Copolymer random, stepwise polymerization

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

Anionic, stepwise polymerization is used to prepare copolymers of alkylene oxides. The relative copolymerization rate is essentially 1. Random or block polymers can be formed, depending on the monomer fed to the reaction. 

Use of a mixed dihalide monomer feed in a conventional polymerization will produce random copolymers. A block copolymer cannot be prepared by a stepwise addition of the monomers. During copolymerization interchange takes place, resulting in randomization. To prepare block copolymers, it is best to prepare individual mercaptan-terminated polymers, blend them in the desired proportion, and then process the blended system by conventional techniques. 

Since these anionic stepwise reactions are essentially living polymerizations (42,43), complete reaction of one monomer can be followed, sequentially, by reaction of a second, giving rise to block polymers with AB or ABA structures. It should also be apparent that block copolymers with random block compositions of different monomer ratios can be prepared. 

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