Syntheses and Ring-Opening Polymerizations of 2, 3- and 2, 5-Dimethyl-7-Oxabicyclo[2.2.1]heptanes

Abstract

New bicyclic ethers, 2,3- and 2,5-dimethyl-7-oxabicyclo[2.2.1]heptanes were synthesized from 2,3- and 2,5-dimethylphenols as starting materials. Each of them was separated by fractional distillation and preparative gas chromatography into the corresponding exo,exo-,endo,exo-, and endo,endo-isomers. Ring-opening polymerizations of these isomeric monomers were carried out in CH2Cl2 using cationic initiators such as BF3—tetrahydrofuran (THF) complex—epichlorohydrin (ECH), SnCl4—ECH, SbCl5—ECH, and FeCl3—SOCl2 systems. The SbCl5—ECH catalyst system brought about high monomer conversions, but it induced a chain-transfer reaction during the polymerization. The exo,exo- and endo,exo- 2,3-dimethyl monomers (1a and 1b) yielded white powdery polymers in yields of 58 and 63 %, respectively, using the SbCl5—ECH system. NMR spectroscopic analysis of poly(1b) showed that it contained a chair-formed trans,trans,trans-1,2,3,4-tetrasubstituted cyclohexane ring in its monomeric unit. Both exo,exo- and endo,endo-2,5-dimgthyl monomers (2a and 2c) yielded grease-like polymers in 16 and 17-% yields, and its endo,exo-isomer (2b) yielded a highly crystalline polymer, which was insoluble in the usual organic solvents, in 26-% yield using BF3–ECH. A monomeric unit in poly(2b) was assumed to be trans,cis,trans-1,2,4,5-tetrasubstituted cyclohexane. In the polymerization of the monomers 1b and 2b, it has been shown that the monomer attacks the propagating end at the bridgehead carbon in position 4 (C-4), but not in position 1 (C-1). Nucleophilicities of the monomers 2a and 2b were compared based on their copolymerizations with the exo-monomethyl analogue (3a). The exo-methyl group relative to the endo-methyl group tends to lower the monomer nucleophilicity. The polymer yield is controlled by three factors; the steric crowding around the bridgehead carbon due to the methyl group, the nucleophilicity, and the ring strain of the monomer.