Studies on a series of bisarylimides containing four phenylene rings and their polymers: 1. Synthesis and characterization of the monomers John M. Barton* Materials and Structures Department, Roya/ Aerospace Establishment, Farnborough, GU14 6TD, UK lan Hamerton, John B. Rose and David Warner Chemistry Department, University of Surrey, Guildford, GU2 5XH, UK (Received 21 December 1989; revised 16 February 1990; accepted 16 February 1990) Two aryl bismaleimidesand the corresponding biscitraconimides,in which the imide groups were attached to the ends of aromatic residues containing four phenylene rings, were prepared and then purified by preparative high-performanceliquid chromatography. Rigorous purification in this way was found to have marked effects on the thermal polymerization characteristics of these compounds as determined by the differential scanning calorimetry technique. The polymerization of bis-4-maleimidophenylmethane and of bis-4-maleimidophenyl ether was also affectedby rigorous purification. Samples of the corresponding two bisnadimides containing four phenylene rings were also prepared. (Keywords: bismaleimides; biscitraconimides; bisnadimides; purification; polymerization; differential scanning calorimetry) INTRODUCTION Many aryl bismaleimides (structure I) have been pre- pared as monomers for crosslinked resin systems, and resins based on bis-4-maleimidophenylmethane (Ar = 4,4'-diphenylenemethane in I) are available commer- ciallya. Most of these compounds have melting points greater than 150°C and close to the temperature at which thermal polymerization starts 2, so that typical d.s.c, scans for these monomers show a sharp melting endotherm quickly followed by a broad exothermic reaction peak due to polymerization and crosslinking. The close proximity of the melting and polymerization tempera- tures may lead to difficulties in using the d.s.c, technique 3 for investigating the kinetics of polymerization of these compounds and can cause problems in their use as monomers for composites 2. Hence, we were interested to find bismaleimides with melting points below 100°C, which were expected to show a substantial temperature gap between melting and the start of polymerization. We also wanted to examine monomers containing a high proportion of phenylene to maleimide residues in the hope of obtaining crosslinked resins of improved thermal and oxidative stability. Another way of increasing the temperature gap between melting and polymerization is to increase the temperature at which polymerization starts. Information on the polymerizability of arylcitraconimides is conflict- ing. Thus, it has been reported 4 that N-phenylcitracon- imide does not polymerize in solution with free-radical initiators, but other workers report that biscitracon- imides polymerize at temperatures lower than those * To whom correspondence should be addressed 0032-3861/91/0200358-06 © 1991 Butterworth-Heinemann Ltd. 358 POLYMER, 1991, Volume 32, Number 2 found for the corresponding maleimides5-s. However, on general grounds citraconimides would be expected to polymerize less readily than the corresponding male- imides, and as the melting points of the two types of compound are similar, e.g. N-phenylmaleimide m.p. 91°C, and N-phenylcitraconimide m.p. 98°C, biscitra- conimides (structure II) should show a larger temperature gap between melting and polymerization than the corresponding bismaleimides. The thermal curing of bisarylnadimides to crosslinked resins is a complex process (see for example ref. 9) in which polymerization is preceded by a reversed Diels-Alder reaction forming maleimide groups, which then act as key monomers in the subsequent polymerization1 o. Reversion of the Diels- Alder reaction requires temperatures in excess of 250°C so that with monomers of this type (structure III) 0 0 N --Ar-- 0 0 O 0 0 0 0 0 I HI B IV=I, Ar=A ; V=I, Ar=B ;VI=II, Ar=A ; VII=II, Ar=B VIII =III, Ar = A ; IX = Ill, Ar = B.