Dimensions of branched polymers formed in simultaneous long-chain branching and random scission

In free-radical olefin polymerizations, the polymer-transfer reactions could lead to chain scission as well as the formation of long-chain branches. The Monte Carlo simulation for free-radical polymerization that involves simultaneous long-chain branching and random scission is used to investigate detailed branched structure. The relationship between the mean-square radius of gyration [s(2)] and degree of polymerization P as well as that between the branching density and P is the same for both with and without random scission reactions-at least for smaller frequencies of scission reactions. The [s(2)] values were larger than those calculated from the Zimm-Stockmayer (Z-S) equation in which random distribution of branch points is assumed, and therefore, the Z-S equation may not be applied for low-density polyethylenes. The elution curves of size exclusion chromatography were also simulated. The molecular weight distribution (MWD) calibrated relative to standard linear polymers is much narrower than the true MWD, and high molecular weight tails are clearly underestimated. A simplified method to estimate the true MWD from the calibrated MWD data is proposed. The MWD obtained with a light scattering photometer in which the absolute weight-average molecular weight of polymers at each retention volume is determined directly is considered a reasonable estimate of the true MWD. (C) 2001 John Wiley & Sons, Inc.*.