Macromolecules 1997, 30, 3141-3146
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Modification of a Hyperbranched Hydridopolycarbosilane as a Route to New Polycarbosilanes I. L. Rushkin, Q. Shen, S. E. Lehman, and L. V. Interrante* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Received November 8, 1996; Revised Manuscript Received February 18, 1997X
ABSTRACT: A general method for introducing various side chains onto a highly branched polysilaethylene (HBPSE) backbone is described. This method starts with a commercial oligomer mixture which has a highly branched structure, consisting of [H3SiCH2], [SiH2CH2], [SiH(CH2)2], and [Si(CH2)3] subunits combined to give an overall “SiH2CH2” average formula. Several alkyl-, aryl-, and allyl-substituted HBPSEs were prepared by first converting the parent HBPSE into a highly reactive bromo-substituted derivative which was then treated with organolithium reagents (RLi, R ) C4H9, C6H13, C8H17, C6H5, C6H4N(Me)2) or Grignard reagents (RMgBr, R ) Et, allyl). The parent HBPSE and its derivatives were characterized by means of NMR and IR spectroscopy, as well as elemental analysis, DSC, GPC, and VPO. It was shown that the allyl-substituted HBPSE can be used as a synthetic platform for attaching functional side chains via the hydrosilation reaction.
Introduction
Scheme 1
Polysilylenemethylenes, PSMs, (polymers having the formula [Si(R1)(R2)-CH2]n), constitute a rather neglected class of organic-inorganic hybrid polymers which are currently undergoing a resurgence in interest. Among the potential advantages that this class of polymers may have as compared to the existing classes of inorganic and organic polymers are a combination of low Tg’s and high synthetic versatility similar to that of such inorganic backbone polymers as the polyphosphazenes and polysiloxanes with good chemical backbone stability similar to that of the polyolefins.1,2 Two general routes have been developed for the preparation of linear PSMs having desired substituents attached to the silicon. One route involves the ringopening polymerization (ROP) of the correspondingly substituted disilacyclobutane. This method works best for alkyl- and phenyl-substituted disilacyclobutanes.3 An alternative route involves the preparation of polymers having reactive (Cl, OR, or H) functionalities attached to the silicon and then introduction of the desired substituents through substitution or addition (hydrosilation) reactions.2 Both routes rely on the use of disilacyclobutane compounds as monomers. Unfortunately, the preparation of the required disilacyclobutanes can be problematic, involving a multistep procedure with low to moderate overall yields.3d,4 This represents a serious obstacle to the full development of this new and interesting class of polymers and their prospective application as functional polymeric materials. Several years ago a rather different approach to the preparation of methylene-bridged polycarbosilanes was reported.5 This approach involves the condensation via a Grignard reaction and subsequent reduction of (chloromethyl)trichlorosilane so as to form a hyperbranched oligomeric/polymeric carbosilane having a nominal “SiCH4” formula on pyrolysis (Scheme 1). This “hydridopolycarbosilane” was found to give nearly stoichiometric silicon carbide (SiC) in high yield. Recently, a new process for the kilogram-scale production of this hyperbranched hydridopolycarbosilane (HPCS) has been developed by Starfire Systems, Inc.6 X
Abstract published in Advance ACS Abstracts, May 1, 1997.
S0024-9297(96)01664-6 CCC: $14.00
In addition to offering the prospect of a more convenient and readily available starting point for the preparation of new substituted polycarbosilanes, this highly branched version of polysilaethylene,5 which we will refer to here as HBPSE (1), is also of independent interest from the perspective of its highly branched structure. Such hyperbranched polymers and their relatives, the dendritic polymers, have attracted a great deal of attention recently and are being widely studied for a variety of potential applications, ranging from solvent-free paints to agents for delivering drugs or genes into cells.7 Examples of dendritic carbosilanes have been reported in the literature; however, the synthesis of these polymers is rather tedious, discouraging their consideration for most potential applications.8 All these considerations led us to realize the importance of developing convenient synthetic methods for the modification of HBPSE (1). This paper describes our first efforts in this direction. Characterization Of The Parent HBPSE. HBPSE (1) was obtained as a viscous, yellowish liquid. The relative amounts of the units that comprise its hyperbranched, oligomeric structure were estimated from its quantitative 29Si NMR spectrum (Figure 1a). The assignments of the peaks in these spectra to the (CH2)4Si, (CH2)3SiH, (CH2)2SiH2, and (CH2)SiH3 units of the oligomers were carried out as previously described.5 The integration data indicate that the relative values of X/Y/Z/L (Scheme 1) are about 2/8/20/11, which is similar to results previously reported.5 As can be seen from its 1H NMR spectrum (Figure 1b), this sample of 1 does not have the extraneous ethyl groups on Si that resulted from cleavage of the diethyl ether solvent in the earlier preparation.5 Elemental analysis also corresponds reasonably well to the CH4Si composition expected for the “average” “[SiH2CH2]n” repeat unit (Calc: C, 27.24; H, 9.08. Found: C, 25.74, H, 9.10.). For © 1997 American Chemical Society
3142 Rushkin et al.
Macromolecules, Vol. 30, No. 11, 1997 Table 1. Reagents Used and Degree of Substitution of the “[Si(R)2CH2]” Polymers oligomer
side chain
reagent used
degree of substitution (%)
2 3 4 5 6 7 8
ethyl allyl butyl hexyl octyl phenyl N,N-dimethylanaline
C2H5MgBr C3H5MgBr C4H9Li C6H13Li C8H17Li C6H5Li Me2NC6H4Li
80a 75a (70b-79c) 90a (74b-82c) 90a (70b-80c) 85a (67b-77c) 85a (71b-84c) 80a
a Determined by integration of the 1H NMR peaks. b Calculated from the elemental analysis assuming the average composition [Si(H)2-x(R)xCH2]n. c Calculated from the elemental analysis assuming the average composition [Si(OH)2-x(R)xCH2].
Scheme 2
Figure 1. (a, top) Quantitative 29Si NMR spectrum of HBPSE. (b, bottom) 1H NMR spectrum of HBPSE.
the sake of simplicity, from this point on we will use this “average” repeat unit “[SiH2CH2]n” to refer to the structure of HBPSE. Typically, the molecular weight of HBPSE (Mp), as estimated by GPC relative to polystyrene standards, was around 600 (corresponding to a degree of polymerization of about 13) and varied slightly from batch to batch. The polydispersity (PD) of HBPSE was found to be around 1.5-1.8. Since the hydrodynamic volume of HBPSE can differ significantly from that of the polystyrene standards used for GPC calibration, a sample of HBPSE with a GPC-estimated molecular weight (Mn) of 720 was examined by vapor pressure osmometry (VPO). The VPO-determined molecular weight was 740, showing that, in this case at least, GPC gives a quite reasonable estimate of the molecular weight. Derivatization of HBPSE. The convenience of the hydrosilation process lead us to first try this reaction as a means of modifying HBPSE.1 However we found that the Si-H bonds in this case are essentially inactive toward addition to olefins by using the usual Pt complex catalysts. For example, after treatment of 1 with 1-hexene and Karstedt’s catalyst for 24 h under reflux in an inert atmosphere, hydrosilation proceeded only to the extent of 3-5%, on the basis of the NMR spectra of the product. Since the most accessible Si atoms in HBPSE have two or even three hydrogen atoms attached to them, this is in line with the reported sluggish Pt-catalyzed hydrosilation of olefins by di- and trihydridosilanes. Catalyst poisoning has been suggested as a possible cause of this problem.9 It is also well-known that electropositive and bulky groups attached to the silicon retard the hydrosilation process.10 We therefore decided to convert HBPSE into a more reactive intermediate. We found that the Si-H bonds of 1 can be conveniently converted to Si-Br bonds by reaction with bromine in chlorinated solvents (Scheme 2).11 The 1H NMR spectrum12 of the brominated HBPSE shows numerous peaks that we attribute to the various
“[SiBr2CH2]” units in the area 0.5-2.7 ppm and a very small amount (
