2854 Macromolecules 1992,25, 2854-2859 Characterization of the Structures and Properties zyx of Poly(dimethylsily1ene-co-di-n- hexylsilylene) Frederic C. Schilling,* Andrew J. Lovinger, Don D. Davis, and Frank A. Bovey AT&T Bell Laboratories, 600 Mountain Avenue, Murray zyxwv Hill, New Jersey zyx 07974 John M. Zeigler Silchemy, Inc., Albuquerque, New Mexico 87112 Received September 18,1991; Revised Manuscript Received December 30, 1991 ABSTRACT: In this report we describe the structures and properties of poly(dimethylsily1ene-co-di-n- hexylsilylene) (PM-co-HS), as determined by solution- and solid-state NMR, X-ray diffraction, DSC, and UV spectroscopy. A comparison is made between the structures and properties of the copolymer and those of the homopolymers, poly(dimethylsily1ene) (PDMS) and poly(di-n-hexylsilylene) (PDHS), formed from each of the comonomers. While the two homopolymers adopt the same chain conformation in the solid state, they differ significantly in their absorption characteristics and in the nature of their solid-state transitions. At room temperature PM-co-HS is found to be mostly disordered with a small amount of a well-ordered crystalline phase in which the silicon backbone adopts an all-trans conformational arrangement zyx as is observed for the PDHS homopolymer. The copolymer does not contain any ordered region similar to that of the PDMS homopolymer. Upon cooling of PM-co-HS, there is a slight increase in the PDHS-like structure, and between -10 and -20 "C the conformationally disordered phase partly crystallizes into a trans-like structure with a much larger intersilicon-backbone spacing than either PDMS or PDHS. The absorption characteristics of the copolymer and PDHS are similar, and, upon heating above 42 zyxwvu "C, the copolymer exhibits the same solid-state transition observed in PDHS. Introduction Recent papers have described the structure and prop- erties of many of the symmetrically substituted poly- (di-n-alkylsilylenes).'-'O Of particular interest in these materials has been the relationship between the silicon bond conformation and the electronic properties of the polymers. In addition, several of these homopolymers exhibit strong thermochromicl~~ and piezochromiclOJ1 behavior. The electronic and photochemical properties of these polymers are summarized in a recent paper.12 The synthesis and solution characterization of several copolymer systems containing asymmetrically substituted monomers have been but only recently have copolymers formed from two symmetrically substituted monomers been e ~ a m i n e d . ~ l - ~ ~ Since the solid-state structures and properties of the symmetrically substituted polysilylene homopolymers can differ significantlydespite strong similarities in molecular structure, the correspond- ing characteristics of their copolymers cannot be predicted a priori. In this paper we report the characterization of poly(dimethylsily1ene-co-di-n-hexylsilylene) (PM-co-HS) by solution- and solid-state NMR, X-ray diffraction, DSC, and UV spectroscopy. This copolymer is of particular interest in that while both of the comonomers adopt an all-trans silicon bond conformation in their respective ho- mopolymers,3.4.9the electronic properties of the homopoly- mers, as reflected in their UV absorption characteristics, differ significantly.5t9 Additional reasons for our interest are that poly(dimethylsily1ene) (PDMS) is the lowest ho- mologue of this family and its methyl side chains have no conformational degrees of freedom to influence the structure of the Si backbone; this is in contrast to poly- (di-n-hexylsilylene) (PDHS), where the long side chains have been shown to play a dominant r ~ l e . ~ ? ~ In this discussion we will contrast the chain conformation and the absorption characteristics of PM-co-HS with the structure and properties of the PDMS and PDHS ho- mopolymers. 0024-929719212225-2854$03.00/0 Experimental Section Synthesis of PM-ceHS. To an oven-dried, Nz-purged three- necked flask equipped with a gas inlet, condenser, magnetic stirrer, and inlet from a syringe pump was added 20 g (74.3 mmol) of freshly fractionated di-n-hexyldichlorosilane and 9.59 g (74.3 mmol) of dimethyldichlorosilane (purified by fractionation from a small amount of diethyl ether to remove trace quantities of methyltrichlorosilane). Dry toluene (90 mL) and dry heptanes (10 mL) were then added, and the resulting solution was heated to brisk reflux. A mineral spirits dispersion of Na (18.78 g of 40% by weight dispersion, 327 mg-atom) was added via a syringe pump at a rate of 200 mequiv/min. Upon completion of the Na addition, the resulting mixture was refluxed for an additional 90 min and then allowed to cool to room temperature. The reaction mixture was then quenched by addition of methanol followed by a large excess of saturated aqueous NaHC03 solution. After centrifugation to aid in separation of the organic phase from the aqueous phase, the layers were separated and the organic phase was filtered through diatomaceous earth to remove insoluble polymer. The solvent was removed from the resulting clear solution and 10 volumes of ethyl acetate added to the viscous residue to precipitate the crude product. This was redissolved in toluene and reprecipitated with ethyl acetate and then re- dissolved again in tetrahydrofuran and reprecipitated again with methanol, with removal by filtration each time of small amounts of polymer which could not be induced to redissolve. Finally, the polymer was redissolved in toluene and 15% by volume of acetone added to fractionate the high molecular weight material from the lower molecular weight polymer and other contaminants. After decantation of the solvent, the precipitated polymer was redissolved again in tetrahydrofuran and precipitated again with methanol to give, after drying, 0.73 g (3.8%) of the pure, white, flocculent title polymer having a monomodal molecular weight distribution of M, = 813 000 (from size-exclusion chromatog- raphy standardized with polystyrene). Methods. NMR data were recorded on a Varian Unity-400 spectrometer operating a t carbon and silicon frequencies of 100.58 and 79.46 MHz, respectively. Solution samples were prepared at a concentration of 5% in toluene-ds, and spectra were referenced to internal hexamethyldisiloxane (HMDS). Between 500 and 2000 scans were recorded for each sample. In order to obtain quantitative data, the spin-lattice relaxation times for 0 1992 American Chemical Society