Chemistry Reference
In-Depth Information
in the relative amounts of cyclic and linear species. At equilibrium, a
Gaussian distribution of molecular weight exists. The cyclic oligomers
that occur most frequently are D4 through D6. The amount obtained de-
pends greatly on the “monomer” and on the polymerization conditions.
D4 through D6 are typically present to the extent of 10-15 wt %. The
lower molecular weight products are generally removed from the polymer
before it is used in a commercial application.
The low molecular weight products are also of interest from a funda-
mental scientific point of view in two respects. First, the extent to which
such products are formed can be used as a measure of chain flexibility.25-27
25-27
Second, the various cyclic species formed can be purified using standard
separation techniques, and then used to test theoretical predictions re-
garding the differences between otherwise identical cyclic and linear
molecules.
28-33
For anionic equilibrations, typical catalysts are alkali metal oxides and
hydroxides, and bases in general.
34-40
Initiation and propagation involve
nucleophilic attack on the monomer, causing opening of the ring followed
by chain extension. As is frequently the case in ionic polymerizations, the
nature of the counter cation, particularly its size, can have a large effect on
the reaction. This polymerization is very different from most others,
8
which are driven by a decrease in enthalpy. That is, the decrease in entropy
that accompanies the linkage of monomers into a chain-like structure is
counteracted and overcome by the decrease in energy generated by the
formation of new chemical bonds. In the siloxane case however the bonds
linking the monomer units into the chain are similar in energy to those
found in siloxane rings, and the net energy change is very small. There is
an increase in entropy, presumably from increased internal molecular
freedom of the siloxane segments in going from the cyclic structures to
the linear chains. It is this increase in entropy that drives the polymeriza-
tion reaction. Ring opening has also been induced by hydride transfer
reactions.
41
A new, low-Tg siloxane thermoplastic elastomer with a functionalizable
backbone was recently synthesized via sequential anionic ring-opening
polymerization and coupling.
42
The attachment of a photoresponsive
liquid crystal led to a rapid thermoplastic photoactuator. Polystyrene was
used as a hard glassy end block, and poly(vinylmethylsiloxane) served as
the soft middle segment in this polystyrene-b-poly(vinylmethylsiloxane)-
b-polystyrene ABA triblock copolymer.
Cationically catalyzed polymerizations
12, 43-45
have not received as
much attention as the anionic variety. Typical cationic (acidic) catalysts in
this case are Lewis acids. Yields and proportions of the various species are
