Bevington 2 Initiation

The efficiency of AIBN in initiating polymerization at 60°C was determined by Bevington et al.t by the following strategy. They measured Rp and v and calculated Rj = Rp/r. The constant was measured directly in the system, and from this quantity and the measured ratio Rp/r" the fraction f could be determined. The following results were obtained for different concentrations of initiator ... 

The success of the multifunctional initiators in the preparation of block and graft copolymers depends critically on the kinetics and mechanism of radical production. In particular, the initiator efficiency, the susceptibility to and mechanism of transfer to initiator, and the relative stability of the various radical generating functions. Each of these factors has a substantial influence on the nature and homogeneity of the polymer formed. Features of the kinetics of polymerizations initiated by multifunctional initiators have been modeled by O Driscoll and Bevington 64 and Choi and Lei.265... 

Fig. 3-3 Effect of styrene concentration on the initiator efficiency of azobisisobutyronitrile. . o and refer to experiments with initiator concentrations of 0.20, 0.50, and 1.00 g I, 1, respectively. After Bevington [1955] (by permission of Faraday Society, and Royal Society of Chemistry, London).

Bevington has continued his studies of the initiation reaction and of the reactivities of monomers towards reference radicals (69—71). A study of the polymerization of substituted styrenes was recorded (72). In methyl methacrylate polymerization by ammonium trichloroacetate in the presence of copper derivatives, the complexities of the initiation and termination reactions were elegantly unravelled by Bamford and Robinson using two differently labelled trichloroacetates (73). Apparently cyclic processes involving alternate oxidation and reduction of copper may arise. 

Oxygen free irradiation of solid polymers was also carried out in the absence of monomer in this case free radicals with relatively long life time are produced which are able to initiate the polymerization of a second monomer (56,113). Several irradiated polymers have been used as initiators of polymerization, and, evidently, if the polymerization of the second monomer is initiated only by these frozen-in radicals, the product will undoubtedly be hundred per cent grafted. Bevington using 14C-acrylonitrile was able to estimate the amount of acrylonitrile grafted on preirradiated polystyrene, polyacrylonitrile, polyethylene terephtha-Iate and nylon yams (36,37). 

Current views on polymerization of acrylonitrile in homogeneous solution are illustrated by a description of the reaction in N,N-dimethyl-formamide (DMF) as initiated by azobisisobutyronitrile (AIBN) at about SO to 60°. Primary radicals from the decomposition of AIBN react with monomer to start a growing chain. About one-half of the primary radicals are effective, the others being lost in side reactions not leading to polymer. Bevington and Eaves (32) estimated initiator efficiency by use of AIBN labelled with C-14, whereas Bamford, Jenkins and Johnson (13) used the FeCls termination technique. Both of these methods require that the rate of AIBN decomposition be known, and the numerical value of this rate has undergone a number of revisions that require recalculation of efficiency results. From recently proposed rate expressions for AIBN decomposition at 60° (22, 136) one calculates an efficiency of about 40% by the tracer technique and 60—65% by the FeCl3 method. 

Barson, Bevington and Eaves have looked into the initiation reaction with azobisisobutyronitrile and with benzoyl peroxide, both in dimethylformamide (23). With AIBN they reported normal initiation. With the peroxide there appeared to be complications, due they thought to hydrogen abstraction from monomer or polymer. By using peroxide labelled in the ring or in the carbonyl group with C-14 they measured the relative importance of the two steps ... 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

Moad G, Solomon DH (1989) Azo and Peroxy Initiators In Allen G, Bevington JC, Eastmond AL, Russo A (eds), Comprehensive Polymer Science, Pergamon Press, Oxford 3(8) 97... 

A variation of the above method involves both radioactive end-group analysis of the polymer and measurement of the polymerization rate (Bevington, 1955). Thus from an assay of the polymer, it is possible to calculate the number of monomer molecules combined in the polymer for each initiator fragment This number is the kinetic chain length (see Section 6.7.1) independent of the mechanism of termination. Dividing the overall rate of polymerization by the kinetic chain length one obtains the rate of initiation, which can then be compared with the rate of production of radicals from the initiator to obtain the efficiency of initiation. 

Bevington et aL have found with the use of 3c-labelled benzoyl peroxide that the polymerization of methyl methacrylate or styrene in the presence of stilbene resulted in a high proportion of the initiator fragment attached to the stilbene unit. Tlie stilbene was found to react readily with oxygen-centred benzoyloxy radical, but was very unreactive to carbon-centred radicals. The stilbene is not polymerized. 

Isotopic enhancement may be also illustrated by Bevington et al. s exploration of the use of the C-enriched free radical initiators 1,1 -azobis(phenylethane) and AIBN in preparing butadiene polymers [51] and the use of dimethyl 2,2 -azobis(isobutyrate) to initiate the polymerisations of styrene, acrylonitrile, methyl methacrylate and methyl acrylate [52]. The signals from the ends are thus rendered more intense, and b ome observable in a standard NMR spectrum, where they display information on the manner in which the initiator radicals have attacked the first monomer to become incorporated at the start of the polymer chain one can thus compare initial and mean tacticities. In a further use of isotope enrichment, Moad and Willing found that selective enrichment... 

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