Acrylonitrile Block and Graft Copolymers

This paper presents a review of the methods available for preparing block and graft copolymers in which one component is acrylonitrile. Methods using polymer ions and polymer radicals are considered together with a useful new technique based on reactions of nitrile groups with amide compounds of group IV transition metals. Because of its polarity and its unusual solubility properties, a polyacrylonitrile component adds great potential interest to a block or graft copolymer system and many reports can be found in the literature of attempts to prepare such copolymers. The methods used embrace a wide range of techniques; some are purely chemical,'-3 others involve ultrasonics4 or high energy radiation.5 My main purpose in this paper is to discuss three of the purely chemical methods which have developed significantly in recent years, and the methods in question involve polymer ions, polymer radicals or organometallic polymer derivatives as key reagents in the formation of the block or graft copolymer. METHODS USING POLYMER IONS The living polymer systems described by Szwarc6 provide the best-known technique for preparing block copolymers but not much is recorded in the literature concerning molecules containing acrylonitrile as one component. The use of acrylonitrile as the first-phase monomer with either lithium butyl of sodium naphthalene seems to give insoluble polymers of little use,7 no doubt due to reactions at the CEN as well as the C=C bonds, but some success has been reported with acrylonitrile as the second-phase monomer, especially in conjunction with styrene. Claes and Smets8 were able to obtain block copolymers in this way free from homopolyacrylonitrile but much homopolystyrene was present, although this was readily removed by solvent extraction. Seymour et al.7 have reported success with a similar system but few details are given about the need to separate the block copolymer. Methyl methacrylate has also been used as the first-phase monomer but evidently it is less satisfactory: Seymour et a!. found that the product after adding acrylonitrile contained less than 10% methyl methacrylate and was quite insoluble in acetone, the inference presumably being that a transfer mechanism was responsible for the formation of much homopolyacrylonitrile. Block copolymers of styrene and acrylonitrile were prepared by Perry9 using a different technique. He made polystyrene in a free radical system with molecular growth controlled by transfer to dibutylphosphine so that he obtained an initial P of about 400 with 95% of the polystyrene chains containing a phosphine residue at one end. Since dialkyl phosphines initiate the anionic polymerization of acrylonitrile, the first-phase polymer is ideally suited to form the basis of block copolymer and acrylonitrile contents in the range 14—76% were obtained. Extraction techniques established a fairly high purity level for the block copolymers thus obtained. METHODS USING POLYMER RADICALS During the investigation of the bulk polymerization of acrylonitrile it was deduced from kinetic observations that under certain conditions a significant, or even substantial, fraction of the radicals generated remain occluded in the precipitated polymer for long periods of time.'0" At temperatures as high as 60°C occlusion affects the velocity constant for the termination process but is not severe enough to trap radicals; at 25°C, on the other hand, it has been estimated that perhaps 1% of the radicals generated in photopolymerization become so tightly embedded in the polymer that they are unable to propagate further, although they are liberated by raising the temperature. Very fine control is possible in this situation because elevation of the temperature to 40°C facilitates propagation butdoesnotsufficientlyopenthestructurethattheoccluded radicals terminate in appreciable numbers, whereas heating to 60°C destroys them all in a shorttime. The environmentof the trapped radicals can be changed by replacement of the residualacrylonitrilebyanotherliquid, andif this contains a monomer the liberated radicals will react with it to produce block copolymer.'2 The precise nature of the block copolymer will then depend upon the mechanism which terminates the new growth, disproportionation or transfer giving an AB type and combination an ABA type of block structure. The type of system discussed here was first described by Bamford and Jenkins,'2 who demonstrated that the nature of the second monomer had a large influence on the extent of second-phase polymer formed. This is, of course, not surprising since the system must be very sensitive to the degree of interaction between the liquid and the polymer, a point taken up later by Seymour and his colleagues.'3 Among the systems examined by Bam.ford and Jenkins,'2 the one which gave the greatest amount of polymer was polyacrylonitrile/methyl acrylate; it was possible here to attain almost complete conversion of the methyl acrylate to polymer because the rate of radical destruction is very low. This observation highlights a key factor in the situation, viz, that if polymer and liquid interact too weakly the occluded radicals will not be liberated enough to propagate, while swelling on too large a scale will allow the radicals to terminate too rapidly to produce much block copolymer. In the former case a remedy is at hand if a small proportion of swelling agent, such as dimethylformamide, is added to the second-phase monomer; the latter problem can possibly be mitigated by mixing the monomer with a liquid which has very little interaction with polymer or by lowering the temperature, but these