Primary silanes, polymerization

Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

With cyclohexene, polymerization occurs more rapidly than hydrosilation. After polymerization has proceeded to completion, there is a slow hydrosilation to introduce cyclohexyl groups onto the polymer chain, to a maximum extent of about 50 per cent of the Si-H groups. With more reactive olefins, such as styrene, hydrosilation occurs more rapidly than polymerization and the polymerization reaction is suppressed. As in the polymerization reaction, the reactivity of primary silanes is much greater than... 

Using the titanocene-catalyzed co-hydrogenation of cyclohexene, we have studied the kinetics of the polymerization of a number of primary silanes ( 20 ). The rate law was found to be ... 

A similar dichotomy was observed in the titanium catalyzed polymerization of primary silanes coupled to the hydrogenation of norbornene (20). At low catalyst concentration (ca. 0.004H), essentially complete conversion of norbornene to an equimolar mixture of norbornane and bis-phenylsilyl- (and/or 1,2-diphenyl-disilyl)norbornane was observed. Under these conditions no evidence for reduction of titanium was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 occurs and the catalytic reaction produces mainly polysilane (DPn ca. 10) and norbornane in ca. 80 per cent yields, and silylated norbornanes in about 20 per cent yield. 

Like many homogeneously catalyzed reactions, the overall cycle (or cycles) in these polymerization reactions probably contains too many steps to be easily analyzed by any single approach. Both kinetics and model compound studies have thrown light on some of the steps. However, as indicated above, many of the model compounds isolated from the reactions of primary silanes with metallocene alkyls and hydrides are too unreactive to explain the polymerization results. 

Dehydrogenative Coupling. Transition-metal catalyzed polymerization of silanes appears to hold promise as a viable route to polysilanes. A number of transition-metal complexes have been investigated, with titanium and zirconium complexes being the most promising (105—108). Only primary silanes are active toward polymerization, and molecular weights are rather low. The dehydrogenative polymerization is depicted in reaction 11, where Cp = cyclopentadienyl ... 

In his first publication4 reporting the use of Cp2TiMe2 to polymerize primary silanes, Harrod also described the condensation of PhMeSiH2 in toluene and the coupling of this secondary silane occurred much more... 

Silanes. Group IVB metallocenes catalyze the stepwise polymerization (dehydrocoupling) of primary silanes, yielding H—(SiHR) —H and n H2 (232). The mechanism appears to he strictly metathetical for such molecules as (Cp)(C5(CH3)5)Hf(X) (R = H, alkyl X = H, alkyl, chloride), wherein the entire process takes place on the neutral manifold (Scheme 11) (233). 

A second class of catalysts was discovered by Harrod and coworkers [8], who showed that titanocene and zirconocene derivatives dehydrogenate primary silanes (RSiH3) to poly silanes with 10-20 monomer units, in high yield. These catalysts therefore appear to be more promising with respect to production of polymers, and have revived interest in coordination polymerization routes to polysilanes. 

Other primary silanes, mainly functionalized aryl and alkyl silanes, were successfully polymerized and some examples are listed in Table 4. 

The problems of polymerizing secondary silanes seem to be not only finding a siiane with a suitable substitution to allow a polymerization but also finding the right catalyst, as in the case for the polymerization of primary silanes. 

Aitken C, Harrod JF, Samuel E (1985) Polymerization of primary silanes to linear polysilanes catalyzed by titanocene derivatives. J Organomet Chem 279 C11... 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

Summarizing, aminosilanes show a fast adsorption on the silica surface. An equilibrium monolayer coating is formed. Modification in aqueous solvent causes polymerization on top of the initial monolayer. For modification from organic solvent, the reactions can be better controlled. With the bifunctional AEAPTS, a secondary silane layer adsorbs on the free primary amine groups of the first monolayer. At high concentration and after long reaction times, for both aminosilane types, a further non-specific deposition occurs. 

The monosilanes that produce polymeric material tend to be limited. Thus far, primary arylsilanes are the only well documented silanes that react to provide polymer. Primary alkylsilanes couple more slowly and tend to give relatively short chains (chain lengths up to about 12 silicon atoms). 

Three primary mechanisms have been suggested for enhanced adhesion via silane coupling agents.5 The classical explanation is that the functional group on the silane molecule reacts with the adhesive resin. Another possibility is that the polysiloxane surface layer has an open porous structure. The liquid adhesive penetrates the porosity and then hardens to form an interpenetrating interphase region. The third mechanism applies only to polymeric adherends. It is possible that the solvent used to dilute and apply the silane adhesion promoter opens the molecular structure on the substrate surface, allowing the silane to penetrate and diffuse into the adherend. 

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