Significance of Stereoregularity

The occurrence of stereoisomerism in polymers plays a major role in their practical utilization. There are very significant differences in the properties of unordered and ordered polymers as well as in the properties of ordered polymers of different types (cis versus trans, isotactic versus syndiotactic). The ordered polymers are dramactically different from the corresponding unordered structures in morphological and physical properties. 

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Non-crystalline polymers are those which include high levels of irregularity within their structure. Typical sources of such irregularity are copolymerisation with significant amounts of at least two co-monomers and also complete absence of stereoregularity, i.e. atactic polymers. 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

Stereospecific polymerization has particular significance for the preparation of stereoregular polymeric dienes. In the radical polymerization of butadiene or isoprene the molecular chains always consist of varying proportions of adjacent cis- and trans-1,4-units as well as 1,2- and 3,4- linked units, depending on the polymerization conditions but it is now possible, using particular ionic initiation systems to make a synthetic natural rubber that contains more than 90% cfs-l,4-isoprene repeating units (see Example 3-21). 

P. Corradini, The Role of the Discovery and Investigation of Stereoregular Polymers in Macromolecular Chemistry. In S. Carra, F. Parisi, I. Pasquon, P. Pino, eds., Giulio Natta Present significance of his scientific contribution. Editrice di Chimica, Milan, 182, pp. 126-146, p.136. 

Currently this technology is of minor commercial significance, but stereoregular forms of numerous polyacrylates have been prepared and characterized These include poly(/-butyl acrylate) (138—141), poly(isopropyl acrylate) (142), and poly(isobutyl acrylate) (143,144). Carefully controlled reaction conditions are usually required to obtain polymers with some measurable degree of crystallinity. In nonpolar solvents the anionic polymerization of acrylates generally yields isotactic polymer, whereas in polar solvents syndiotactie polymerization is favored. The physical and chemical properties of the various forms are often quite different. A general review covers these and other aspects of the anionic polymerization of acrylates (145). 

Many polymers are now manufactured on a, commercial scale using Ziegler-Natta catalysts. Indeed, stereoregular (isotactic) polypropylene of high molecular weight, which cannot be made by free-radical or ionic polymerization, has already achieved the status of a commodity plastic. The scientific and practical significance of Ziegler and Natta s work earned them the joint award of the Nobel Prize in Chemistry in 1963. 

The major end-use of iodine is in catalysis (e.g., the Monsanto process for producing acetic acid). Titanium tetraiodide and aluminum iodide are also significant in the dehydrogenation of butane and butene to butadiene, and in the preparation of stereoregular polymers. The second major end-use of iodine is as a stabilizer in the manufacture of nylon, for converting resins, tall oil and other wood products to more stable forms, while the third major use is as additives for animal and human food (iodization of salt and mineral mixtures). 

The essential requirement for crystallinity in polymers is some sort of stereoregularity. This is not to say that the entire collection of macromolecules within the sample needs to be all isotactic or syndiotactic however, it is essential that regions along the backbone of a significant number of the macromolecules do have such regularity. 

Very recently using the chromatographic technique on optically active supports elaborated by Italian school, Marchetti and Chiellini in collaboration with us (30) performed the separation of different samples of stereoregular polymethylthiiranes and obtained significant separations in two fractions of opposite sign. 

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