Julia Krstina

Living free radical polymerization with reversible addition - fragmentation chain transfer (the life of RAFT)

Free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) is discussed with a view to answering the following questions: (a) How living is RAFT polymerization? (b) What controls the activity of thiocarbonylthio compounds in RAFT polymeriza- tion? (c) How do rates of polymerization differ from those of conventional radical polymerization? (d) Can RAFT agents be used in emulsion polymerization? Retardation, observed when high concentra- tions of certain RAFT agents are used and in the early stages of emulsion polymerization, and how to overcome it by appropriate choice of reaction conditions, are considered in detail. Examples of the use of thiocarbonylthio RAFT agents in emulsion and miniemulsion polymerization are provided. # 2000 Society of Chemical Industry

“Weak links” in polystyrene—thermal degradation of polymers prepared with AIBN or benzoyl peroxide as initiator

Abstract Polystyrene has been prepared with 13 C-labelled initiators and 13 C NMR spectroscopy used to evaluate the role of initiator-derived functionality in the thermal degradation at 300°C. Changes in the molecular weight and molecular weight distribution were measured by GPC. For polymers prepared with benzoyl- carbonyl - 13 C peroxide or benzoyl-α- 13 C peroxide, the end groups incorporated by head addition or transfer to initiator/primary radical termination (secondary benzoate esters) undergo rapid elimination at 300° and must be considered potential “weak links”. However, benzoate end groups formed by “tail” addition to monomer (primary benzoates) are relatively stable at 300° and are not “weak links”. Phenyl end groups are also stable at 300°. Experiments with model compounds show that the unsaturated ends of the type formed by elimination of the benzoate end groups are labile at 300°. Studies on polymers prepared with AIBN-α- 13 C show that both cyanoisopropyl ends and copolymerized methacrylonitrile appear stable at 300° and are unlikely to be “weak links”. Possible causes of anomolous behaviour of high conversion polystyrene prepared with benzoyl peroxide with respect to the variation in molecular weight distribution on thermolysis are discussed in terms of the initiator-derived and other structural irregularities.

Further studies on the thermal decomposition of AIBN : implications concerning the mechanism of termination in methacrylonitrile polymerization

Abstract The products formed during thermal decomposition of AIBN-α- 13 C at 60° in benzene and dimethyl sulphoxide have been identified and quantified by 13 C-NMR. Analysis of the oligomeric products demonstrates that there is selectivity in the reaction of oligo(methacrylonitrile) radicals (5) with cyanoisopropyl radicals (1). There is a marked preference for C-N coupling over C-C coupling and the major product observed results from combination of the nitrogen of 5 with the carbon of 1. The reaction kinetics of AIBN decomposition and product formation have been analysed to yield absolute rate constants (60°, benzene) for ketenimine decomposition and for the addition of 1 to methacrylonitrile. The implications of our findings are discussed with respect to the kinetics and mechanism of methacrylonitrile and other polymerizations.

Effects of solvent on model copolymerization reactions. A 13C-NMR study

Abstract Relative reactivities of methyl methacrylate (MMA) styrene and vinyl acetate (VAc) towards the 2-methyl-2-(methoxycarbonyl)ethyl (1) and cyanoisopropyl (2) radicals have been determined in three solvents (benzene, ethyl acetate and methanol). The experiments involved 13 C-NMR analysis of the 13 C-labelled end-groups of copolymers formed with either azo-bis-(methylisobutyrate-α- 13 C) or azo-bis(isobutyronitrile-α- 13 C) as initiator. Relative reactivities of styrene vs MMA and MMA vs VAc were the same in benzene and ethyl acetate. Use of methanol as solvent was found to enhance slightly the relative reactivity of styrene vs MMA towards both radicals. The significance of these results is discussed in relation to the mechanism by which solvents modify reactivity ratios in radical copolymerization.

Effect of ethyl aluminium sesquichloride on the specificity of the reactions of 1-methyl-1-methoxycarbonylethyl radical

SummaryCopolymerizations of styrene and methyl methacrylate (MMA) have been carried out with azo-bis-(methyl isobutyrate-α-13C) (AIBMe-α-13C) as initiator and in the presence of ethyl aluminium sesquichloride (EASC). The specificity of the 1-methyl-1-(methoxycarbonyl)ethyl radical (I) for reaction with styrene over MMA is substantially enhanced by the presence of very low concentrations of EASC. This behaviour is consistent with the involvement of a complexes radical and suggests that such species may play an important role in Lewis acid induced alternating copolymerization.

Supercritical carbon dioxide as a solvent for deposition of a tailored dye in dye sensitized solar cells

We report a new technique whereby dyes can be deposited onto metal oxide surfaces using supercritical carbon dioxide (scCO2) for use in solar cell applications. This process eliminates the need for hazardous organic solvents and waste solvents generated during the dyeing process. The solubility of a perylene anhydride dye in scCO2 is enhanced by the incorporation of fluorinated alkyl subsituents and the use of masked carboxylic binding groups. This allows fast deposition of the dye onto the TiO2 photoanode, resulting in efficient photovoltaic performance. The unreacted dye is then easily recovered in a solid form after the deposition process by venting the carbon dioxide.