Termination constants vinyl ethers

The equilibrium constants with nucleophiles such as tertiary amines are so large, that carbenium ions practically do not exist. Thus, tertiary amines and pyridine apparently react with carbenium ions irreversibly and therefore terminate carbocationic polymerizations. Somewhat weaker nucleophiles such as 2,6-dimethylpyridine (lutidine), sulfides, and tris(p-chlorophenyl)phosphine are good deactivators in vinyl ether polymerizations because they react reversibly with monomer, thus maintaining a low concentration of carbenium ions without causing elimination. However, the equilibrium constants in styrene and isobutene polymerizations with amines, sulfides, and phosphines are too large to generate a sufficient stationary concentration of carbenium ions to complete polymerization in a reasonable amount of time. 

The first detailed study of a cationic polymerisation of vinyl ethers induced by stable carbenium salts was reported in 1971 by Bawn et Isobutyl vinyl ether was polymerised with trityl and trc ylium hexadiloroantimonates and trityl fluoroborate. From calorimetric measurements of the rate of polymerisation, it was concluded that all the initiator used was consumed roon after mixii and the assumption was made that an equal number of active species was formed in this fast initiation reaction. Propagation rate constants were thus obtained and attributed to the action of free ions. It was als) claimed that no significant termination took place during the polymerisation ce successive monomer additions produced polymerisations having the same propagation rate constant. Later work performed in the same laboratory on other vinyl ethers... 

Figure 5. Correlation of rate of hydration of terminal alkenes with

Depending on the mole ratio of reactants used, the polyacetals have either both terminal vinyl groups, both terminal hydroxyl groups, or one vinyl ether and one hydroxyl group. The reaction can be run at -10° to 20°C using acid salts of strong acids, e,g, alkali metal bisulfate or a-haloethers. Organic acids with dissociation constants of 10 to 10 are used if a reaction temperature of 20°-180°C is desired. 

R=HVice versa, in terms of initiation, an increase in the rate constant of initiation has been observed in the order X=Clbased systems generally results in the formation of polymers that predominantly contain tram-vinylene units. For a detailed discussion on the stereochemistry of ROMP-derived polymers and the determination of tactidty refer to the chapter by J. G. Hamilton in Ref. 25 and the references cited therein. Polymerizations initiated by Gmbbs-type initiators are best terminated by the use of ethyl vinyl ether, yielding vinyl-terminated polymers and Ru-(ethoxymethylidenes) with very low ROMP activity. ... 

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