Catalysis silicone polymers, cross linking

Catalysis. Platinum-catalyzed hydrosilation is used for cross-linking silicone polymers and for the preparation of functionally substituted silane monomers (250). The most widely used catalyst is chloroplatinic acid (Spier s catalyst), H2PtCl6. Other compounds that catalyze the reaction include Pt(II)... 

These two steps are the basic reactions in silicone polymerization. It will be recognized that fhe presence of two —OH groups on silicon allows the formation of linear polymers three or more —OH groups allows the formation of cross-linked polymers. There is no evidence that hydroxyls on the same silicon ever condense to give =Si=0 for this reason the polymer is free to grow. Acid or base catalysis is generally used to speed up the reaction. 

Weak P-donor interactions could be the reason for the frequently reported high reactivity of hydroxylaminosilanes. These include claims for new cross-linking agents and cold-curing catalysts for silicone polymers [6], and more recently, the catalysis of the alkoholysis of Si-H functions in poly-phenylsilane [7]. 

Hexachloroplatinic acid and other platinum complexes are mainly used as soluble catalysts for the additive cure. The most active catalyst used recently for vulcanization of silicon rubber is the platinum-alkenylsiloxanes complex, mainly the platinum-vinylsiloxane complex (Karstedt s catalyst) (4). One important approach to the activated cure of silicone rubber makes use of various inhibitors or moderators added to the platinum catalyst to reduce, or temporarily inhibit, its catalytic activity in the presence of the alkenyl- and hydropolysiloxanes (see catalysis by Pt complexes). The catalyst is usually added to the reaction mixture in quantities related to the number of unsaturated (e.g., vinyl) substituents in the polysiloxane. Vinyl-terminated polydimethylsiloxane polymers (viscosity > 200 cSt) are typically cross-linked by methylhydrosiloxane-dimethylsiloxane copolymer with 15-50 mol% of polymethylhydrosiloxane. A typical catalyst is a platinum complex in alcohol, xylene, divinylsiloxanes, or cyclic vinylsiloxanes. The system is usually prepared in two parts (part A, vinylsiloxane -I- Pt (5-10 ppm) part B, hydrosiloxane -I- vinylsiloxane). Inhibitors stop the platinum catalyst they are volatile or react with silicone hydride cross-linker to become a part of the polymer network. Some of them are decomposed by heat or light (UV). A single-component system contains fugitive inhibitors of Pt. 

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