Cyclic monomers, redistribution

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]. 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

This overview analyzes the electron redistribution that takes place in two interacting monomers upon H-bond formation. The systems selected (shown in Fig. 1) focus on the two most important classical H-bonds, O-H- O and N H- O, and cover both donor and acceptor roles of every monomer. They are the following (a) water dimer (WD), (b) methanol water complex (MW), (c) water methanol complex (WM), (d) formic acid dimer (FAD), (e) forma-mide dimer (FD), and (f) formamide-formic acid complex (FFAC). Systems (a)-(c) are bound by one single O-H- 0 bond whereas cyclic dimers (d)-(l) are linked by two H-bonds. In FAD and FD homodimers, both monomers behave simultaneously as donor and acceptor while in the FFAC heterodimer, for-mamide and formic acid play competing roles. 

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. 

The product mixture from a typical MCS reaction is subjected to several distillation and isolation steps. The product mixture can be roughly divided into monomers and residue. The monomers are separated from the residue stream by distillation the residue contains siloxanes and disilanes. Some monomers can be recovered by various redistribution reactions of the residue mixture (15). The individual monomers are separated by distillation where the separation of Di from Tri is difficult. With Di as an example, equation 3 shows the hydrolysis and condensation to form linear and cyclic polysiloxanes. Another useful material is hexamethyldisiloxane (MM) which forms from hydrolysis/condensation of Me3SiCl (mono), equation 4. 

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