Cyclosiloxanes sequential polymerization

With the purpose of increasing the range of available block copolymers, comonomers other than methacrylates and acrylates can also be involved in sequential polymerization, provided that they are susceptible to anionic polymerization. Dienes, styrene derivatives, vinylpyridines , oxiranes and cyclosiloxanes are examples of such comonomers. The order of the sequential addition is, however, of critical importance for the synthesis to be successful. Indeed, the pX a of the conjugated acid of the living chain-end of the first block must be at least equal to or even larger than that of the second monomer. Translated to a nucleophilicity scale, this pK effect results in the following order of reactivity dienes styrenes > vinylpyridines > methacrylates and acrylates > oxiranes > siloxanes. 

Sequential polymerization consists of first polymerizing a given cyclosiloxane to the desired chain length whereupon the comonomer cyclosiloxane is then introduced into the reaction medium and proceeds to grow from the living chain end. The reaction is terminated at the desired conversion129,130. The active reaction centre must only react with the monomer and not with the linear chains already formed. Under these conditions pure block copolymers will be produced. 

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. " ... 

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-... 

AB and ABA block copolymers have been prepared by the sequential addition of hexaphenyltricyclosiloxane and octaphenylcyclotetrasiloxane on a PDMS with one and two lithium silanolate chain-ends, respectively [153-157]. Some redistribution reactions occurred due to the harsh conditions required for the ROP of the perphenyl cyclic monomers. Finally, some multiblock copolymers have been prepared by the anionic equilibrium polymerization of cyclosiloxanes, which leads to a random distribution of different units-that is, diethyl, diphenyl, methylphenyl siloxy units [157], dimethyl, diphenyl siloxy units [158], and dimethyl, methylvinyl-siloxy units [159]. 

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