Stereoisomerism in vinyl polymers

FIGURE 3.5 Stereoisomers of polystyrene showing isotactic, syndiotactic, and atactic structures. 

Atactic A random arrangement of the X groups. Its lack of regularity has important 

Iso tactic The structure in which all the X groups are lined up on the same side of the backbone plane. 

Syndiotactic Alternating placement of the X group on either side of the plane. 

These three terms were coined by Dr. Giidio Natta, who shared the 1964 Nobel Chemistry Prize for his work in this area. Although atactic polymers are certainly most common, the arrangement and packing (and thus the properties) of stereoregular (syndiotactic and isotactic) polymers makes them important for certain applications. Methods to synthesize the stereoregular isomers will be discussed in Part 11 of this book. 

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Fig. 2. Stereoisomerism in vinyl polymers (a) isotactic (b) syndiotactic and (c) atactic.

Figure 1.1 shows the typical representation proposed to distinguish easily between the different types of stereoisomerism present in vinyl polymers. Viewing the main chains as lying down on a plane, there are (i) isotactic sequence of configurations, (ii) syndiotactic sequence of configurations, and (iii) no order present, atactic sequence of configurations. 

Stereoisomerism occurs in vinyl polymers when one of the carbon atoms of the monomer double bond carries two different substituents. It is formally similar to the optical isomerism of organic chemistry in which the presence of an asymmetric carbon atom produces two isomers which are not superimposable. Thus glyceraldehyde exists as two stereoisomers with configurations shown in 4-13. (The dotted lines denote bonds below and the wedge signifies bonds above the plane of the page.) Similarly, polymerization of a monomer with structure... 

A line of research that has aroused much interest in recent years is the study of head-to-head, tail-to-tail polymers (96-98). Their direct synthesis has little likelihood of being successffil as head-to-tail sequences usually predominate in vinyl polymerization. One possibility for their preparation is through the chemical modification of suitable preformed polymers. In the case of the head-to-head, tail-to-tail polypropylene, different stereoisomeric forms have been isolated, depending on the method of preparation. In the general scheme, the precursor is an unsaturated polymer obtained by polymerization of the disubsti-tuted butadiene (2,3-dimethylbutadiene or 2,4-hexadiene) then, by chemical or catalytic reduction, this polymer is converted into the desired polypropylene, whose stmcture can then be examined by NMR spectra. Head-to-head, tail-to-... 

The problem is further complicated for vinyl polymers with their problems of stereoisomerism. The first descriptions of the conformational state of isotactic polypropylene in solution go back 25 years (178, 179, 192, 193). Corradini, Allegra, and Ganis proposed a model, still essentially valid today, according to which macromolecules possess a local helical structure analogous to that observed in the crystalline state. The helix segments are rather short, only a few monomer units, after which an inversion of the helix sense occurs, with simultaneous alteration of its direction (Figure 15). As a whole this disordered con-... 

Nuclear magnetic resonance has been extensively used to investigate stereoisomerism in polymers such as polypropylene, polyacrylates, vinyl polymers. 

Optical Activity in Polymers Stereoisomerism in polymers is formally similar to the optical isomerism of organic chemistry. In a vinyl polymer with the general structure shown in (XIH) every other carbon atom in the chain, labeled C, is a site of steric isomerism, because it has four different substituents, namely, X, Y, and two sections of the main chain that differ in length (Rudin, 1982). 

Linear macromolecules having a constitutional repeating unit such as -CH2-CHX- (X 7 H) show two further stereoisomerisms, i.e., optical isomerism and tacticity. The stereoisomerism named tacticity has its origin in the different spatial arrangements of the substituents X. When we arrange the carbon atoms of the polymer main chain in a planar zigzag conformation in the paper plane, X is either above or below that plane ( Natta projection ). If the substituents X of the vinyl polymer are either all above or all below that plane (case I), the polymers are... 

A polymer in which at least one chain atom in a mer can exhibit stereoisomerism (e.g., -CH2C HX-), but has no preference for one particular configuration, e.g., atactic vinyl polymers (PVC or PS), atactic polypropylene, and PP. 

The definitions of polymer chirality are also closely connected with the clear description of stereoisomerism in macromolecules. The sites of isomerism in vinyl... 

In G.A.F. laboratories at Easton, Pa., our evaluation of German methods for polymerizing alkyl vinyl ethers to obtain pressure sensitive adhesives led to the discovery of stereoregulation. I first disclosed in 19 6 that differences in cationic polymerization conditions gave differences in properties in vinyl isobutyl and vinyl methyl ether polymers which could not be attributed to molecular weight but must result from different chemical structures, e.g., stereoisomerism . Although Staudinger had not predicted stereo-... 

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