| A relatively benign method has been developed to attach 1-pyrenyl chromophores covalently to interior sites of polyethylene (PE) films. Because covalent attachment can restrict the orientations of chromophores and impede their translocations during film stretching, attached probes have been employed to investigate morphological changes (induced by film stretching) and temperature-dependent relaxation processes of PE films, as well as to determine the origin of the orientation and orientational distribution of guest molecules before and after film stretching. From the results, it is concluded that: (1) the optimization of non-covalent interactions between aromatic molecules and surrounding polymeric chains through adjustment of their relative spatial orientations (i.e., random in the amorphous regions and aligned on the crystal surfaces, especially (110) faces, of the interfacial regions) is responsible for the site orientations and bulk orientational distributions of attached or doped species inside PE; (2) changes in the rates of polymeric chain motions and the onsets of specific types of chain motions induced by increasing temperature can be sensed with varying degrees of sensitivity by covalently-attached aromatic probes.; Remarkable regio- and stereo-selectivities of prochiral radical-pair combinations have been demonstrated by the photochemistries of 1-naphthyl (R)-2-phenylpropanoate and 1-naphthyl (R)-1-phenylethyl ether in PE and poly(vinyl acetate) films and n-alkanes of varying viscosities. The results indicate that the radical-pair intermediates are able to maintain at least some orientation memory of their chiral precursors during their lifetimes. The competition among several basic motional processes by radicals of a radical-pair, including translational diffusion to escape from a cage and translations and tumbling motions within a cage, determines the regio- and stereo-chemistries of the products. The relative orientation of the initially formed radical-pair and other factors, including crystallinity and stretching of polymer films, solvent polarity and viscosity of the n-alkanes, and temperature, influence these motions. Especially in PE, very high microviscosities and "templating" effects of reaction cavities can attenuate and direct radical motions. The amorphous and interfacial sites of partially crystalline PE can affect combination of a prochiral radical-pair in subtly different ways. |
