Reverse Iodine Transfer Polymerization (RITP) of Methyl Methacrylate

The polymerization of methyl methacrylate (MMA) initiated by 2,2‘-azobis(isobutyronitrile) (AIBN) as radical initiator in the presence of iodine (I₂) was studied at different temperatures (T = 70 °C and 80 °C). This process, called reverse iodine transfer polymerization (RITP), is based on the direct reaction of radicals with molecular iodine. RITP efficiently controls the molecular weight (determined by size exclusion chromatography, SEC) and the structure of the polymer chains (confirmed by ¹H and ¹³C NMR spectroscopy). For instance, PMMA samples of M_n,SEC = 2300 g mol^-1 and polydispersity index PDI = M_w/M_n = 1.5 (M_{n,theoretical} = 2300 g mol^-1), M_n,SEC = 4600 g mol^-1 and PDI = 1.6 (M_{n,theoretical} = 4700 g mol^-1), M_n,SEC = 9600 g mol^-1 and PDI = 1.6 (M_{n,theoretical} = 10 000 g mol^-1), and M_n,SEC = 19 200 g mol^-1 and PDI = 1.5 (M_{n,theoretical} = 18400 g mol^-1) were successfully prepared by RITP. The polymerization was followed by on-line ¹H NMR spectroscopy, and the conversion of iodine was followed by titration with a thiol (thioglycolic acid). It was shown by SEC, titration of iodine, gas chromatography, and NMR analyses that RITP was split into two different periods:  a first “inhibition” period during which iodine is consumed to form very short ω-iodotelomers, and a second period where the polymerization follows the kinetics of a conventional free radical polymerization governed by degenerative chain transfer. The degenerative chain transfer constant was estimated to be C_ex ≈ 2.6 at T = 80 °C. Last, the iodine-end-capped structure of the polymers was demonstrated by different analytical techniques, and the iodine functionality of the PMMA chains was up to 95%.