AbstractGraft copolymers of butyl rubber (BR) with polytetrahydrofuran (PTHF) were prepared from bromobutyl rubber by a cationic “grafting‐from” reaction initiated by silver perchlorate at allylic bromine sites on the rubber. By studying the influence of variables, such as temperature etc., it was shown that initiation at −20°C for 30 min, followed by polymerisation at 0°C with a [AgClO4]/[Br]ratio of ca. unity, produced near optimum reaction conditions with grafting efficiencies>90%. Copolymers containing upto 40% of grafted PTHF were obtained by this means. The BR‐g‐PTHF copolymers have a two‐phase morphology comprising amorphous BR with crystalline domains of PTHF which function as points of reinforcement in the rubber