Dehydrocoupling polymerization

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

Transition Metal Catalyzed Dehydrocoupling Polymerization Reactions... 

The synthesis of comblike organoboron polymer was examined (see scheme 2) by dehydrocoupling polymerization of hydroborane monomers bearing an... 

To avoid problems associated with these polymer reactions, the direct synthesis of lithium borate polymer electrolytes37 was undertaken by dehydrocoupling polymerization using lithium mesitylhydroborate40 (scheme 7). 

A variety of organoboron polymer electrolytes were successfully prepared by hydroboration polymerization or dehydrocoupling polymerization. Investigations of the ion conductive properties of these polymers are summarized in Table 7. From this systematic study using defined organoboron polymers, it was clearly demonstrated that incorporation of organoboron anion receptors or lithium borate structures are fruitful approaches to improve the lithium transference number of an ion conductive matrix. 

Kawakami, Y. Imae, I. Synthesis and Characterization of Siloxane-Containing Polymers Obtained by Dehydrocoupling Polymerization. In Synthesis and Properties of Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds. ACS Symposium Series 838 American Chemical Society Washington, DC, 2003 pp 61-71. 

The functionalization of polymers is another useful feature of the hydrosilation reaction. The introduction of highly fluorinated alkyl chains by the hydrosilation of Si-H groups of the polymer with fluorinated alkenes is a typical example. Poly(phenylsilanes) obtained by the dehydrocoupling polymerization of phenylsilane undergo AIBN initiated free radical hydrosilation with alkenes, ketones and aldehydes. Cross-linking and branching of polymers can also be easily accomplished using hydrosilation. 

Cross-dehydrocoupling polymerization of 1,3-dimethyl-1,3-diphenyl-1,3-disiloxa-nediol with 1,3-dihydro-l, 3-dimethyl-l,3-diphenyl-l,3-disiloxane in the presence of a catalyst gave poly(methylphenylsiloxane). This reaction was applied to the synthesis of stereoregular poly(methylphenylsiloxane) in the presence of rhodium... 

In studies of the reaction of TiMe2(dmpe)2 with PhSiHs Girolami and coworkers could isolate two interesting oligosilane titanium complexes (Scheme 34) which can be considered as intermediates in a dehydrocoupling polymerization... 

Mw = 56,170, PDI = 1.67). The electron-withdrawing nature of the fluorinated aryl substituents appeared to have had an important influence on the reactivity, as the dehydrocoupling process was observed to occur efficiently at the mildest temperatures observed with such polymerizations to date.49... 

More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

It seems likely that further improvements in this coordination polymerization technique will require design of more active catalysts. The catalyst systems currently available are very amenable to modifications that dramatically influence dehydrocoupling rates. Significantly, results so far indicate that there is no inherent limitation to molecular weight control via polymerizations of this type, and in principle it should be possible to identify conditions and catalysts that allow production of high polymers. 

Scheme 8 o--Bond metathesis polymerization mechanism proposed in Zr-catalyzed dehydrocoupling. 

As described in Section II.E, polystannanes, which can be prepared by the Wurtz coupling of chlorostannanes with alkali metals and by the dehydrocoupling of hydrostannanes, exhibit intriguing physical properties. The Sita group reported a peculiar polymerization of -Bu2SnH2 with a RhH(CO)(PPh3)3 catalyst [Eqs. (21a) and (21b)].63... 

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