Does bimolecular termination dominate in benzoyl peroxide initiated styrene free-radical polymerization?

It is believed that polymer chains mainly terminate through bimolecular termination in free-radical polymerization, however, there is few convincing evidence to support it. In benzoyl peroxide (BPO) initiated styrene free-radical polymerization, our studies demonstrate that polymer chain terminates dominantly through primary radical termination rather than by bimolecular termination as described in polymer textbooks. Even at very high primary radical concentration or at very low monomer concentration, primary radical termination still dominates (FPRT&CTI>70%) besides the occurrence of some bimolecular coupling terminations. Two kinds of model polymers were prepared, one for bimolecular coupling termination, and the other for primary radical termination. The 1H-, 13C-, DEPT135-, and 2D HSQC NMR spectra of the polystyrene initiated by BPO are almost the same as those of the model polystyrene for primary radical termination, but they are quite different from those of the model polystyrene for bimolecular coupling termination, demonstrating directly that primary radical termination rather than bimolecular coupling termination dominates in BPO-initiated styrene free-radical polymerization. [Display omitted] •Primary radical termination dominates in styrene free-radical polymerization.•Steric effect of the polymer chain radical end restrains bimolecular termination.•Steric effect mainly comes from the polymer chain radical coil itself.•The longer the polymer chain, the greater the steric effect.•As for other monomers, bimolecular termination also most likely does not dominate.