Scale-free power-law distribution of nonlinear polymers formed in a homeostatic system
Abstract A power-law distribution of molecular weight is found for low-density polyethylene formed in a one-zone autoclave reactor. This reactor could be modelled as a continuously stirred tank reactor (CSTR) operated under homeostatic conditions where the concentration of each component as well as temperature and pressure are kept constant. A simple stochastic model is used to investigate the molecular weight distribution (MWD) formed in free-radical polymerization involving chain transfer to polymer. The MWD profiles are dominated by the Pb value, which represents the probability that the chain end is connected to a backbone chain. By increasing Pb, MWD changes from an exponential to a powerlaw distribution. A scale-free power-law distribution is found for Pb > 0.5. Because the power of the weight fraction distribution, W(M) ~ M -γ, is 1 < γ < 2, the number average molecular weight will reach a stationary-state value but the weightaverage may continue to increase unlimitedly under ideal CSTR conditions.