Regiospecific polymerisation

One can consider the anionic polymerisation of PO as a regiospecific polymerisation, because, due to the preferential attack of the a-carbon oxiranic ring atom, the microstructure of the chain is predominantly head-to-tail (H-T) type [4, 69, 74] ... 

Polymers resulting from successions of (prevailingly) equal additions, either 1,2 [scheme (39)] or 2,1 [scheme (40)], irrespective of the choice of configuration, are said to be regioregular (from a regiospecific or regioselective polymerisation... 

Regiospecificity of Ziegler-Natta and related coordination catalysts for a-olefin polymerisation implies non-equivalence of the two insertion modes, primary and secondary [schemes (39) and (40) respectively], owing to electronic and/or steric factors. 

The syndiospecific polymerisation of propylene with soluble vanadium-based Ziegler Natta catalysts is not completely regiospecific [389 392], i.e. the monomer unit enchainment is not entirely head-to-tail. In addition to syndiotactic stereoblocks, the polymer also contains sterically irregular stereoblocks. The whole polymerisation can be thus described as a copolymerisation with four head-to-tail and tail-to-tail stages [2,379]. 

The non-covalent imprinting process is perhaps best considered in terms of the sequential nature of the individual steps involved, which are summarised in Figure 6.8. The key to MIP formation is a series of regiospecific non-covalent interactions between the template and functional monomers to form a pre-polymerisation complex in the liquid phase. 

The purpose of the functional monomer is twofold. Firstly, it undergoes regiospecific, weak, complementary, interaction with a particular moiety of the template molecule. Secondly, it contains a polymerisable unit. The choice of functional monomer should therefore be based upon the functionality of the molecule to be imprinted. 

Figure 6.16 Schematic representation of an example of pre-polymerisation complex showing a range of regiospecific solvation interactions. In bold is a hypothetical template molecule, functional monomers are 4-VPy and MAA whilst the cross-linker is EGMA.

The prospects for reducing cross-reactivity (or increasing specificity) will depend upon investigations which address the basic science of imprint formation. One possible answer, currently under investigation in these laboratories, involves a more biomimetic approach where multiple regiospecific interactions combine to yield cavities of greater specificity. Such co-operativity is, of course, dependent upon the the formation of the required complex in the liquid phase, followed by retention of the complex during the polymerisation step. 

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