Configurations of Vinyl Polymers

The addition of a vinyl monomer to a free radical during the polymerization process (propagation phase) can take place In either of two ways, as shown in Fig. 3.22. 

The CHX group is considered the head of the monomer and the CH2 group is the tail. If only reaction (1) occurs, the resulting polymer Is said to have a head-to-tail configuration, where the X group is attached on every other carhon in the main chain, as shown in Fig. 3.23. 

If both reactions (1) and (2) take place, both head-to-head and tail-to-tail configurations occur randomly in the molecule, along with head-to-tail configurations, as shown in Fig. 3.24. 

Flgure 3.24 Vinyl polymer with irregular configuration (sometimes head-to-head, sometimes head-to-tail) 

If only reaction (2) occurs, the molecule would have a head-to-head/tail-to-tail configuration. However, this generally does not occur. 

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In the past decade, the two-dimensional NMR method has been progressively developed(4,5). This method has been successfully used for the assignmental problem. In this paper, we would like to show the usefulness of the two-dimensional NMR method for characterizing synthetic polymers. We focus our attention to the characterization of tlie stereo-configuration of vinyl polymer, in particular, poly(vinyl alcohol)(PVA)(6,7). 

F. A. Bovey, The sterochemical configuration of vinyl polymers and its observation by nuclear magnetic resonance, Acc. Chem. Res. 1, 175 (1968). 

It is well known that the mechanical and physical properties of vinyl polymers are dependent upon their stereochemical configuration. It is critical, therefore, that the stereoregularity of poly(TBTM/MMA) be determined accurately and conveniently if the field performance of the material is to be predicted with any certainty. The effectiveness of organometallic polymers as an anti-... 

When the configurations at the centers are more or less random, the polymer is not stereoregular and is said to be atactic. Polymerizations which yield tactic polymers are called stereospecific. Some of the more important stereospecilic polymerizations of vinyl polymers are described briefly in Chapter 9. 

It was Staudinger (19b) who first recognized the concept of stereoregularity of vinyl polymers. PVC may be represented as a chain in which the carbon atoms lie in the plane of the slide, and atoms shown with a hatched line lie below the plane and those with the bold line lie above the plane. Three configurations can thus result ... 

Depending on the type of catalyst and other polymerization conditions, the molecular structure of PP homopolymer can consist of any of the three different types of stereochemical configurations for vinyl polymers isotactic, syndiotactic, and atactic. 

Fig. 4.2 Imagined planar zigzag conformations of vinyl polymers (a) the carbon backbone, (b) a regular isotactic configuration, (c) a regular syndiotactic configuration and (d) a random atactic configuration. The planar zigzag conformation might not be possible for the real chain because of steric hindrance. ( Cambridge University Press 1989.)...

As Randall emphasizes in his book, there are close similarities between the determination of vinyl polymer tacticity and of copolymer sequence structure. It is convenient therefore to use the n-ad notation (dyad, triad, tetrad, etc.) originally applied to the former to describe the latter also. When there is any possibility of ambiguity, the term compositional -ad is used to refer to sequences of n units differing in chemical structure in a copolymer (AA, AB, BB, etc.), and the term configurational -ad is used to refer to stereochemically distinct sequences, following Johnsen et al. ... 

Today, I would like to discuss some recent work in our laboratory dealing with the configuration and conformation of vinyl polymers. Of the several vinyl polymers that might be discussed, I have chosen two polypropylene and polyvinyl chloride. These polymers are not only of major technological importance, but they also present quite challenging problems of analysis and interpretation. It may be said at the outset that use of the 220 MHz superconducting spectrometer and of computer simulation of the spectra are essential in order to be able to draw unambiguous conclusions. 

Distribution of the monomer units in the polymer is dictated by the reactivity ratios of the two monomers. In emulsion polymerization, which is the only commercially significant process, reactivity ratios have been reported (4). IfMj = butadiene andM2 = acrylonitrile, then = 0.28, and r2 =0.02 at 5°C. At 50°C, Tj = 0.42 and = 0.04. As would be expected for a combination where = near zero, this monomer pair has a strong tendency toward alternation. The degree of alternation of the two monomers increases as the composition of the polymer approaches the 50/50 molar ratio that alternation dictates (5,6). Another complicating factor in defining chemical stmcture is the fact that butadiene can enter the polymer chains in the cis (1), trans (2), or vinyl(l,2) (3) configuration ... 

Preferential addition to one end or the other of a vinyl (CH2=CHX) or substituted vinyl (CH2=CXY) monomer seems to be the rule to which exceptions are rare. This generalization appears to apply to ionic as well as to free radical polymerizations. The polymers of mono-unsaturated compounds consequently are characterized by a high degree of head-to-tail regularity in the arrangement of successive units. Little is known concerning the sequence of d and I configurations of the asym-... 

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