Polymerization of siloxanes

An interesting feature of the ring opening polymerization of siloxanes is their ability to proceed via either anionic or cationic mechanisms depending on the type of the catalyst employed. In the anionic polymerization alkali metal hydroxides, quaternary ammonium (I NOH) and phosphonium (R POH) bases and siloxanolates (Si—Oe M ) are the most widely used catalysts 1,2-4). They are usually employed at a level of 10 2 to KT4 weight percent depending on their activities and the reaction conditions. The activity of alkali metal hydroxides and siloxanolates decrease in the following order 76 79,126). 

Typical initiators for living anionic polymerization of siloxanes include conventional organoalkali compounds and lithium siloxanolates22). Initiators containing lithium counterions are preferable to sodium or potassium counterions due to the lower catalytic activity of lithium in siloxane redistribution reactions. Living anionic polymeriza-... 

Amine-terminated siloxane oligomers have also been utilized in the synthesis of various siloxane-amide and siloxane-imide copolymers, High molecular weight siloxane-amide copolymers have been synthesized by the solution or interfacial co-polymerization of siloxane oligomers with sebacoyl chloride or terephthaloyl chloride respectively 1S5,165). In some reactions diamine chain extenders have also been utilized. Thermal and dynamic mechanical characterization of these copolymers have shown the formation of multiphase systems160). Compression molded films displayed very good elastomeric properties. 

Benzyldimethyldodecylammonium hydroxide (BDDOH), in emulsion polymerization of siloxanes, 22 561 Benzyl formate, physical properties, 6 292t Benzylic carboxylic esters, cleavage of,... 

A. Rembaum, E. E. Eostick and M. A. Deisz Block polymerization of siloxanes and vinyl monomers. Amer. chem. Soc. 133th Meeting, San Francisco, 1958 Abstracts of Papers, p. 5R. 

Both protonic and Lewis acids initiate ring-opening polymerization of siloxanes. In spite of the practical importance [240] and the very extensive studies of this system, the mechanism of the polymerization is not completely understood. Even the nature of ionic propagating species is still a matter of controversy. The silicenium [241] or oxonium [242,243] ion structure has been postulated. 

For small product quantities, the polymerization of siloxane oligomers is carried out in mixing tanks in which batches of up to 15 t can be easily managed. 

Recent studies have shown that the polymerization of siloxanes initiated by strong acids such as CF3S03H follows a complex rate law in which it is necessary to invoke both of the mechanisms represented by equations 15 through 19 in an interrelated manner59,60,63. 

The relative contribution of the specific chain growth mechanism (i.e., activated monomer vs. oxonium ion addition) may depend on ring strain of monomer, nucleo-philicity of anion and solvating power of solvent (ability to stablize ions). Many of these factors have been quantiatively determined in the polymerization of cyclic ethers and acetals, where the concentrations of the tertiary and secondary oxonium ions were simultaneously determined by the phosphine cation-trapping method (cf. Adv. Polymer Sci. 37). This method seems to be also applicable in the polymerization of siloxanes, but has not yet been evaluated. 

Two other classes of silicones deserve mention. These are the water-based silicones that are used in sealant and coating applications and the silicone pressure-sensitive adhesives. Water-based silicones can be prepared by anionic polymerization of siloxanes in water using a surface-active catalyst such as dodecylbenzenesulfonic acid [4]. The resulting emulsion can then be cross-linked in several ways, including the use of alkoxysilane copolymerization or tin catalysts in conjunction with colloidal silica. The result is essentially an emulsion of cured PDMS in water. Various fillers and other components are added, resulting in a sealant composition. Upon evaporation of water. 

Silicone oils are linear polysiloxanes obtained by the following two-step process. The first is that siloxanes are prepared by hydrolysis of trimethylchlorosilane or dimethyldichlorosilane. The second is a polymerization of siloxanes with acid or alkali as shown in eqs. (8.38)-(8.40) [32,33]. 

Kirk [52] described the use of tetrabutylphosphonium hydroxide, which brings about rapid rearrangement and polymerization of siloxanes at temperatures up to 100 C. After the polymerization is completed, the phosphonium catalyst is decomposed by heating the polymer above 130°C, as shown in Eq. (20). 

Polymerization of siloxanes in water emulsion attraas srill growing interest and has recently been reviewed by Ganachaud and Boileau. Hyde and Wehrly were the first to demonstrate a possibility of ROP of cyclosiloxanes in water emulsion. High MW polymer may be obtained, either in the presence of anionic initiator and cationic emulsifier or using... 

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