Premium
On the nature of active species in cationic ring‐opening polymerization of cyclodimethylsiloxanes
Author(s) -
Sigwalt Pierre,
Stannett Vivian
Publication year - 1990
Publication title -
makromolekulare chemie. macromolecular symposia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.257
H-Index - 76
eISSN - 1521-3900
pISSN - 0258-0322
DOI - 10.1002/masy.19900320118
Subject(s) - cationic polymerization , polymerization , oxonium ion , monomer , polymer chemistry , chemistry , silanol , polymer , ring opening polymerization , kinetics , activation energy , organic chemistry , catalysis , ion , quantum mechanics , physics
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 (D 3 , D 4 , D 5 , D 6 ) and to the type of initiation (chemically, or radiation induced). For polymerizations with either protonic or non‐protonic initiators, made in CH 2 Cl 2 near room temperature, the smaller cycle D 3 behaves quite differently from D 4 , D 5 and D 6 . D 3 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 D 3 on different sites such as oxonium or silanol groups. Polymerization of D 3 , D 4 and D 5 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 D 3 , D 4 and D 5 , which is attributed to propagation occurring on very reactive silicenium ions, either free or in the same solvation state.