New thermosetting poly(2-allyl-6-methylphenol-co-2,6-dimethylphenol)s (3) have been developed by oxidative coupling copolymerization of 2-allyl-6-methylphenol (1) with 2,6-dimethylphenol (2), followed by thermal curing. Copolymerization was conducted in nitrobenzene in the presence of copper(I) chloride and pyridine as the catalyst under a stream of oxygen, producing high molecular weight copolymers (M-n similar to 50,000) with broad molecular weight distributions (M-w/M-n similar to 35). The structure of resulting copolymers 3 was characterized by IR, H-1, and C-13 NMR spectroscopy. Cross-linking reactions of copolymers were carried out by thermal treatment in the absence or presence of a peroxide (3 wt%, 2,5 -dimethyl-2,5 -di(tert-butylperoxy)-3-butane). The 10% weight loss and glass transition temperatures of the cured copolymers were 436 degreesC in nitrogen and 235 degreesC, respectively after curing at 70 degreesC for 1 h and 300 degreesC for 1 h. The average refractive index of the cured copolymer (3b) film was 1.5407, from which the dielectric constant (epsilon) at 1 MHz was estimated as 2.6. The epsilon and dissipation factor of copolymer-films at 1 MHz were directly measured from their capacitances as 2.5-2.6 and 0.0015-0.0019, respectively. (C) 2003 Elsevier Ltd. All rights reserved.