Helical molecular chain conformation

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

As pointed out above with relation to the data at 87 °C, the Tic of the crystalline-amorphous interphase is appreciably longer than that of the amorphous phase, suggesting the retention of the helical molecular chain conformation in the interphase. We also note that a Tic of 65-70 s for the crystalline phase is significantly shorter than that for other crystalline polymers such as polyethylene and poly-(tetramethylene oxide), whose crystalline structure is comprised of planar zig-zag molecular-chain sequences. In the crystalline region composed of helical molecular chains, there may be a minor molecular motion in the TiC frame, with no influence on the crystalline molecular alignment that is detected by X-ray diffraction analyses. Such a relatively short TiC of the crystalline phase may be a character of the crystalline structure that is formed by helical molecular chain sequences. 

A perfect helical main chain conformation always leads to a rodlike or cylindrical external shape. But each monomeric unit in such a rod contributes a certain flexibility. So, the flexibility of the rod, as a whole, must increase with increasing degree of polymerization, even when the flexibility per monomeric unit remains constant. A macroscopic example of this would be the flexibility of steel wires of equal diameter but different lengths. Thus, even a perfect helix will adopt coil shape if the molecular mass is very high. Because of this, helically occurring macromolecules, and other stiff macromolecules, can often be well represented by what is known as the wormlike screw model for macromolecular chains at low molecular masses, the chains behave like a stiff rod, but for high molecular masses, the behavior is more coil-like. Examples are nucleic acids, many poly(a-amino acids), and highly tactic poly(a-olefins). 

Region I here the molecular chains partly assume helical (stiff) conformation from random coils where the persistence length gradually increases. 

The conformation of the chains of isotactic polymers in the crystalline state is generally helical and corresponds to a succession of nearly trans and gauche torsion angles, the exact values depending on the bulkiness of the side groups. Molecular mechanics calculations have been extensively used for the prediction of the chain conformation of polymers in the crystal.29... 

In this part, two series of 44 copolymers with coiled main-chain structures and 45 copolymers with stiff main-chain conformations were described. It was concluded that both optically inactive 42 and 43 adopt helical conformations with an equal proportion of P and M screw senses by means of UV and CD spectra as well as molecular mechanics calculations. A marked positive cooperative induction effect of the preferential screw sense in 44 and 45 copolymers was found. However, there is a marked difference in the helical cooperativity between 44 and 45, probably because of the differences in their global and local conformations. This difference can be related to the persistence of the helical conformation against defects allowing change of... 

Recently, a very interesting example of solvatochromism was reported by Fujiki and co-workers.206 Poly(methyl-3,3,3-trifluoropropylsilylene), 87, synthesized via Wurtz coupling, showed solvatochromism as a result of weak, non-covalent intramolecular Si- -F-G interactions which rendered the conformation of the polysilane uniquely controllable by solvent choice and molecular weight. UV, shown in Figure 18, photoluminescence, NMR, and viscosity studies on the polymer indicated a 73 helical rod-like conformation at room temperature in non-coordinating solvents (e.g., toluene and decane), since the intramolecular interaction resulted in constraining the chain in a rigid helix. 

At thermal equilibrium, the helical fraction and all other quantities characterizing the conformation of a helix-forming polypeptide are fluctuating from time to time about certain mean values which are uniquely determined by three basic parameters s, a, and N. The rates of these fluctuations depend on how fast helix units are created or disappear at various positions in the molecular chain. Recently, there has been great interest in estimating the mean relaxation times of these local helix-coil interconversion processes, and several methods have been proposed and tested. In what follows, we outline the theory underlying the dielectric method due to Schwarz (122, 123) as reformulated by Teramoto and Fujita (124). 

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

This review has summarize the applications of neutron inelastic scattering to the study of pol3uners. The technique has proven useful for measuring and characterizii low-frequency intramolecular and inter-molecular vibrations, particularly for three systems, such as polyethylene and the n-paraffins, for which theoretical calculations of phase-frequency relations are available. More calculations of this type, and extension of them to include the effects of departures of chain conformations from their ideal transplanar or helical configurations, are needed for an optimum application of the method. 

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