Graft polymerization is a widely studied field, used to alter the surface properties of a material for the purpose of expanding its applicability. The characteristic of the modified surface does not depend only on the grafted layer thickness and the polymer properties, but also on the uniformity and grafting density of the layer. Controlled radical polymerization is used to create an even polymer layer with predetermined characteristics. The control in this polymerization is provided through equilibrium between radical and dormant chains. One type of controlled radical polymerization is reversible addition fragmentation chain transfer polymerization, RAFT.
In their new work, Shiping Zhu and co-workers at McMaster University and at Jiaxing University propose mechanisms by which radical chain ends migrate around the surface in RAFT polymerization. The first mechanism is termed “rolling” migration because the radical does not leave the surface during migration. It moves by reacting with neighboring surface dormant. The rate of “rolling” migration depends on how crowded the surface is.
The second mechanism is termed “hopping” migration because the mechanism involves surface radicals reacting with solution dormant and for solution radicals reacting with surface dormant. The rate of “hopping” migration depends on the solution dormant chains concentration.
The radical migration promotes the irreversible termination by increasing the probability at which a radical comes into the vicinity of the other. Significant termination affects the properties of the polymer layer, and could be used to explain experimental discrepancies in the literature. The effects of graft density and concentration of solution dormant chains on the properties of the grafted polymers are studied through simulation.