Benzoxazine polymers containing phenyleneethynylene and polypropylene glycol (PPG) in the main chain, poly(1)230, poly(1)400 and poly(1)2000, were synthesized by a Mannich reaction of the corresponding ethynylenebisphenol, paraformaldehyde and PPG diamines with Mn = 230–2,000. The curing temperature of poly(1) decreased from 212 to 182 °C as the Mn of the PPG chain decreased from 2,000 to 230. Poly(1)230–2000 was heated at 200–250 °C to obtain the corresponding polymers, and poly(1)′230–2000 was cured by ring-opening polymerization of the benzoxazine moieties. The structures of the polymers were elucidated by 1H-NMR and IR spectroscopies before and after curing. Poly(1)′230–2000 became flexible upon increasing the Mn of the PPG chain. Poly(1)′230 showed Tg as high as 253 °C. Poly(1)′230–2000 was thermally stable at approximately 300 °C, presumably due to the existence of rigid phenyleneethynylene moieties.