Effects of Reaction Media

The reaction medium does not affect the mode of placement in free-radical polymerizations. There were some early reports that an influence was detected in the polymerizations of vinyl chloride in aliphatic aldehydes at 50 C. This was not confirmed in subsequent studies. The rate of polymerization, however, can be influenced by the pH of the reaction medium in polymerizations of monomers like methacrylic acid. Also, the rate of polymerization and solution viscosities increase in polymerizations of acrylamide and acrylic acid with an increase in water concentration. It is not quite clear whether this is due to increases in the speeds of propagations or due to decreases in the termination rates. In the free-radical polymerization of vinyl benzoate the rate of propagation varies in various solvents in the following order  

Similarly, the rate of photopolymerization of vinyl acetate is affected by solvents. In most cases, however, the rate of polymerization is proportional to the square root of the initiator concentration and to the concentration of the monomer.  

There is a controversial suggestion that the solvent affects the propagation step in some reactions by forming hot radicals. These radicals are supposed to possess higher amounts of energy. At 

The rate expression for the polymerization is then expressed as follows  

In comparing free-radical polymerizations of ethyl acrylates in benzene and in dimethyl formamide at 50 the rates were found to be proportional to the square roots of the initiator concentration. They were not proportional, however, to the concentrations of the monomer. This was interpreted in terms of hot radicals.  

De Sterck and coworkers [90] studied solvent effect on tacticity of methyl methacrylate in free-radical polymerization. They observed that solvents CH3OH and (C[F3)3COH, which are H-bonded with the carbonyl oxygens and are located on the same side of the backbone of the growing polymer radical hinder the formation of isotactic poly(methyl methacrylate) to some extent. Methanol is less effective in reducing the isotacticity because of its small size and also because of the relatively loose hydrogen bonds with the carbonyl oxygens. 

There is a difference in the rate constants of propagation for hot and ordinary radicals so two different reaction schemes were written [91]  

The major species in most cationic polymerization systems are covalent, including halides and esters. There has been considerable controversy as to whether covalent species undergo propagation by direct monomer insertion into a covalent bond or only through ionization to 

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The study of lactonization via an intermediate phenonium ion has been further pursued for several methyl 4-aryl-5-tosylhexanoates (55) as substrates. The intermediate phenonium ion (56) has two possibilities for ring closure, yielding products (57) or (58). In all the substrates, Ar contained one or two methoxy groups and sometimes also a methyl group. The effects of reaction medium, temperature, and time on the product ratios were examined. It was concluded that substrates (55) give y-lactone (57) selectively under thermodynamic conditions, but 5-lactone (58) under kinetic conditions. Substituents in Ar influence the selectivity through their electronic effects. 

Table 3 Effect of reaction medium on selectivity of nitrogenous 2-methylcyclohexanylidene (27) precursors3...

Suba C, Niyazymbetov ME, Evrms DH (1997) Addition of electrochemicaUy-generated anions to aldehydes emd olefins effect of reaction medium and anion basicity. Electrochim Acta 42 2247-2255... 

Figure 5.5 Effect of reaction medium on the relative reactivity of hydroxyl and bromide ions with a lipophilic sulphonate ester.

Figure 4. Effect of reaction mediums on solution polymerization.

Table 1 Effect of Reaction Medium on the Polymerization of Pyrrole via the FeCl3 Process... 

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