Studies on a series of bis-arylimides containing four phenylene rings and their polymers: 3. Kinetic analysis of the thermal polymerizations John M. Barton* Materials and Structures Department, Defence Research Agency (Aerospace Division), RAE, Farnborough, Hampshire GUI4 6TD, UK and lan Hamerton, John B. Roset and David Warner Department of Chemistry, University of Surrey, Guildford, Surrey GU2 5XH, UK (Received 23 September 1991; revised 7 November 1991; accepted 18 November 1991 ) A series of aryl bis-maleimides (BMIs) and bis-citraconimides (BCIs) were characterized by d.s.c, using both temperature scans and isothermal experiments. Impurities in the BMIs tend to increase the temperature at which thermal polymerization starts, while the converse is true for BCIs. This was attributed to the presence of an itaconimide impurity in the BCIs. In particular, the thermal polymerization kinetics of the compounds were investigated. Effects of structure and monomer purity on the thermal polymerization characteristics were identified. ( Keywords: bis-maleimides; bis-citraconimides; differential scanning calorimetry; thermal polymerization; kinetics; purity ) INTRODUCTION Bis-maleimide (BMI) and to a lesser extent bis- citraconimide (BCI) monomers are increasingly being used in structural composite and adhesive applications where an enhancement in operational temperature is required relative to epoxy resins. In general the BMI and BCI systems are cured by thermal polymerization to produce a network by chain extension and crosslinking. The maleimide or citraconimide ring is susceptible to polymerization by a free radical or an anionic mechanism 1. The polymerization rate may be enhanced by free radical initiators, or retarded by inhibitors such as hydroquinone. On the other hand basic compounds such as imidazoles are also effective accelerators 1 in the thermal polymerization, so that both radical and anionic mechnisms may be involved. Bulk thermal polymerizations of this type show complex kinetics. The relative rates of free radical inhibition, transfer and termination are significant, and in the presence of air inhibiting peroxide species may be involved 2. Further complications are reductions in molecular mobility due to increasing viscosity with conversion and leading ultimately to the formation of a glass where chemical reactions involving diffusion essentially stop. A convenient way of monitoring the thermal polymerization is by d.s.c, in which the rate of evolution © 1992 Controller HMSO London *To whom correspondence should be addressed t Present address: 8 Hillier Road, Guildford, Surrey GU1 2JQ, UK 0032-3861/92/173664-06 © 1992 Butterworth-Heinemann Ltd. 3664 POLYMER, 1992, Volume 33, Number 17 of heat due to reaction is measured a. This does not, however, give direct information on the concentration of chemical species. Kinetic models may be used to rationalize the data without necessarily revealing the exact chemical kinetic mechanism. This is of value for rationalizing relative reaction rates and temperature dependence of reactions, and it is of significance in the technological processing of resin and composites. In this paper d.s.c, is applied to the kinetic study of four BMIs (structures IV, V, VI and VII in Scheme 1) and two BCIs (structures VIII and IX in Scheme 1 ). The general synthesis and thermal properties of these materials have been described elsewhere 4-6. EXPERIMENTAL The synthesis based on a method first reported by Searle 7 and purification by h.p.l.c, of the BMIs (IV-VII) and BCIs (VIII and IX ) is outlined in our earlier publication 5. Dynamic d.s.c, was performed at a range of heating rates (5, 10, 15 and 20K min -1) under nitrogen (40cm a min- 1) using a Du Pont 910 calorimeter interfaced with a Du Pont 9900 computer/thermal analyser. Isothermal d.s.c, was carried out at four to five different temperatures over a 20K range using a Perkin-Elmer DSC-7 calorimeter interfaced with a Perkin-Elmer 7300 computer. Monomer samples (10 + 2 mg) were run in open aluminium pans. After conversion of the raw data to ASCII files using dedicated software, kinetic analysis was carried out using in-house programs 8 on a Hewlett-Packard HP-86 computer.