Understanding the roles of chemically-controlled and diffusion-limited processes in determining the severity of autoacceleration behavior in free radical polymerization

This study examined the underlying processes determining the severity of autoacceleration behavior in the free radical polymerization (FRP) process. Using a simple kinetic model for describing FRP, analysis of experimental methyl methacrylate (MMA) polymerization data indicates that the severity of autoacceleration behavior for this polymerization system is described by the diffusion limitations of a typical termination event. This study identified the length of the radical chain determining the severity of autoacceleration behavior for the polymerization of MMA. A radical chain that is no more than 10 to 15% the length of the polymethylmethacrylate being produced controls the severity of the autoacceleration. This result quantitatively supports the short-long picture for the termination reaction.; Identification of the radical chain determining the severity of MMA autoacceleration behavior utilized the results of a detailed study of the concentration dependence of polystyrene (PS) diffusion. Analysis of PS diffusion data both measured experimentally via pulsed field gradient-NMR and found in the research literature illustrates a universal complex chain-length dependence for the concentration dependence of diffusion. For unentangled PS diffusion, the concentration dependence of diffusion increases for short chain lengths before reaching an asymptotic level at longer chain lengths. Beyond this asymptotic limit, a strong chain-length dependence of the concentration dependence suggests transient entanglement formation slows the rate of diffusion.; This is the first study to utilize experimentally measured data for the concentration dependence of polymer diffusion to create a kinetic FRP model that incorporates a chain-length dependent termination rate parameter. Analysis of MMA polymerization data with this second kinetic model indicates that the distribution of short radical chain species determines the severity of the autoacceleration behavior. Examination of additional polymerization systems indicates that the more typical autoacceleration behavior is less severe than the classically-studied MMA autoacceleration behavior. Experimental studies support previous results that indicate that chemically-controlled transfer reactions can weaken autoacceleration behavior by altering the distribution of radical species. Analysis of weak autoacceleration behavior observed for styrene polymerization using the FRP models indicates that a variety of chemically-controlled processes contribute to determine the severity of this autoacceleration behavior.