Cationic Chain Growth

Several fluorinated monomers have been polymerized via homogeneous cationic polymerization in compressed carbon dioxide. For example, vinyl ethers with fluorinated side chains were polymerized in SCCO2 at 40 °C using adventitious water initiation with ethylaluminum dichloride as the Lewis acid coin- 

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The most industrially significant polymerizations involving the cationic chain growth mechanism are the various polymerizations and copolymerizations of isobutylene. In fact, about 500 million pounds of butyl rubber, a copolymer of isobutylene with small amounts of isoprene, are produced annually in the United States via cationic polymerization [126]. The necessity of using toxic chlorinated hydrocarbon solvents such as dichloromethane or methyl chloride as well as the need to conduct these polymerizations at very low temperatures constitute two major drawbacks to the current industrial method for polymerizing isobutylene which may be solved through the use of C02 as the continuous phase. 

Lewis acids initiate cationic chain-growth polymerizations. There are several possible chain propagation reactions, and the mechanism of cationic chain growth is still open to... 

In the polymerization of eCLM only a few percent of lactam is incorporated into the polymer through hydrolysis and bimolecular condensation. Most monomer enters the polymer through stepwise addition, i.e. the cationic chain growth 22) ... 

Polymerizations in Dense Carbon Dioxide 317 4.S.3.2 Cationic Chain Growth... 

The carbon cation which forms cannot initiate a cationic polymerization of excess alkene molecules that are present in the reaction mixture, due to steric hindrance. The less hindered alkenes, however, like isobutylene, polymerize rapidly under the same conditions with antimony pentafluoride. It was therefore suggested that initiation involves an opening of a previously formed n complex of SbFs with the double bond, through halide participation. The a,y -haloalkylantimony tetrafluoride ionizes with excess Lewis acid to the related carbon cation that is capable of initiating cationic chain growth ... 

Explain the differences and similarities in anionic and cationic chain-growth polymerizations. 

Summarize the initiation process in cationic chain-growth polymerization. 

Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

In this type of polymerisation an initiating molecule is required so that it can attack a monomer molecule to start the polymerisation. This initiating molecule may be a radical, anion or cation. Chain growth polymerisation is initiated by free-radical, anion or cation proceeded by three steps initiation, propagation and termination. The chemical nature of the substituent group determines the mechanism. 

Ionic polymerization uses anionic or cationic catalysts. Monomers with electron-accepting groups at the double bonds such as vinyls are catalyzed by anionic catalysts such as organometallic compounds [16, 17]. Monomers with electron-donating groups at the double bonds are catalyzed by cationic catalysts such as Lewis acids, Ziegler catalysts, and Friedel Crafts catalysts [16, 17]. Polyacetals are linear polymers that can be produced by anionic or cationic chain-growth polymerization of formaldehyde [2, 14]. 

Cationic Poiymerizations. Homogeneous and heterogeneous cationic polymerizations of various monomers in liquid and SCCO2 have been performed (Table 2). Several different catalyst systems were employed in these polymerizations, all of which proved to be effective. The early experiments conducted using monomers such as isobutylene (67) and formaldehyde (70-72) normally resulted in a low yield of low molecular weight products. Nonetheless, proof of concept for the use of CO2 as an effective solvent for cationic chain-growth reactions was achieved. Other polymers based on different substituted alkenes (73), vinyl ethers (74,75) (eq. (4)), and oxetanes (75) (eq. (5)) have been synthesized. 

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