Living free radical and photo initiation studies of acrylate, methacrylate and itaconate polymerization systems

摘要

This thesis work has focused on the study of itaconate monomers and photo-initiationprocesses in acrylate, methacrylate and itaconate monomer systems. Novel informationpertaining to photo-initiator derived radical species and their reactivity, as well as thebehaviour of itaconate polymerization systems is presented in detail. The knowledge gainedfrom the photo-initiation studies is utilized as a precursor to mark polymer chains usingnitrones as radical spin traps and the applicability of this technique discussed.The sterically hindered monomers dimethyl itaconate (DMI), di-n-butyl itaconate (DBI)and dicyclohexyl itaconate (DCHI) were polymerized via reversible addition fragmentationchain transfer (RAFT) free radical polymerization. The RAFT mediated polymerization ofthese monomers displayed hybrid living behaviour (a mix of conventional and living freeradical polymerization characteristics) of varying degrees depending on the molecularstructure of the RAFT agent employed. DCHI was also polymerized using atom transferradical polymerization (ATRP). The resulting molecular weight distributions are broad forthe RAFT mediated systems (1.2 ≤ PDI ≤ 3.4). The molecular weight distributionsgenerated via the ATRP of DCHI are narrower (1.2 ≤ PDI ≤ 1.5). Chain transfer tomonomer constants for the itaconate monomers DMI, DBI and DCHI have beendetermined at 60 °C (CDMI = 1.4⋅10-3, CDBI = 1.3⋅10-3 and CDCHI = 1.0⋅10-3) and are relativelylarge in comparison to similar 1,1-disubstituted systems, suggesting that the transfer tomonomer reaction is significant. PREDICI® simulations confirm that a significant chaintransfer to monomer step results in broad molecular weight distributions. Viscosity of thepolymerizing system has also been shown to be an important factor in the resulting widthof the molecular weight distributions.Chain extension of RAFT capped pDCHI and pDBI yield molecular weight distributionsthat progressively shift to higher molecular weights. Thermogravimetric analysis (TGA) ofpDCHI-block-pStyrene copolymers indicates thermal degradation in two separate steps forthe pDCHI and pStyrene blocks.Conventional pulsed laser polymerization coupled with size exclusion chromatography(PLP-SEC) as well as multi-pulse pulsed laser polymerization (MP-PLP) has beenemployed to study the depropagation kinetics of DMI, DBI DCHI and di(4-tertbutylcyclohexyl) itaconate (DBCHI). The effective rate coefficient of propagation, kpeff,was determined for DMI, DBI and DCHI in bulk and solution of cyclohexanone (DCHI),N-methylformamide (DMI and DBI) and anisole (DBCHI) for monomer concentrationsbetween 0.7 cM0 7.1 mol L-1 in a wide temperature range (0 T 90 °C). The resultingArrhenius plots (i.e. ln kpeff vs. 1/RT) displayed a significant curvature in the highertemperature regimes and were analyzed in their respective linear and curved sections toyield the activation parameters of the forward and reverse reaction.Mark-Houwink-Kahn Sakurada parameters for pDBI and pDBCHI were determined intetrahydrofuran at 40 °C using triple detection gel permeatation chromatography.High resolution Electrospray Ionization - Quadrupole Ion Trap Mass Spectrometry (ESIMS)was applied to study the polymeric product spectrum generated by the pulsed laserpolymerization (PLP) of methyl methacrylate (MMA), methyl acrylate (MA), butylacrylate (BA) and DMI at temperatures ≤ 0 ºC in the presence of various photo-initiatorsincluding 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzoin, benzil, benzoin ethylether (BEE) 2,2-azobisisobutyronitrile (AIBN) and bis(2,4,6-trimethyl-benzoyl)-phenylphosphinoxide (Irgacure 819) to study the reactivity of primary and potentialsecondary derived radical fragments from photolytically induced fragmentation.Termination products, both combination and disproportionation, were identified with highaccuracy. Results have been compiled in a user friendly table presenting the reactivity ofthe various photolysis product fragments towards the different monomers. Energydeposition into the MA/photo-initiator systems is found to have no influence on the productdistributions of the MA polymers produced via photo-initiation under the conditionsexamined. For various photo-initiators employed, products congruent to that of chaintransfer to monomer species in the DMI photo-polymerizations are observed, conclusivelyillustrating that chain transfer to monomer is a significant reaction pathway in itaconatefree radical polymerizations. Both the benzoyl and acetal fragments generated as a result ofDMPA photo cleavage were found to initiate and highly likely terminate polymerization.Under the conditions studied, the acetal radical produced upon DMPA photolysis fragmentfurther to yield methyl radicals which seem to act predominantly as terminating moieties.Both the benzoyl and ether fragments produced as a result of benzoin photo cleavage werefound to act as initiating and probable terminating species, indicating that the ether radicalfragment does not act exclusively as a terminating species. Additionally, increasing laserintensity and/or irradiation repetition rate (i.e., energy deposition into the system) results inmore complex product distributions of the MMA polymers produced via photo-initiation(with the exception of AIBN). Temperature was determined to have a minor influence onthe resulting product distribution under the conditions examined. Polymerization systemsutilizing Irgacure 819 give complex product spectra due to the formation of secondgeneration radical species resulting in several initiator fragments incorporated into a singlepolymer chain.A novel method utilizing PLP in free radical polymerization has been developed formarking of polymer chains with radical spin traps. By introducing a so-called “marker”(nitroxide derived from a nitrone), which specifically terminates propagating radicals viacombination, a polymer subdistribution is generated which can be measured by ESI-MSand may potentially be utilized to determine propagation rate coefficients of ultimateaccuracy. The general methodology of the technique in which such marker radicals aregenerated via reaction of an initiating radical with a nitrone is demonstrated on theexamples of butyl acrylate (BA) and vinyl acetate (VAc).