Shape Control over the Polymer Molecular Weight Distribution and Influence on Rheological Properties

The shape, breadth, and average molecular weight of the overall molecular weight distribution (MWD) largely define polymer properties. In conventional free-radical polymerization, control over this distribution is through the many competing kinetic pathways dominated by radical termination events. "Living" radical polymerization mechanistically minimizes these termination events, providing a facile route to a desired Gaussian distribution with the distribution breadth dependent upon the activity of the catalyst or modulating agent. However, producing unusually shaped distributions can only be achieved through modeling of the complex polymerization kinetics and invoking feeding and other methods. Here, we construct square, slanted, and chair-like MWDs by blending two to four polymers made using a low-reactive RAFT agent with dispersities close to 2. The synthesis of these polymers, unlike that of polymers made with high-reactive RAFT agents, is simple, scalable, and importantly reproducible as the MWD is independent of conversion, making this polymerization method virtually and kinetically model-free. The blending method described here overcomes many of the difficulties in producing unusually shaped MWDs and allows control over the shape and breadth of the MWD. The concept further provides a general synthetic strategy for studying important structure–property relationships of polymers with desired processing and performance characteristics. This is demonstrated by measurement and modeling analysis of the linear viscoelastic properties of selected samples, which provides a way to tailor the properties of polymers by controlling the form of their MWD via blending. Unlike conventional approaches analyzing the effects of the MWD, its actual shape is considered and its effect on the properties is addressed.