Tacticity of polymer

The application of NMR spectroscopy to tacticity determination of synthetic polymers was pioneered by Bovey and Tiers.9 NMR spectroscopy is the most used method and often the only technique available for directly assessing tacticity of polymer chains. "2 7 8 0JI The chemical shift of a given nucleus in or attached to the chain may be sensitive to the configuration of centers three or more monomer units removed. Other forms of spectroscopy (e.g. TR spectroscopy l2 lJ) are useful with some polymers and various physical properties (e.g. the Kerr effect14) may also be correlated with tacticity. 

The chain structure (length, entanglement, branching, tacticity,...) of polymers is discussed further in the next chapter (Chapter 4) by Beaucage and Kulkarni. 

The structural regularity in such polymers gives rise to optical (D-L isomerism) and tacticity of polymers and geometrical cross-trans isomerism. 

Optical Isomerism of (D-L-Isomerism) and Tacticity of Polymers Optical isomerism has its origin in the way different substituents occupy positions on an asymmetric carbon atom in a polymer molecule. For example, polyethylene molecule has fully saturated carbon atoms as shown in the following chemical formula ... 

Note 1 Terms referring to the tacticity of polymers (tactic, ditactic, tritactic, isotactic, cistactic, etc.) can also be applied with similar significance to chains, sequences, blocks, etc. 

Another promising approach to the study of microporosity of zeolites involves the measurement of the isotropic 13C NMR chemical shift which, as has been shown in the studies of the tacticity of polymers, is highly sensitive to the environment of the nucleus. In the first study of this kind, Boxhoorn et al. (329) observed that the C-3 carbon resonance from the tetrapropylam-monium cation enclathrated in the framework of zeolite ZSM-5 in the course of synthesis is split into two components of equal intensity. The reason for this is that the cation is located at the cross-section of the two nonequivalent... 

Figure 5.6. The tacticity of polymer chains. Illustrated are (a) isotactic, (b) syndiotactic, and (c) atactic polymers.

For polymers produced by radical polymerization, while one of these structures may predominate, the idealized structures do not occur. It is necessary to define parameters to more precisely characterize the tacticity of polymer chains. 

Table 1. cis—traws Contents and Tacticities of Polymers Prepared by ROMP Using Well-Defined Group VI... 

Since the insertion of alkenes into M-C bonds proceeds via a four-center transition state, some requirements have to be accomplished namely (1) the alkene and the hydrocarbyl group have to take cis coordination sites (2) the double bond and the M-C bond have to become coplanar and (3) the 1,2-addition is cis, and this controls the relative stereochemistry at both carbons in case of prochiral alkenes (Scheme 6.33). The regio and stereoselectivity of this process controls the tacticity of polymers. 

The empirical quantity Yc is defined as the value of Yl (surface tension of the wetted liquid) at the intersection point of the straight line cos 0 against Yl with the horizontal at cos 0. This extrapolation corresponds to complete surface wetting made possible by the liquid. In general, the Yc value depends on the length, composition, branching and terminal groups of perfluoroalkyl side chains, and the crystallinity and tacticity of polymers (Pittman, 1972). 

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