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发帖数: 176 | 1 http://onlinelibrary.wiley.com/doi/10.1002/masy.19900320118/pdf
On the nature of active species in cationic ring-opening polymerization of
cyclodimethylsiloxanes
Authors
Pierre Sigwalt,
Vivian Stannett
First published: February 1990Full publication history
DOI: 10.1002/masy.19900320118View/save citation
Cited by: 5 articlesCitation tools
Abstract
Contrarily to cationic ring-opening polymerization of cyclic ethers and of
some other cyclic monomers, for which direct identification of the various
types of active centres has been made in a few cases, the nature of the
species active in the polymerization of cyclo-dimethylsiloxanes is not yet
known. However, some provisional conclusions about the possible mechanisms
may be deduced from the wide variation in the types of products and in the
kinetics observed according to either the size of the cyclic monomer (D3, D4
, D5, D6) and to the type of initiation (chemically, or radiation induced).
For polymerizations with either protonic or non-protonic initiators, made in
CH2Cl2 near room temperature, the smaller cycle D3 behaves quite
differently from D4, D5 and D6. D3 is more reactive in both homo- and
copolymerizations. It gives small cycles of other types and the effect of
water on the reaction may be quite different. A discussion of the data leads
to the conclusion that polymer growth for most cyclosiloxanes involves
activated esters, while it may occur for D3 on different sites such as
oxonium or silanol groups.
Polymerization of D3, D4 and D5 initiated in bulk at 90°C by high energy
radiation, in high purity conditions, has also been shown to be cationic but
the active centres concentration is much lower, and the propagation rate
constants much higher, than in chemically initiated polymerizations. The
global rates, the monomer reactivities in copolymerization and the types of
cycles are similar for D3, D4 and D5, which is attributed to propagation
occurring on very reactive silicenium ions, either free or in the same
solvation state. |
