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 (I-2) was studied at different temperatures (T = 70 degrees C and 80 degrees 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 H-1 and C-13 NMR spectroscopy). For instance, PMMA samples of M-n,M-SEC = 2300 g mol(-1) and polydispersity index PDI = M-w/M-n = 1.5 (M-n,M- theoretical = 2300 g mol(-1)), M-n,M-SEC = 4600 g mol(-1) and PDI = 1.6 (M-n,M- theoretical = 4700 g mol(-1)), M-n,M- SEC = 9600 g mol(-1) and PDI = 1.6 (M-n,M- theoretical = 10 000 g mol(-1)), and M-n,M-SEC = 19 200 g mol(-1) and PDI = 1.5 (M-n,M-theoretical = 18400 g mol(-1)) were successfully prepared by RITP. The polymerization was followed by on-line H-1 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 omega-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 approximate to 2.6 at T = 80 degrees 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%.