Polystyrene conformational transition

It is obvious, that the conformation of a section of polystyrene which is required to make two neighboring phenyl residues lie parallel to one another (at a distance of about 2.5A°) would be characterized by a prohibitive potential energy and such a conformation would not be expected to be significant in the ground state. This conclusion is also supported by the fact that polystyrene exhibits the normal UV spectrum of alkylbenzene, while benzene rings constrained to lie parallel to one another at such a short separation have characteristically distorted absorption spectra (14). We must, therefore, conclude that excimers are formed by a conformational transition during the lifetime of the excited state. This transition is assisted by the large exother-micity of excimer formation which is characterized by AH values of -5.1 and -3.9 kcal/mole for benzene and toluene, and by -5.8 and -6.9 kcal/mole for naphthalene and the methylnaphthalenes (11). 

Luminescence behavior and electron transfer of various transition metal complexes with 4,4 -bipyridyl, EDTA or 1,2-diaminoethane in the interaction with polyelectrol5des have been studied in solution in detail [31,32]. The systems were prepared by mixing solutions of the polymers (in excess) and the metal complexes. The measurements showed a monomolecular distribution of the metal complexes and allowed the study of the pH-dependent conformational transition of the polymer chains as shown in Fig. 8-2. Polystyrene was doped with small amounts of porphyrins 2 (M = 2H R = -CsHs and —CeFs) by casting... 

The viscoelastic nature of the srdvent may limit the usefulness of the BD technique for the study polymer chains with very fast dynamics (e.g., polybutadiraie, polyisoprene, and PEO)l At this time, it is not clear to what extent various features of the dynamics in sudi a BD simulation will be unrealistic as a result of the assumption of a viscous continuum. Careful comparisons between BD simulations atul MD simulations will be required to address this question. Studies such as the one by Yun-Yu et al. [49] (see Sect 4.3) could address this point if longer trajectories were run and conformational transition rates were compared. It seems likely that the BD technique will be adequate to simulate the behavior of polymer chains with relatively slow dynamics (e.g., polystyrene). 

Several papers are concerned with motions of polymers containing backbone sulphur atoms. - > In poly(phenyl thiirane), it is found that backbone correlation times are an order of magnitude shorter than in polystyrene. An unusual feature > of relaxation in poly(alkene sulphones) is that C Ti s are independent of molecular weight whereas dielectric relaxation times are not. This has been rationalized in terms of specific conformational transitions which re-orient C—H bonds but not the sulphone dipole. 

Literature data concerning conformational transition rates of polystyrene in dilute solution... 

It must be noted that the end-to-end distance of the brushes is also affected by adsorbing them flatly on a hard wall. In monolayers (d = 2) the molecules are constrained to two dimensions which favors extension and parallel aUgnment [ 163 ]. This is particularly pronounced if the side chains get tightly adsorbed and will be discussed in detail below. Yet even in the case of a weak interaction with a substrate, the chains are extended. In contrast, in thick films the individual chain can adopt a less straight conformation by transition between layers in the z-direction. Figure 24 clearly demonstrates the difference in the ordering of cylindrical brushes of polystyrene depending on the film thickness [78]. 

The two examples of adsorbed side chain substituted macromolecules, i.e., the poly(n-butyl acrylate) brush and the tris(p-undecyloxybenzyloxo) benzoate jacketed polystyrene, demonstrate two rather complementary aspects of the interaction of such molecules with a planar surface. In the first case the two-dimension to three-dimension transition results in a cooperative collapse of an extended coil conformation to a globule. The second case shows a rather high degree ordering with a distinct orientation of the backbone in the substrate plane. Combination of both effects and partial desorption can lead to a repta-tion-hke directed motion as depicted schematically in Fig. 36. 

GPC fractionation and ozonisis of the products, the yield of cyclic polymer was estimated to be ca. 90%. The morphological transition of the cyclic block polymer depends on composition in essentially the same manner as that of the linear triblock copolymers, whereas the domain spacing of polystyrene-block-polyisoprene cyclic block copolymers were all smaller than those of the corresponding SIS linear triblock copolymers, which is attributed to looped chain conformation. 

Hydrodynamic radii of poly(benzyl ether) dendrimers are shown in Fig. 5. Data for monodendrons with a hydroxyl focal group and tridendrons fall on the same curve. The value of the exponent v in Eq. (4) is 0.46 of low MW. At high MW it is 0.26 [48]. Data on low MW linear polystyrene in benzene [74] have been included in Fig. 5 for comparison. They highlight the little difference in the actual values of the hydrodynamic radii of linear polystyrene and low MW poly(benzyl ether) dendrimers. Deviations are observed only when MW>5xl03. Furthermore, the MW dependence of the radii of polystyrene and poly(benzyl ether) dendrimers are the same at low MW. This indicates that it remains impossible to draw major conclusions about the conformation of the low MW dendrimers from their global properties. The low values of the hydrodynamic radii of the high MW dendrimers, on the other hand, attest to their compact conformation. A similar transition to more compact dendrimers has recently been shown in a direct comparison of linear and dendritic poly(benzyl ethers) [75]. 

Monocyclopentadienyl complexes of titaninm (Cp TtXs) perform poorly as catalysts for ethylene or propylene polymerization, bnt in the presence of MAO, they polymerize styrene to stereo- and regioregnlar syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a Ti complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

The standard molecular structural parameters that one would like to control in block copolymer structures, especially in the context of polymeric nanostructures, are the relative size and nature of the blocks. The relative size implies the length of the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more elaborate description that includes its solubility characteristics, glass transition temperature (Tg), relative chain stiffness, etc. Using standard living polymerization methods, the size of the blocks is readily controlled by the ratio of the monomer concentration to that of the initiator. The relative sizes of the blocks can thus be easily fine-tuned very precisely to date the best control of these parameters in block copolymers is achieved using anionic polymerization. The nature of each block, on the other hand, is controlled by the selection of the monomer for instance, styrene would provide a relatively stiff (hard) block while isoprene would provide a soft one. This is a consequence of the very low Tg of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain. 

---