Cyclosiloxanes copolymerization

Thus, Andrianov et al. (26) attempted to catalyze polymerization of a number of alkyl and alkyl/aryl cyclosilazanes using catalytic amounts of KOH or other strong bases at temperatures of up to 300°C. In general, the reactions proceed with evolution of NHj, hydrocarbons and the formation of intractable, crosslinked, brittle products even at low temperatures. Contrary to what is observed with cyclotri-siloxanes, no evidence was found for the formation of linear poly-silazanes. Copolymerization of mixtures of cyclosilazanes and cyclosiloxanes gave somewhat more tractable polymers with less evolution of hydrocarbons or ammonia, however very little was done to characterize the resulting materials. 

Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. 

The above-mentioned materials indicate that cyclolinear organosiloxane copolymers are usually synthesized by homopolycondensation and HFC reactions of difunctional organocyclosiloxanes or organopoly(cyclosiloxanes) with a,to-dihydroxy (diamino, dichloro)diorganosiloxanes. As known from the literature, polymerization and copolymerization of organopoly(cyclosiloxanes) not always give soluble polymers and their yield is usually low [99-103],... 

The consumption of cyclosiloxane during polymerization and copolymerization was monitored by GC. The conversion of D3 was nearly 100 % (Fig. 1). In contrast, the ring-opening polymerization of d/ yielded a conversion of about 60 % only (Fig. 2). During the copolymerization with D3 the cyclic vinylsiloxanes showed a similar behavior as observed during homopolymerization. 

The classical Mayo-Lewis scheme relating comonomer feeds to relative reactivity ratios (5i) is often applied to copolymerization of cyclosiloxanes. This scheme presumes that no depropagation of the copolymer occurs, that the copolymerization rate constants depend only on the ultimate comonomer units, and that instantaneous comonomer feed ratios and copolymer compositions are used in the analysis of data. When these assumptions hold, the Mayo-Lewis method is very useful for the analysis of copolymerization data. 

The lack of reliable information on kinetic parameters for the copolymerization of important cyclosiloxanes with ring sizes exceeding three is a serious deficiency. Industrial and routine laboratory syntheses rely almost entirely on such monomers, and in many cases, the processes are copolymerizations. An understanding of these constants would give valuable information on copolymer microstructure and its control. Unambiguous studies... 

Knowledge of the polymerization and copolymerization of cyclosiloxanes and the structure of the polymers so formed is fundamental to understanding the structure-property relationships of polysiloxanes. Such knowledge in the areas of polymer stability, reactivity and surface activity is of prime importance in the industrial application of polysiloxanes. 

ROP of functional cyclosiloxanes is a convenient route to all-siloxane copolymers. " " This may be realized in several ways. Sequential ring-opening copolymerization involves polymerization of one cyclosiloxane monomer to the desired chain length, then the second monomer is introduced, which adds to the growing chains. Diblock and triblock copolymers in the case of bifunctional initiator are produced provided the process is kinetically controlled. Otherwise, a copolymer with random or partly ordered sequencing is obtained. The kinetic control of the copolymerization process imposes the use of cyclotrisiloxanes as monomers. Thus, sequential copolymerization of two or more cyclotrisiloxanes is a good method for the precise synthesis of all-siloxane block copolymers. " ... 

Simultaneous polymerization of a mixture of cyclosiloxanes gives polymers whose microstructure is not easily predictable. Typically, the kinetics of ring-opening copolymerization is analyzed in terms of the Mayo-Lewis copolymerization equations (Scheme 5). The aim of such analysis is to determine the Mayo-Lewis reactivity ratios rD = feDD/feox and rx = fexx/fexD, which define the composition of a copolymer." The task is relatively easy when the propagation reactions are irreversible. 

Equilibrium copolymerization of unstrained cyclosiloxanes leads to random copolymers.Thus, for example, equilibrium copolymerization of diethylcydotetrasiloxane with cydotetrasiloxanes having various R R SiO units (R = R = methyl or phenyl, or R = methyl, and R = phenyl) gave copolymer of units randomly distributed along the chain. ° The regioselectivity of the ROP of unsymmetrically substituted cydotetrasiloxanes such as 2,2,4,4,6,6-hexamethyl-... 

The copolymerization of a functional cyclotrisiloxane with D3 leads to a copolymer of dimethylsilojane units and sUoxane-containing functional group units which-in similar manner to the copolymer obtained by ROP of cyclosiloxane with mixed units-contains macromolecules of uniform structures with regards to the size, monomer unit composition and sequencing [117]. However, the distribution of units along the chain is different The more reactive comonomer enters the chain... 

The sequential copolymerization of cyclosiloxanes is particularly suitable for the synthesis of well-defined diblock and triblock all-siloxane functionalized copolymers. Using a monofunctional initiator leads to diblock copolymers, while triblock copolymers are formed with bifunctional initiators. Some typical examples of AB block copolymers are shown in Table 3.7. It is recommended that the less-reactive monomer be polymerized in the first step, and this is the case of the examples in which D3 and V4 are polymerized first. The comonomer should be introduced not later than at 90-95% of conversion of the first monomer, in order to avoid the processes that lead to chain randomization. Those monomers which have func-... 

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