Tacticities of Selected Homopolymers

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

With 1,3-diene based polymers, greater scope for structural variation is introduced because there arc two double bonds to attack and the propagating species is a delocalized radical with several modes of addi tion possible (see 4.3.2). 

For mono- and 1,1-disubstituted monomers, steric, polar, resonance, and bond-strength terms (see Section 2.3) usually combine to favor a preponderance of tail addition Le. an almost completely isoregic structure. Fiowever, the occurrence of 

In view of the potential problems associated with discriminating between the various types of head-to-head linkages, it is perhaps curious that, while much effort has been put into finding hcad-to-hcad linkages, relatively little attention has been paid to applying spectroscopic methods to detect tail-lo-lail linkages where no such difficulty arises. 

Even allowing for the above-mentioned complication, the number of head-to-head linkages is unlikely to equate exactly with the number of tail-to-tail linkages. The radicals formed by tail addition (T ) and those formed by head addition (H ) arc likely to have different reactivities. 

Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

Most monomers have an asymmetric substitution pattern and the two ends of the double bond are distinct. For mono- and 1,1-disubstitiited monomers (Section 

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