Platinum catalysts, hydrosilylation using

The SiH functional PDMS polymers are grafted onto this surface by platinum catalyzed hydrosilylation, using DC platinum 4 catalyst ... 

Hydrosilylation with 2,4,6,8-tetramethylcyclotetrasiloxane has sometimes given uncontrollable exotherms or explosions in the laboratory. Use of safety screens and the lowest possible (<20 ppm) level of platinum catalyst are recommended. See VINYLSILOXANES... 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

The catalytic hydrosilylation by soluble platinum compounds was discovered by Speier in the late 1950 s. The catalyst he used was H2PtCl6. Other well-known catalysts are those developed by Lukevics [7] and Karstedt [8], The crystal structure of the latter complex was solved by Lappert [9], The catalysts are depicted in Figure 18.5. 

Another method for preparing polysiloxane networks is to use the hydrosilylation reaction an SiH group is added to a double bond e.g., an allyl group with the help of a catalyst (platinum catalyst) ... 

The original use of hydrosilylation [117] in semi-IPN formation is attributed to Arkles [118-120]. His approach was to mix two polysiloxanes with the thermoplastic. One contained hydride functions and the other vinyl functions. The subsequent introduction of a platinum catalyst induced crosslinking. Depending on the number of hybrid and vinyl functions per chain, either chain-extension or crosslinking was observed. As pointed out by Arkles [119], according to theory, these phenomena should be obtained for a 1 1 hydride vinyl group ratio, however, in practice the hydride functions were consumed by several side reactions so that a higher hydride ratio was required. 

We have described a new system of polymeric snpports based on multifunctional, exceptionally sterically hindered carbosilane moieties, grafted with block poly(vinylmethyl-co-dimethyl)siloxane arms. They offer uniformly periphery-distribnted active sites (-CH = CH moieties) and can be used for preparation of novel catalysts. Platinum was thus attached to the polymers via coordination to vinyl gronps. The materials used in hydrosilylation of vinylsilanes can be considered as an alternative for traditional platinum catalysts. The utility of the catalysts seems to be dependent on the ratio [D]/[V] in the copolysiloxane arm. Those with too high amonnt of vinyl groups suffer from poor solubility and catalytic performance dne to excessive inter/intra-chain coordinative cross-linking. 

Platinum compounds and complexes are the most important and commonly used catalysts for hydrosilylation processes [7 - 9]. Platinum catalysts tolerate a variety of functional groups, but some impurities may interact with them leading to catalyst poisoning [10]. This has stimulated much research aimed at employing other transition metal compounds as potential catalysts. For example, Rh(I) complexes are selective and active hydrosilylation catalysts [11] and more resistant to poisoning than the platinum ones [12]. 

On the other hand, there is a need to use inhibitors of the platinum catalysts temporarily to reduce their catalytic activity in the presence of hydro- and vinyl-polysiloxanes in order to stop the curing process at room temperature, but to allow the platinum catalyst to be activated at elevated temperature. Among the principal types of compounds reported are alkenyl derivatives, esters of unsaturated acids, crown ethers, organic nitrogen compounds, phosphines, linear and cyclic vinyl-siloxanes, and poly(vinyl)siloxanes [2], and recently fumarate [44] and maleinate [33]. New co-activators of the catalysts (precursors) have been revealed in the 1990s to reduce to ppm the levels of platinum required to effect hydrosilylation curing [45, 46]. 

Non-conjugatively linked 9,9/-dibenzosilole polymers have also been reported [44,45]. In an early paper, a dehydrogenation catalyst, bis(l,5-cyclo-octadiene)palladium, was used in the synthesis of poly(9,9-dibenzosilole) 52 (Scheme 6) [45]. Recently, poly(dibenzosilole-vinylene)s 54 were obtained from the platinum-catalysed hydrosilylation of 9,9-dihydrodibenzosilole 51 with 9,9-diethynyldibenzosilole 53 (Scheme 6) [44]. 

The catalyst, hydrogen hexachloroplatinate IV) hydrate, (H2PtCl6) may be used in the hydrosilylation reaction. If this catalyst is used then the reaction mixture must be left stirring at 60-70X overnight. Sometimes a trace amount of colloidal platinum may be present in the side-chain liquid crystal cyclic siloxane tetramer. This can be removed by adding a small amount of triphenylphosphine (0.1 g) at step 11. 

The second cure system uses a hydrosilylation reaction for crosslink formation, and is typically supplied as a two-part product. In the crosslinking reaction, a silicon hydride group adds to a vinyl group, typically using a platinum catalyst.(4-6)... 

The franr-1-silyl isomer is exclusively produced through cir-addition. This is characteristic of the hydrosilylation of alkynes in the presence of platinum catalysts. In contrast fran5-addition giving the c/r-adduct is the predominant process on using free radical initiators . Dichlorosilane also adds to 1-alkynes in the presence of platinum catalyst giving a mixture of 1 1 and 1 2 adducts ... 

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