Contrasting melt memory of homopolymers and random ethylene copolymers using halogen substitution with precision placement or random distribution

Polyethylenes with Cl or Br atoms placed at an equal distance of 21 or 15 backbone carbons are known to crystallize as homopolymers, accommodating the halogen in layered crystallites. In contrast, analogs with a random distribution display a crystallization path dominated by sequence-length selection. A consequence of the sequence selection of random copolymers is a constrained interlamellar region and broader melting peaks displaced at higher temperatures than systems with the precise placement. Precision and random ethylene-vinyl halides are excellent models to contrast the strong melt-memory behavior observed in random ethylene 1-alkene copolymers with lack of melt-memory seen in linear polyethylene. While precision polyethylenes with Cl or Br placed on each 21^st or 15^th backbone carbon show negligible deviation in crystallization rate above the observed melting, the increase in crystallization rate of analogs with the random distribution is observed even from melts 60 degrees above the observed melting point. These data give further evidence of the sharp difference of melt-memory behavior between homopolymers and random copolymers.