Polymer linking method

The microgel method (C) is advantageous over the multifunctional initiation (A) and termination (B) methods in that the product polymers may have a large number of arm chains, often exceeding 50 or sometimes 100. Because the microgellation process is statistical, however, there is by definition a distribution in the arm number in a particular polymer sample. In these regards, the polymer-linking method (C) is complementary with the multifunctional initiation (A) and the multifunctional termination (B). 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

Scheme 2.4 Polymer support silylene linking method for hindered hydroxyl-bearing glycals.

LCB polymers can be formed by chemically linking preformed polymers (arm first or polymer first method) or by growing polymer chains from a multifunctional initiatior (core first method). In both cases living polymerization techniques are preferred because they provide better control over MW, MW distribution and the final branching architecture. However, highly selective coupling reactions e.g. with multifunctional isocyanates, or dicyclohexylcar-bodiimide (DCC) coupling, have also been successful. 

Another important application of the polymer reagent method is in the synthesis of cyclic peptides. In this approach, the amino-protected linear peptide (7(5) is converted into the corresponding polymeric active ester (17) by coupling it to a cross-linked hydroxyl function containing polymer (eg. 12) on deprotection of the amino group and subsequent neutralization, the cyclic peptide (18) is obtained in good yields 109 110> ... 

A number of multidimensional analyses have been developed that provide powerful methods for characterizing these polymers. Linking a liquid chromatogram to a pyrolysis gas chromatograph [l 9] can determine the breadth of the composition distribution, as the method fractionates the SAN copolymer before pyrolysis. This information is useful for determining the source of variation in SAN copolymer properties. Composition drift towards high acrylonitrile-containing fractions can lead to undesirable yellow color, and excessively broad composition drift can cause opacity and brittleness in the material due to phase separation... 

The third method is based on a polymer-linking reaction where the liner polymers are obtained by the living radical polymerization with divinyl compounds. This can afford star polymers with a relatively large number of arms, up to several hundred per molecule, while the number of arms by definition involves a statistical distribution in a single sample. 

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