Purification procedures radical polymerization

The purification procedures to be applied depend on the monomer, on the expected impurities, and especially on the purpose for which the monomer is to be employed, e.g., whether it is to be used for radical polymerization in aqueous emulsion or for ionic polymerization initiated with sodium naphthalene. It is not possible to devise a general purification scheme instead the most suitable method must be chosen in each case from those given below. A prerequisite for successful purification is extreme cleanliness of all apparatus (if necessary, treating with hot nitrating acid and repeatedly thorough washing with distilled water). 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

Purification procedures which depend primarily on the removal of interfering materials by partial polymerization of the monomer are usually not described in detail [22]. It appears that the monomer is generally warmed with a typical free-radical initiator until a slight increase in viscosity is observed. Then the impolymerized monomer is distilled off in an inert atmosphere through a fractionating column. The monomer may be predried with a zeolite such as Linde 4A prior to partial polymerization and fractional distillation [29]. 

ATRP has also been applied to block copolymer synthesis [16]. Both sequential monomer addition and two-step procedures were used. The former involves the simple addition of a second monomer to the reaction medium after complete consumption of the first monomer. In the latter case the first monomer, after isolation and purification, was used as macroinitiator for the polymerization of a second monomer in its usual manner. Macroinitiators suitable for ATRP may also be prepared by a polymerization technique other than radical polymerization. This way block copolymers of monomers with different chemical structures are prepared. Such examples include cationic to radical and condensation to radical transformation reactions [20,21]. 

SynQiesis of graft copolymers wifli TBMA. The free radical polymerizations were carried out as described for the reaction with TBCS, however, using a feed of lSwt% or 7wt% of the macromers (with respect to TMBA). A reaction period of 4 days was used. Dialysis techniques were applied to remove any unreacted mat ial. Standard procedures were used for purification. 

Organotellurium CTAs Te-7 and Te-8 were prepared by reacting AIBN with dimethyl- and diphenylditelluride, respectively, but in low yields (8-18%). This route has the practical advantage that both AIBN and ditellurides are stable in air and can be handled without special precautions. Furthermore, the air-sensitive CTAs formed can be directly used for polymerization without purification because the only side product is a dimer of AIBN-derived radicals, which does not affect the polymerization reaction. Despite the simplicity of this procedure, the use of purified initiators is the method of choice for obtaining living polymers with the highest level of control of MWD. 

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