No abstract

No abstract

Poly(methyI methacrylate) is an example of a disubstituted vinyl chain in which the substituents CO-OCH3 and CH3 differ in size and shape; the former is noncylindrical, and the latter resembles the CH2 skeletal group in its steric interactions. Owing to the planarity of the ester group, severe steric overlaps involving one or the other of its O atoms occur in the g conformation of a skeletal bond irrespective of the rotational states of neighboring bonds. Conformational energy calculations indicate the g state to be at least 7 kcal mol-1 higher than the t and the g states. With the exclusion of the former conformation, all interactions of long range are eliminated, and the statistical weight matrices U' and U" for the respective skeletal bond pairs reduce to 2 X 2 order. The contour energy surface computed as a function of bond rotations <£,• and

Thermoreversible organogels were prepared from carbamates with alkyl side chains of different lengths. Gelation was possible only up to an alkyl side chain length of 12 carbons, beyond which crystallization occurs, due to the dominant van der Waals interaction between the alkyl chains. This is in contrast to other alkane-based organogels, in which gelating efficiency increased with the length of the alkane chain (see Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237). The critical concentration for gelation decreases drastically with an increase in the side chain length. Xerogels of these show birefringent fibers with uniform cross section and unlimited growth in one direction. The extent of this unlimited growth is affected by the length of the alkyl side chain in the carbamate, which finally ceases the gel formation ability of the carbamate. Cryogenic scanning electron microscopy images of the gels are similar to those of xerogels. From X-ray diffraction of the fibers, we propose that the growth direction is along the plane of hydrogen bonds between the carbamate molecules. The thickness of the fibers depends on the length of the alkyl side chain. Morphological differences are seen between gels prepared by slow cooling and quenching of the solution. Thus, the morphology of the fibrous xerogels of the carbamates can be tailored for specific applications, by the choice of the alkyl side chain length and the rate of cooling the solution.

The primary chiral elements of a vinyl polymer chain CHS(CRR'CH2)ZH, in which R' ^R, are the skeletal bonds rather than the substituted carbons as conventional terminology implies. The conformation in relation to any specified stereochemical configuration may be treated with full generality in terms of the chiralities of skeletal bonds. Statistical weight matrices are formulated in a parallel manner for both mono-(R' = H) and disubs tituted chains, with emphasis on the latter. As an indirect consequence of the copious second-order interactions (involving groups separated by four bonds) when the sizes of both R and R' are commensurate with or larger than CH3, interactions of higher order (involving groups separated by as many as eight bonds) are not averted by interactions of second order, as is the case when R' = H. Significant interactions of longer range in disubstituted chains may introduce interdependence between consecutive pairs of skeletal bonds, the relevant pairs being the two that adjoin each substituted carbon. The complications thus arising are easily managed by resort to statistical weight matrices (of orders 7 X 7 for three states per bond) that relate the combined state of one such bond pair to that of the neighboring pair. Previous methods are adapted to the formulation of generator matrices suitable for calculation of configuration-dependent properties on the basis of these pair-pair statistical weight matrices. Simplifications for symmetric chains are introduced.The spatial configurations of monosubstituted vinyl polymer chains, CH3[CHRCH2]IH, are greatly restricted by steric interactions if, as is usual, the substituent R is comparable to or greater than CH3 in size.12 The predominant interactions are those involving pairs of groups from the set CH2, CH, and R that are separated by four bonds. The distance between the centers of two groups thus related depends on the angles of rotation about the central pair of the sequence of four bonds; see Figure 1 with R' = H. Hence, following Birshtein and Ptitsyn,s we refer to these as interactions of second order.1 2 The mutual orientation of the two groups depends, of course, on the rotations about the two outer, or pendant, bonds of the set of four. In particular, for groups like CH2 and CH3 the mutual disposition of the peripheral hydrogen atoms, which are the atoms most directly involved in the interaction, depends on these rotations. To an approximation that is often adequate, groups such as these may be treated as spherical domains. Rotations about the outer two bonds of the sequence (1) P.