Order-disorder transitions polysilylenes

Order-Disorder Transitions and Thermochromism of Polysilylenes in Solution... 

Order-disorder, or rod-to-coil , transitions in dilute solution have been reported for polydiacetylenes (2, 5-11), polysilylenes (12-15), and alkyl-substituted polythiophenes (16). The interpretation of the experimental observations has been the subject of considerable controversy with respect to whether the observations represent a single-polymer-molecule phenomenon or a many-chain aggregation or precipitation process (3-16). Our own experimental evidence (12, 13) and that of others (5-8, 10, 16) weigh heavily in favor of the single-chain interpretation. In our theoretical interpretation, we will assume that the order-disorder transitions observed in dilute pol-ysilylene solutions represent equilibrium, single-chain phenomena. 

In this chapter, the theory of conformation-dependent polymer-solvent interactions, which was developed in detail by Schweizer (20-22) for soluble TT-conjugated polymers, will be used to explain both qualitatively and quantitatively a large body of observations on the polysilylenes (23, 24). The same theory has been used recently to interpret qualitatively order-disorder phenomena and the electronic thermochromism of TT-conjugated-polymer solutions and films (25, 26). The study presented in this chapter represents part of an ongoing effort to understand in a unified fashion both the optical properties (27-30) and order-disorder transitions (20-24) of flexible, conjugated-polymer solutions. 

Order-Disorder Transitions. General Features, Experimental data are summarized in Table II, and representative thermochromic behaviors are shown in Figure 2. For the dialkyl-substituted polysilylenes the transition is very sharp, with a barely discernible coexistence region and an approximate isosbestic point. On the other hand, the asymmetrically substituted polymers, except poly(n-dodecylmethylsilylene), display very smooth behavior only in n-hexane solution and a broad but clearly discernible transition in dilute toluene solution. The transition width (ATc) in toluene solution was taken to be the interval between departure from the extrapolated, smooth, high-temperature behavior and the onset of peak absorption wavelength saturation at low temperature. The transition temperature (Tq) is defined arbitrarily as the midpoint of this region. 

Table II. Order-Disorder Transition Temperatures and Widths of High-Molecular-Weight Polysilylenes ...

Symmetrical Dialkyl-Substituted Polysilylenes Because of their extremely sharp order-disorder transitions, the nonpolar, symmetrical dialkyl-substituted polysilylenes are almost ideal systems with which to test the predictions discussed earlier. The predicted solvent dependence of Tc was tested by performing a series of experiments with high-molecular-weight poly(di-n-hexylsilylene) in dilute solution. Experimental results for six solvents are listed in Table II, and the theoretically defined solvation coupling constants and solvent parameters are collected in Table III. 

The predicted intrinsic width of the order-disorder transition of a mono-disperse, flnite-molecular-weight polymer solution was also tested. The average molecular weights of dialkyl-substituted polysilylenes are in the order of 6 X 10, which implies that N is 3000-5000 silicon atoms. With equation 9, the theory predicts that ATq/Tc is 0.004-0,006, which for Tc = -30 corresponds to an intrinsic width of roughly 1 or 2 C. This result is in good agreement with the experimental observations summarized in Table II. 

In this chapter we describe results of TOF measurements on aliphatic polysilylenes and poly(di-n-butylgermylene) carried out over a broad range of temperature encompassing the glass transition temperature Tg, and the phase I - phase II (order - disorder) transition caused by side chain ordering and melting, which is accompanied by a blue shift in the uv spectra of the polymers. 

The polysilylenes continue to be of interest because of the novel electronic and optical properties of these macromolecules. These properties stem from the presence of a o-delocalized -Si-Si- polymer backbone. The interplay between conformational states, side-chain order/disorder transitions, and electronic states creates interesting temperature and pressure dependent optical phenomena. These novel properties have stimulated interest in improved synthetic methods. Thus, the first three papers in this symposium focus on understanding and improving catalytic methods for generating silylene chains. These efforts are followed by Worsfold s research on improving the traditional Wurtz coupling method of synthesis. 

Finally, solid films of some polysilylenes exhibit thermochromism and undergo true thermodynamic order-disorder phase transitions at much higher temperatures 18, 19, 47, 48) than in solution (typically, Tq 40-80 °C). In the context of the theory, a larger refractive index of the neat solid compared with that of the dilute solution results in a higher predicted Tq 21). However, we do not believe that the observed high TqS in films can be explained solely by this effect. Previous explanations have been made exclusively in terms of side-chain crystallization 18, 19, 48). Packing effects should be more important in the solid state, but both intramolecular and intermolecular packing effects must be carefully considered. Indeed, the fact... 

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