Ring-opening polymerization with electrophilic

Poly (dimethyl siloxane) obtained by anionic ring-opening polymerization of hexamethylcyclotrisiloxane (D3) can also be end-capped with vinylbenzyl bromide or other electrophiles, such as p. (chlorodimethylsilyl)styrene80). 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

Suzuki has shown that vinylcyclopropane 143 behaves both as an electrophile and a nucleophile and thus undergoes palladium-catalyzed ring-opening polymerization as shown in Equation (66). Vinyl cyclopropane 143 first reacts with palladium(O) to induce ring opening of the cyclopropane ring and forms zwitterionic TT-allylpalladium/molonate anion species. Repeated intermolecular attack of the malonate anionic moiety to the 7r-allylpalladium part through bond formation of an r/i -carbon atom affords finally the polymer 142. ... 

Most carbocationic and cationic ring-opening polymerizations are chain processes proceeding with carbocations and/or onium ions as the active species. Nonchain processes which occur via cationic and electrophilic intermediates will be discussed in Chapter 7. 

Isomerlzed unit took place during the polymerization. A polymer sample having a molecular weight of 3400 was prepared. It melted at 269 C and decomposed at 465°C. This Is the first example of an electrophilic ring-opening polymerization of cyclic phosphorus(III) monomer that yields a polymer with a clear-cut structure. 

Figure 6. (A) Radical ring-opening polymerization of vinylcyclopropane with trimethylsiloxy groups and acidic hydrolysis of the poly(silyl enol ether)s. (B) Macromolecular reaction of the poly(silyl enol ether) with electrophile.

Star structures have been prepared by means of an elegant approach that uses metal-polypyridine complexes as metalloinitiators. The first results in this area were reported in 1997 when Fe and Ru tribipyridyl complexes with electrophilic halogenomethyl functionalities (X=Cl, Br or I) were used as multifunctional initiators for the ring-opening polymerization of oxazolines to afford stars 7.30 and 7.31 (Eq. 7.7) [57]. 

Braune and Okuda have shown the possibility to substitute porphyrin initiators by simpler systems based on the association of ammonium salts with a bulky aluminum bis(phenolate) electrophile. According to their study, the ring-opening polymerization of PO cannot occur at simple Lewis add centers, but that nucleophilic ate complexes must be present at the same time. However, so far, only the synthesis of PPO oligomers with MWs less than 5000 has been reported for this system. The important contribution of Braune and Okuda is that they confirmed the Vandenberg binudear mechanism earlier proposed for epoxide polymerization (see Figure 12). °- ... 

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