Multifunctional termination

In the second case, first the arms are synthesized separately and then linked together using either a well-defined multifunctional terminator or a difunctional monomer leading to a cross-linked core. The free end of the branches may contain functional groups by using a functional initiator for the preparation of the arms. 

Coupling of living polymers with a multifunctional terminator... 

In the multifunctional initiation (A) and the multifunctional termination (B), living polymers grow outward from the initiator core and inward into the terminator core, respectively, but both processes lead to similar polymers. If they operate properly, these methods give multiarmed polymers that carry arms in a precisely controlled or predetermined number per molecule (= the functionality number of the initiator or terminator). 

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). 

When compared with the multifunctional initiators, the corresponding terminators are less available in cationic polymerization [202]. The situation is in sharp contrast to anionic living polymerization, where a variety of multifunctional terminators are developed (e.g., Cl2MeSiCH2CH2Si-MeCl2) [203,204]. However, a series of multifunctional silyl enol ethers were recently found to be effective in multiple termination of living cationic polymers of vinyl ethers [142,147,205,206] and a-methylstyrene [159,207] (Scheme 10). 

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. 

There are two basic synthetic routes for star polymers (Scheme 12)-the core first method (polymerization from multifunctional initiators or microgels) and the arm first method (where growing polymer chain ends are reacted with a multifunctional terminating agent or a divinyl compound). Whereas the use of multifunctional initiators leads to stars with a well-known (but often low) number of arms, the use of microgels or divinyl compounds leads to a rather broad arm number distribution, where the average arm number can be quite high. 

Previously, several groups had attempted to synthesize regular stars via a linking reaction of living anionic polymers with BnX-based multifunctional terminators. Unfortunately, however, a series of undesirable side reactions was often encountered that resulted in mixtures of fewer-armed or extra-armed stars, in addition to the target star-branched polymers. Such side reactions have been eUminated by using DP E-end-capped living anionic polymers in THE at —40°C (or lower... 

Triblock and star-shaped polymers are available if the polymerization of one monomer is started with a monofunctional initiator, then the second monomer is added. Finally, the polymerization is terminated by adding a two- or multifunctional terminator. 

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