Living anionic polymerizations vinyl ethers

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

Cationic Polymerization. For decades cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition (see Elastomers, synthetic-BUTYLRUBBEr). More recently, development has led to the point where living cationic chains can be made, with many of the advantages described above for anionic polymerization (27,28). 

It is to be noted that N-vinylcarbazole (NVC) undergoes also living cationic polymerization with hydrogen iodide at —40 °C in toluene or at —78 °C in methylene chloride and that in this case no assistance of iodine as an activator is necessary 10d). NVC forms a more stable carbocation than vinyl ethers, and the living propagation proceeds by insertion between the strongly interacting NVC-cation and the nucleophilic iodide anion. 

Trapping experiments with malonate anions in the controlled/living polymerization of vinyl ethers initiated by mixtures of hydrogen iodide (HI) and Lewis acids revealed that the total concentration of the growing species (the sum of dormant and active) stays nearly constant and equal to that of the introduced initiator when monomer is present in the reaction mixture. However, after complete monomer consumption, the concentration of the growing species decays relatively rapidly [251,252]. 

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). 

Living cationic polymerization techniques are also capable of producing well defined star-block copolymers. An approach similar to the DVB method described above for the case of anionic polymerization was employed in order to prepare amphiphilic star-block copolymers [20]. In one case, living diblock copolymers of vinyl ethers and ester-containing vinyl ethers, prepared by the initiating system Hl/Znh in toluene, were reacted with a small amount of a difunctional vinyl ether to produce star shaped block copolymers (Scheme 5). 

The advantage of the system arises from the fact that, since the poly(chloroethyl vinyl ether) (PCEVE) backbone and PSt grafts can be prepared by both living cationic and anionic polymerization, it is possible to synthesize graft copolymers that possess both a backbone with controlled dimensions and an adjustable number of branches of precise length and narrow molecular weight distribution. 

With a few exceptions [26, 27], living cationic polymerization is initiated by the initiator/coinitiator (Lewis acid) binary system. Selection of an initiating system for a given monomer is of crucial importance, as there are no universal initiators such as organoHthiums in anionic polymerization. For example, while weak Lewis acids such as zinc halides may be necessary to effect living polymerization of the more reactive vinyl ethers, they are not effective for the living polymerization of the less reactive monomers, such as IB and St Detailed inventories of initiating systems for various monomers are well described in recent publications [25, 28, 29). 

A dual initiator was employed by Cramail etal to synthesize block copolymer of St and chloroethyl vinyl ether (CEVE) via anionic-to-cationic transformation. The anionic living polymerization of St was initiated by lithiopropionaldehyde diethyl acetal in the presence of tetramethylene diamine. The resulted polymer with acetal functionality was converted into the corresponding a-iodo-ether with trimethylsilyl iodide (TMSI),... 

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