Studies on the polymerisation of acrylonitrile

Abstract

The thesis contains the results of the investigations on the kinetics and mechanism of polymerization of acrylonitrile. It is divided into two parts: the first part deals with the initiation of polymerization by ferric chloride and the second part deals with chain transfer of solvents in acrylonitrile polymerization.

PART I Unlike other metal-based initiator systems, ferric chloride behaves in a complicated fashion in vinyl polymerization. The behaviour of ferric chloride is very much governed by the conditions existing and it has been found to act both as an initiator and terminator. The results published so far indicate some kind of complex formation or interaction between the monomer and the ferric salts is responsible for initiation. Although ferric chloride is reported to form a complex with acrylonitrile, no polymerization has been reported so far. In this perspective, the present investigation was carried out and the results emerged out of the investigation are given below. Ferric chloride hexahydrate behaves as an initiator for the polymerization of acrylonitrile in the concentration range 1×10?41 \times 10^{-4}1×10?4 to 1×10?31 \times 10^{-3}1×10?3 moles per litre. The RpR_pRp? increases at first and then decreases with increase in the initiator concentration. An initiator exponent of 0.75 was obtained. The nominal deviation from the usual square root dependency may be attributed to the occlusion of the active radical ends and other kinetic complications. The polymerization reaction proceeds through a free radical path as evidenced by the complete inhibition of the reaction by DPPH and hydroquinone. The initiator is highly selective to the nature of the monomer. The overall activation energy is found to be 21.16 and 19.39 Kcal/mole at two different initiator concentrations. The unusually low degrees of polymerization observed in this case are attributable to the terminating action of the ferric species present in the system. Fairly high values of SSS, values of PPP and the complexity of the initiating species tend to suggest that ferric chloride takes part in the termination reaction. Another interesting feature is the high dependency in DMF. Ferric chloride hexahydrate has a pronounced tendency to rapidly exchange its water molecules with other ligands. When FeCl?·6H?O is dissolved in acrylonitrile, this ligand exchange reaction can occur because acrylonitrile is a stronger ligand than water. A stepwise equilibria may be assumed to exist, involving step-by-step replacement of water by acrylonitrile. The exact nature of the species present in the system is not certain. Any of the species present may be responsible for initiation and termination and hence high values of monomer exponent are quite possible. Ferric chloride is reported by Kern to form a complex. This complex was prepared and found to be capable of initiating polymerization of acrylonitrile. When this complex was used as the initiator, the rate of polymerization was found to be proportional to M3/2M^{3/2}M3/2, indicating thereby that the high order for monomer concentration is due to the involvement of the monomer in the complex formation leading to initiation.

PART II Although extensive study has been carried out on the determination of chain transfer constants of solvents in vinyl polymerization, most of it has been restricted to homogeneous phase. The introduction of the "mixed solvent technique" opened up the possibilities of studying the chain transfer of solvents immiscible with the monomer or precipitants for the polymer. Das et al. have determined the chain transfer constants of solvents in acrylonitrile polymerization in the heterogeneous phase although the kinetic relationships employed were based on solution polymerization. How far this treatment is justifiable is not clear. But, using the mixed solvent system it is possible to investigate the chain transfer of solvents in the homogeneous phase. With this background we have studied the chain transfer of different solvents in acrylonitrile polymerization with the object of determining the chain transfer constants of substances which usually cause precipitation of the polymer. It was also intended to compare the data with those obtained in heterogeneous polymerization of acrylonitrile and other vinyl monomers. Our investigations revealed that the rate does not vary much. The solvent effect on the degree of polymerization may be assumed entirely due to chain transfer reaction. A comparison of the chain transfer constants in homogeneous and heterogeneous phases reflects the conspicuous difference between the two sets of values, all the values being higher in the homogeneous phase. In a poor solvent in which the polymer separates out, the monomer is preferentially absorbed on the polymer, causing sub-microscopic phase separation and effectively blanketing the active radical ends. Consequently, the degree of polymerization and overall rate becomes higher in such cases.