Anionic polymerization disadvantages

The initiation of this type of polymerization can be achieved with a polymeria initiator, e.g. a polymer containing peroxide or azo end groups. As the different types of synthesis which have been described for the preparation of such polymeric initiators, have generally the disadvantage to need several reaction steps (3- 5), it seemed interesting to prepare these initiators in a one Step reaction, by keeping the different advantages of the anionic polymerization. 

All these methods have several disadvantages multistep endgroup derivatizations are required for the synthesis of the telechelic PIB initiator precursor, and lithiation and thus the subsequent initiation of anionic polymerization is not quantitative or it requires extreme conditions. As a consequence of less than 100 % initiating efficiency these methods yielded mixtures of homopolymers and block copolymers. This is indicated by the fact that it was claimed that PMMA-Z -PIB-6-PMMA was only obtained by selective extraction ... 

Although the living anionic polymerization technique is the most acceptable method for synthesis of well-defined azo BCs because of the living nature of the end groups, this technique has some disadvantages too ... 

Another disadvantage of anionic polymerization of acrylonitrile is formation of cyanoethylate as a side reaction. This, however, can be overcome by running the reaction at low temperatures. An example is polymerizations initiated by KCN at-50 °C in dimethylfonnamide, or by butyllithium in toluene at -78 Both polymerizations yield white, high molecular weight products that are free from cyanoethylation. 

Cyanoacrylate adhesives J GUTHRIE Anionic polymerization, additives, applications, advantages and disadvantages... 

Many ferrocenylsilanes can be polymerized by anionic initiators such as n-butyllithium, phenyllithium or ferrocenyllithium (Fig. 8.21). The reaction occurs at ambient temperature and affords living polymers. The utility of anionic polymerization is that the molecular weights can be controlled and also that block copolymers can be prepared. The main disadvantage of the anionic polymerization is that the monomer and the solvent should be rigorously purified and should be free of acidic impurities including water. Even traces of impurities can be detrimental [53]. 

The anionic polymerization methacrylic acid esters using such classical initiators as bulky RLi requires low temperatures and has other disadvantages. A breakthrough was reported by Webster, et al, at Dupont in 1983. They used 0-silyl ketene ketals as an initiator in combination with nucleophilic catalysts (fluorides) at room temperature. The process also has two other positive features 1) The polymers are living prior to aqueous workup, and 2) The molecular weight distribution is very narrow (D=M /Mn 1.2) ... 

In 1997, before Bayer reached the commercial stage with their HTPBD, direct competition emerged with the introduction of HTPBD produced by anionic polymerization of butadiene. This material was developed by the Czech company Kaucuk and was marketed under the trade name Krasol . The lower feedstock cost of butadiene versus cyclooctadiene gave Krasol a cost advantage in this already cost-disadvantaged specialty polyurethane market segment. Bayer abandoned its line of ROMP-derived HTPBD before a commercial launch, whereas Krasol was successfully commercialized and is now marketed by Cray Valley. 

Block copolymers can be s)mthesized by alternative techniques to CCTP, but these have disadvantages. Cationic polymerization is limited by the amount of monomers applicable anionic polymerization requires low temperature, which is not feasible for application to plant reactors and GTP requires protection of any acidic monomers. Certain living free radical techniques have also been investigated but drawbacks still remain with respect to conversion, color, and catalyst removal. CCTP provides a new way to prepare hydroxyl-functional graft copolymers that show excellent drying. 

A disadvantage recognized at the outset was the expected difficulty of copolymerizing vinyl ethers (which undergo ready cationic polymerization) with acrylic monomers which are polymerized using radical or anionic Initiators but which resist cationic Initiation (Scheme Ill). 

Anionically Initiated Polymerization. The disadvantages of radical polymerization of cyanoprene result from the operating conditions (temperatures) too many side reactions, chain-terminating reactions, and consecutive reactions occur. Because of this and the dimerization tendency of cyanoprene, catalysts had to be found that could fulfill two contradictory requirements. They should be so reactive that it would be possible to work at temperatures that exclude dimerizations as com-... 

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