Linear siloxane polymers polymer yield

Cross-linking is a predominant process during irradiation of siloxane polymers. Chain scissions are negligible. ° ° The cross-link density increases linearly with a dose up to 160 Mrad (1,600 kGy). ° At 5.0 MGy (500 Mrad) the G(X) value is 0.5. Free radical scavengers, such as n-butyl and frrf-dode-cyl mercaptan and diethyl disulfide, are the most effective antirads. ° - ° At a concentration of 10%, two-thirds of the cross-links were prevented from forming however, the scission yield was also increased. 

The first synthetic route explored to produce cydic polymers made use of ring-chain equilibrium. This approach involves the natural equilibrium that occurs between linear and cydic polymers during condensation polymerizations although, inevitably, this yields linear byproducts and broad polydispersities. As a result, precipitation or preparative gel-permeation chromatography (GPC) was required to obtain cyclic polymers of sufficient purity for further study. This approach is amenable to a broad range of polymerization chemistries, induding the preparation of cyclic polyesters [7,8], polyethers [9], poly(dibutyltin dicarboxylates) [10,11], and poly(siloxanes) [12-15]. 

This state is, by definition, independent of the starting siloxane substrates and of the initiator used (either anionic or cationic). The polymer yield and its characteristics are not related to the polymerization kinetics. Instead, the knowledge of the thermodynamics of the process is essential. The final state of the reaction involves complex equilibria between the polymeric species of two homologous series, cydic and linear polysilox-anes. The equilibrium state may be described by general eqn [5]. 

High temperature thermosets and ceramics have been synthesized by heat treatment of various blends of poly (siloxane-acetylene) and poly (carborane-siloxane-acetylene). The polymeric blends give high char yields on pyrolysis, and the resultant chars show excellent oxidative stability to at least 1500 °C. The thermosets and ceramic chars show similar oxidative stability to previously studied copolymers containing varying amounts of siloxane, carborane, and acetylene units within the backbone. It has been determined that only a small percentage of carborane is necessary to provide this oxidation protection. Thus, these precursor linear hybrid polymers are more cost-effective than previous polymers which contained carborane in each repeating unit. 

Atomic Structure. The control of atomic structure is fundamental to any system, and an incomplete understanding of atomic structure can limit advancement. For example, our understanding of preceramic polymers, up through the formation of networks, is improving but the full exploitation of this chemistry is still limited by the lack of detailed knowledge of the structure of the resulting ceramic at the atomic level. Even with more familiar silicone polymer systems, synthetic barriers are encountered as polymers other than poly(dimethylsiloxane) are used. Stereochemical control is inadequate in the polymerization of unsymmetrical cyclic siloxanes to yield novel linear materials. Reliable synthetic routes to model ladder systems are insufficient. 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

As usual, there are some limitations in attachment of various organic groups to siloxane backbone. In some instances, direct hydrosilylation of an olefin derivative of the intended functional group can lead to serious side reactions it may give poor yield or may simply call for unusual and difficult to prepare intermediates. In any case, hydrosilylation of polymers with Si—H is a good source of preceramic polymeric materials (4,18). A variety of linear polysilanes and polycarbosilanes have been reported, but because they tend to depolymerize on heating and afford little ceramic jdeld, they are not useful as SiC precursors. To increase the... 

Clark employed poly(dimethyl siloxane) (PDMS) homo-IPNs to make improved adhesives. Three separate linear PDMS chains were mixed, each with reactive groups. Polymers I and II reacted to form a network, yielding a semi-IPN. The remaining linear polymer provided the adhesive properties. After adhering the two required surfaces together, raising the temperature initiated a self-crosslinking of polymer III to form the IPN. 

The relative amounts of the two types of polymers are determined by reaction conditions. Hydrolysis with water alone yields 50—80% linear polydimethyl-siloxane-a,co-diols and 50—20% polydimethylcyclosiloxanes. Hydrolysis with 50—85% sulphuric acid gives mostly high molecular weight linear polymers with only small amounts of cyclosiloxanes. Conversely, the hydrolysis of dimethyl-dichlorosilane with water in the presence of immiscible solvents (e.g., toluene, xylene and diethyl ether) results in the preferential formation of lower polycyclosiloxanes. Such solvents, in which the organochlorosilane is readily soluble, lead to a reduction in concentration of dimethyldichlorosilane in the aqueous phase and thus intramolecular condensation is favoured over inter-molecular condensation. Further, the hydrolysis products are also soluble in the organic solvent and the cyclic compounds are protected from the action of the aqueous acid. (See later.)... 

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