A semicrystalline poly(trimethylene monothiocarbonate) (PTMMTC) has been synthesized via the selective and alternating copolymerization of carbonyl sulfide and oxetane. This reaction was catalyzed by (salen)CrCl accompanied by organic bases over a wide range of temperatures from 40 to 130 degrees C. PTMMTC is shown to exhibit similar crystallization behavior to high-density polyethylene (HDPE), be., being spherulite and possessing melting temperatures (T-m) up to 127.5 degrees C and a degree of crystallinity (X-c) of up to 71%. Moreover, PTMMTC has a wide processing temperature window of ca. 100 degrees C.