Sequence-controlled polymers cationic polymerization

In addition to the repeat unit sequence, another area of current interest in polymer structural control (Fig. 1) may be the spatial or three-dimensional shapes of macromolecules. In fact, the recent development of star [181-184] and graft [185] polymers, as well as starburst dendrimers [126], arborols [186,187], and related multibranched or multiarmed polymers of unique and controlled topology, has been eliciting active interest among polymer scientists. In this section, let us consider the following macromolecules of unique topology for which living cationic polymerizations offers convenient synthetic methods that differ from the stepwise syntheses (polycondensation and polyaddition) [126,186,187]. 

When a difunctional trityl ion salt was used to initiate the living cationic polymerization of cyclopentadiene, block copolymers were formed through charge neutralization with living anionic ct-MS. By varying the functionality of the a-MS polymers, controlled block sequences were generated in the copolymers. 

Figure 3.1 Synthesis of sequence-controlled pol3uners by living cationic polymerization. (A) Copolymers containing acid-degradable acetal units. (B) Block copolymers with an acid degradable segment. (C) and (D) Synthesis of star-shaped polymers with acid-degradable acetal units in the core.

Apparently electrostatic association of cationic monomers 1 with polyanions does not provide strong enough complexation to control tacticity or other aspects of microstructure in the daughter polymer. This is likely due to the solvation of template, monomers and growing daughter chain by the reaction medium. Conditions that organize solution phase monomers at an ordered, nonsolvated template surface would be more likely to transfer tacticity, or chirality, or to control monomer sequence during polymerization. 

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