A method of making a polyisobutylene polymer in a recirculating loop reactor with one or more reaction tubes in contact with a heat transfer medium includes: (a) providing a feed mixture consisting essentially of polymerizable monomer and catalyst to a residual reactor stream at a feed rate to form a reaction mixture, the reaction mixture containing less than 5% by weight diluent components; (b) recirculating the reaction mixture in the one or more reaction tubes of the loop reactor at a recirculation rate greater than the feed rate utilizing a recirculating pump operating at a pressure differential, delta P, corresponding to a recirculating flow; (c) polymerizing the reaction mixture in the one or more tubes of the loop reactor to convert the feed mixture to polyisobutylene polymer while cooling the one or more tubes of the loop reactor with the heat transfer medium; and (e) withdrawing polyisobutylene polymer from the loop reactor.