloxane cages.
97
2.4 STRUCTURAL FEATURES
Several structural features make the siloxane backbone one of the most
flexible in all of polymer science.
98-99
Figure 2.1 illustrates the reasons
for this extraordinary flexibility. First, because of the nature of the
bonding
25 -27,
100-101
the Si-O skeletal bond has a length (1.64 Å) that is
significantly larger than that of the C-C bond (1.53 Å) of paramount im-
portance in most organic polymers. As a result, steric interference and
intramolecular congestion are diminished.
25-27
This circumstance is true
for inorganic and semi-inorganic polymers in general. Almost any single
bond between a pair of inorganic atoms (Si-Si, Si-C, Si-N, P-N, etc.) is
longer than the C-C bond. Also, the oxygen skeletal atoms are unencum-
bered by side groups and they are as small as an atom can be and still
have the divalency needed to continue a chain structure. In addition, the
