Tacticity in polymers

Tacticity in Polymers As explained above, the C atoms in (XHf) are pseu-dochiral and hence do not give rise fo optical activity the two mirror image configurations remain distinguishable, however. The different possible configurations or spatial arrangements that occur as a consequence are called the tacticity of the polymer (Rudin, 1982 Cowie, 1991). 

The usual way to picture the configurational nature of a vinyl polymer is to consider the polymer backbone stretched out so that the bonds between the main chain carbons form a planar zigzag pattern in the plane of the paper and the X and Y substituents on successive pseudoasymmetric or pseudochiral carbons lie either above or below the plane of the backbone, as shown in Fig. 2.11. 

If the substituents, say X, are then all above or all below the plane [Fig. 2.11(a)], the configurations of successive pseudoasymmetric carbons are the same the polymer is termed isotactic and designated as dddd... or lllL... If, however, a given substituent appears alternately above and below the reference plane in the planar zigzag conformation [Fig. 2.11 (b)], the polymer is termed syndiotactic and 

It is important to note at this point that completely tactic and completely atactic polymers represent extremes of stereoisomerism and polymers often possess intermediate degrees of tacticity. The most powerful tool for measuring, the extent of stereoregularity in polymers is nuclear magnetic resonance (NMR) spectroscopy (see below). 

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In what follows we first review the cis content of the polymers formed over a range of catalyst/monomer systems, then consider the cis/trans double bond distribution, next the HT bias that sometimes occurs with asymmetric monomers, and finally we collect and discuss the evidence on tacticity in polymers formed from various monomers. 

In addition to identihcation of unknowns, NMR can be used for conformational and stereochemical analyses. This includes the determination of tacticity in polymers, that is, whether the side chains are arranged regularly (isotactic and syndiotactic) or randomly (atatic) along the polymer backbone. Fundamental studies of bond distances from dipolar coupling and molecular motion from relaxation time measurements are used by physical chemists and physical organic chemists. In biology and biochemistry, the use... 

Schrock, R. R. Lee, J.-K. O Dell, R. Oskam, J. H. Exploring factors that determine cis/trans structure and tacticity in polymers prepared by ring-opening metathesis polymerization with initiators of the type syn- and anfi-Mo(NAr)(CHCMe2Ph)(OR)2. Observation of a temperature-dependent cis/trans ratio. Macromolecules 1995,28,5933-5940. 

During the period covered by this article a number of books and review articles have been published. Some of these are fairly general. Others deal with one polymer, for example polyisoprene, or one aspect of the subject such as the effects of tacticity in polymer reactions or pKjlymer modifications with polymerizable monomers. ... 

In this brief review an attempt has been made to indicate how molecular weight and chemical structure can influence the relaxation properties of polymer molecules in solution. Other important effects which can be considered are those of tacticity in polymers such as a polyiiiHe styrene ( ZZ ) and poly methyl methacrylate 21 ) and solvent effects in segmental relaxation. 

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