Polymers Formed by Free Radical Mechanisms

Diene Polymers Formed by Free Radical Mechanisms.—Various structural and configurational arrangements of the units in polymers obtained from 1,3-dienes are possible. To take a comparatively simple example, consider the addition of butadiene to the chain free radical represented by . Addition to a terminal carbon atom of the diene may be taken for granted in view of the resonance stabilization of the product radical. 

Now this resonance hybrid may react by the addition of another monomer at either carbon 4 or 2. The former alternative leads to the 1,4 structure 

whether the remaining ethylenic bond of a given unit is incorporated within the chain (V) or in a pendant vinyl group (VI) external to the chain is governed by the manner of addition of the succeeding unit. The 1,4 unit may occur as either the cis or the trans isomer, i.e. 

A chemical method developed by Kolthoff and Lee for quantitatively determining the proportion of 1,4 and 1,2 units in a polybutadiene involves measurement of the rate of oxidation of the residual double bond by perbenzoic acid. 

Quantitative differentiation between the two types of unit depends on the roughly 25-fold greater rate of reaction of the symmetrically substituted ethylenic group of the 1,4 unit. 

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Free-radical polymerizations of 1,3-butadiene usually result in polymers with 78-82% of 1,4-type placement and 18-22% of 1,2-adducts. The ratio of 1,4 to 1,2 adducts is independent of the temperature of polymerization. Moreover, this ratio is obtained in polymerizations that are carried out in bulk and in emulsion. The ratio of trans-1,4 to cis-1,4 tends to decrease, however, as the temperature of the reaction decreases. Polybutadiene polymers formed by free-radical mechanism are branched because the residual unsaturations in the polymeric chains are subjects to free-radical attacks ... 

Methacrylonitrile polymerizes readily in inert solvents. The polymer, depending on the initiator and on reaction conditions, is either amorphous or crystalline. Polymerizations take place over a broad range of temperatures from ambient to —5°C, when initiated by Grignard reagents, triphenyl ethylsodium, or sodium in liquid ammonia [264]. The properties of these polymers are essentially the same as those of the polymers formed by free-radical mechanism. 

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