NMR studies on the mechanism of iodine mediated polymerisation

[摘要] ENGLISH ABSTRACT: In reverse iodine transfer polymerisation (RITP), chain transfer agents (CTAs) aregenerated in situ from the reaction between 2,2'-azobis(isobutyronitrile) (AIBN) andmolecular iodine. This stage of RITP is the inhibition period, which ends when alliodine has been consumed. The evolution of CTAs was studied for thepolymerisation reactions of n-butyl acrylate and styrene respectively. RITP of n-butylacrylate was performed at 70 °C. In situ 1H nuclear magnetic resonance (NMR)experiments were carried out to study the evolution of CTAs during the inhibitionperiod of n-butyl acrylate polymerisation and the structures A-I and A-Mn-I (where Arepresents the moiety originating from AIBN, M represents the monomer unit and n isthe mean number degree of polymerisation) were observed. A polymer with thegeneral structure A-Mm-I is formed. The molecular weight of poly(n-butyl acrylate)(PnBA) was evaluated with size exclusion chromatography (SEC) and NMR.Structural analysis of PnBA was done using NMR spectroscopy and matrix-assistedlaser desorption/ionisation time-of-flight (MALDI-ToF) mass spectrometry. Similarconditions to those used for n-butyl acrylate polymerisation were used for RITP ofstyrene. The evolution of CTAs during the inhibition period of styrene polymerisationwas studied using in situ 1H NMR. The inhibition period of styrene polymerised byRITP was much shorter than expected. This is due the consumption of iodine in thereaction between styrene and iodine which reversibly forms 1,2-diiodo-ethyl benzene.The CTAs A-I and A-Mn-I are formed, as well as 1-phenylethyl iodide (1-PEI). Themolecular weight of polystyrene (PS) was determined using SEC and NMR and thefunctionality was evaluated using 1H NMR. The structure of PS was confirmed with1H NMR and MALDI-ToF mass spectrometry. By increasing the temperature of thereaction, the inhibition period can be shortened. Both polymerisation systems retaincontrol over molecular weight with an increase in temperature, however, n-butylacrylate is limited due to the possible formation of mid-chain radicals. The formationof an A–Mm–A population (direct combination of the initiator and styrene) in RITP ofstyrene results in more initiator being consumed than for n-butyl acrylate, despitelimited conversion of styrene to polymer.

[发布机构] Stellenbosch University