Polymerizations of Alkenylsilanes Using Early Transition Metal Catalysts
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POLYMERIZATIONS OF ALKENYLSILANES USING EARLY TRANSITION METAL CATALYSTS
JOHN MASNOVI*,
XIN Y. BU*,
PAULA CONROY*,
A. HARRY ANDRIST*,
FRANCES I.
HURWITZt AND DOUG MILLER# * Cleveland State University, Cleveland, OH 44115 t NASA Lewis Research Center, Cleveland, OH 44135 * Cedarville College, Cedarville, OH 45314
ABSTRACT Oxidative coupling of primary alkylsilanes catalyzed by Group IVB metallocene complexes leads predominantly to linear polysilanes, as first reported by Harrod. We have investigated the polymerization of ethylsilane and vinylsilane using dimethyltitanocene in order to determine the suitability of such polymers as precursors to Si-C based ceramics for application as coatings or composite matrices. A photochemical procedure for initiation of polymerization is described. Ethylsilane forms polysilanes (which contain a -Si-Si-Si-Si- backbone) by a step growth mechanism. Vinylsilane shows some Si-Si formation; however, polymerization by several different routes, including formation of polycarbosilanes (which contain a -Si-C-SI-C- backbone) by hydrosilation reactions and crosslinking via metathesis, predominate. The carbosilane polymer has high char yield (60-75%) and appears advantageous for conversion into silicon carbide, as determined by X-ray diffraction.
INTRODUCTION Polymerization of primary alkylsilanes (R-SiH 3 ) by dimethyltitanocene (cP 2 TIMe 2 ) and related catalysts was discovered by Harrod several years ago [1]. Linear polysilanes with n - 10 (Eq. 1) were reported to form by a process which occurs in two stages. In the first stage, step-growth polymerization proceeds via a radical process. Following an induction period, a second stage, characterized by formation of bimetallic Ti(III) species, proceeds to form the linear polymer without indication of short chain oligomers [2]. R n
R-SiH 3 - - -
cP 2 TiMe 2 - - - ..
---
II H-SiI
H
R [-bl-]JnI
h
R I -Si-H I
+
(n-l)
H2
(1)
H
Our goal in this study was to determine the suitability of polysilanes produced in this manner for use as precursors to silicon carbide coatings
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
780
and composite matrices [3]. An immediate problem associated with characterization of the polymers concerns the variable nature of the induction period preceding the second stage of the polymerization. Different polymers apparently result during each of the two stages [2]. The properties of bulk polymer would be expected to depend upon the extent of polymerization which occurs during each stage. Therefore, a means of initiating the second stage of polymerization was sought initially in order to provide more uniform and reproducible polymerization. Furthermore, crosslinks generally confer enhanced structural integrity to polymers. Features which promote crosslinking would be expected to benefit char yield, by reducing loss of small silane fragments on pyrolysis, and thus enhance the conversion of polymer into silicon carbide [3]. Alkenyl functional groups may form crosslinks by several
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