Random copolymers, polysiloxanes

Styrene isoprene block copolymers Styrene butadiene block copolymers Styrene butadiene random copolymers Polyisobutylene Polysiloxanes... 

The same reaction (11.4) is currently used to obtain silicon emulsifier for flexible and rigid PU foams, by the reaction of polydimethylsiloxane of relatively high MW (3000-5000 daltons or more) having several -Si-H groups in the main polysiloxanic chain and a propylene oxide (PO) - ethylene oxide (EO) copolymer, block or preferably random copolymers, having minimum 50% EO units (reaction 11.6). 

Polysiloxane random copolymers (with molecular weights of 5000 to 10,000) with pendant ferrocene groups have been synthesized (2.26) by hydrosUylation of vinylferrocene or 9-ferrocenyl-l-nonene with a poly(methylhydrosiloxane)-r-poly(dimethylsiloxane) random copolymer (Eq. 2.11). These materials were used as amperometric biosensors for the detection of glucose [59]. In this application, the polymers effectively mediated electron transfer between reduced glucose oxidase and a conventional carbon paste electrode. The response of the sensor to glucose was dependent upon the nature of the polymeric backbone. The optimal response was achieved by finding a compromise between increased polymer flexibility and decreased spacing between individual relay (i.e. ferrocene) sites. 

Apart from the hydrolysis route to polysiloxanes there have been efforts to prepare appropriate difunctional silicon derivatives that can be used for step-growth polymerization. For example, diaminosilanes are good syn-thons for condensation with diorganosilanediols. This method is quite effective because of the labiUty of the Si-N bond. Thus, the reaction of Me2Si(NMe2)2 with diphenylsilanediol affords a random copolymer [26] (see Eq. 6.20). The randomization occurs because of the catalytic action of the liberated amine to cleave Si-O bonds. 

The polyoxyalkylene units in the copolymer have a molecular weight below 500, and the polysiloxane units have 3 to 50 silicon atoms. The resin has a phenol/aldehyde ratio of 2 1 to 1 5 and an average molecular weight of 500 to 20,000 Dalton. The composition shows synergistic demulsification activity when compared with the individual components. The siloxane units can be either in blocks [979,980] of the polyoxyalkylene-polysiloxane copolymer or randomly distributed [728,729]. 

The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies. 

Not all copolymers which are produced by step-growth processes are random in nature. Block copolymers are also of major interest. An example of the synthesis of elastic polyurethane fibers was given in Section 1.5.4. Block and graft copolymers of polysiloxane-poly(alkylenc ethers) with segments like5-9 are used as surfactants in the production of polyurethane foams with uniform cell sizes. 

In Chapter 3, the chemistry and technology of the most important oligo-polyols used for elastic polyurethanes fabrication, in fact high MW oligomers (2000-12000 daltons) with terminal hydroxyl groups and low functionality (2-4 hydroxyl groups/mol) were discussed. Polyalkylene oxide polyols (homopolymers of PO or copolymers PO - EO, random or block copolymers), polytetrahydrofuran polyols, filled polyols (graft poly ether polyols, poly Harnstoff dispersion - polyurea dispersions (PHD) and polyisocyanate poly addition (PIPA) polyols), polybutadiene polyols and polysiloxane polyols were all discussed. The elastic polyurethanes represent around 72% of the total polyurethanes produced worldwide. 

Random siloxane copolymers with epoxy are hydrophobic, have a low (-120 °C), are flexible, are thermally stable and are oxidatively stable. However, the simplest polysiloxanes, such as polydimethylsiloxane, polydiethyl siloxane or polydiphenylsiloxane have solubihly parameters veiy diflerent fiom epo ... 

The polyimide siloxane copolymers are generally synthesized, as shown in Scheme 1, from a combination of a dianhydride and diamine monomer and an aminopropyl terminated polysiloxane to form the segmented polyamic acid. The polyamic acid is then cyclodehydrated by either thermal or solution imidization to give randomly segmented polyimide siloxane copolymers (4). This is indeed the classical approach of reacting one flexible oligomer with two monomers which form the "hard segment (9). 

It is widely recognized that the properties of random, alternating, and block polysiloxane copolymers dilfer. These properties depend on the sequencing of the substituents on silicon i.e., regularity of the polymer microstructure. As a result. 

The principle of random block polymerization has been used for polysiloxane block copolymer synthesis as in equation (27) where the block connection is made by condensation between two types of silanol groups. This reaction, which is catalyzed by weak bases such as hexylamine carboxylate salts or tetramethylguanidine to avoid Si—O bond randomization, leads to multiblock polymers (35) having p-phenylene groups in the main chain. ... 

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