Branching determination from radius of gyration contraction factor in radical polymerization

- Zimm & Stockmeyer's model (1949) strictly applies to end-to-end branching.
- New stochastic 'backtracking' model links chemistry to branching topology.
- Polymers with ldPE-like branching topology are more compact.
- Disproportionation termination leads to compacter polymers than recombination.
- Branching characterization by SEC-MALS sensitive to chemical mechanism assumed.

This paper proposes a set of models to calculate contraction factor, which to maximum extent accounts for the kinetics of radical polymerization with transfer to polymer and recombination termination. The models are alternatives to the Zimm and Stockmayer's (1949) analytical expression of contraction factor for molecules with terminal branching. The results, being representative for polymers like as low-density Polyethylene (ldPE), show significantly stronger contraction than predicted by the model of Zimm and Stockmayer. In the case of termination by disproportionation only, molecular sizes turn out to be smaller by a factor of almost two. In presence of recombination termination molecules are less compact. It is shown that the interpretation of contraction factors as measured by the Size Exclusion - Multi-Angle Light Scattering to find the branchedness of ldPE, with the new model would lead to a considerably lower estimate of branching than by using Zimm and Stockmayer's model.