The rheological properties of a polyisobutylene (PIB) having a molecular weight five times its entanglement molecular weight were measured, along with interrupted shear flow experiments to quantify the time required for structural recovery following steady state shearing in the non-Newtonian regime. The reentanglement kinetics was substantially slower (by more than an order of magnitude) than expectations based on the linear relaxation time of the fully entangled material. This result is in accord with published interrupted shear flow results for concentrated polymer solutions. The retarded recovery of entanglements may underlie other anomalies, for example, the delayed development of autoadhesion (tack) in rubber, the behavior of freeze-dried dilute solutions and the limiting behavior of the molecular weight dependence of the viscosity. It also offers a route to more processable polymers and, being reversible, will not affect the ultimate physical properties.