Optimum reaction conditions for the synthesis of macromonomers via the high-temperature polymerization of acrylates

Macromonomers are valuable synthetic building blocks: They can be copolymerized with low molecular weight monomers to generate brush-like structures or serve as conjugation substrates in pericylic, metathesis, and thiolene reactions. Based on earlier reports on the facile high temperature formation of macromonomers from acrylates, a complex kinetic model is developed which accounts for the key reactions constituting the macromonomer formation process. On the basis of the kinetic model, the important rate coefficients governing acrylate polymerization (e.g., β-scission and termination rate coefficients of midchain radicals, backbiting and intramolecular chain transfer rate coefficients) as well as the reaction conditions (e.g., initial monomer concentration, reaction temperature, radical flux) are systematically varied and their influence on the synthetic success is critically evaluated. The systematic coefficient variation reveals that there exist optimum reaction conditions under which the high temperature macromonomers formation may be conducted with maximum success. The present study provides a concise summary of these conditions. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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

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 &lt cM0 &lt 7.1 mol L-1 in a wide temperature range (0 &lt T &lt 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)

Mapping free radical reactivity: A high-resolution electrospray ionization-mass spectrometry study of photoinitiation processes in methyl methacrylate free radical polymerization

High-resolution electrospray ionization-mass spectrometry (ESI-MS) was applied to study the polymeric product spectrum generated by the pulsed laser polymerization (PLP) of methyl methacrylate (MMA) at temperatures ≤0 °C in the presence of the photoinitiators 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzoin, benzil, benzoin ethyl ether (BEE), and 2,2- azobisisobutylnitrile (AIBN). Termination products, both combination and disproportionation, were identified with high accuracy. Both the benzoyl and acetal fragments generated as a result of DMPA photocleavage were found to initiate and highly likely terminate polymerization. Under the conditions studied, the acetal radical produced upon DMPA photolysis fragments further to yield methyl radicals which seem to act predominantly as terminating moieties. Both the benzoyl and ether fragments produced as a result of benzoin photocleavage were found to act as initiating and probable terminating species, indicating that the ether radical fragment does not act exclusively as a terminating species. Additionally, increasing laser intensity and/or irradiation repetition rate (i.e., energy deposition into the system) results in more complex product distributions of the MMA polymers produced via photoinitiation (with the exception of AIBN). Temperature was determined to have a minor influence on the resulting product distribution under the conditions examined. © 2007 American Chemical Society

Depropagation kinetics of sterically demanding monomers: A pulsed laser size exclusion chromatography study

Conventional pulsed laser polymerization coupled with size exclusion chromatography (PLP-SEC) as well as multipulse pulsed laser polymerization has been employed to study the depropagation kinetics of the sterically demanding 1,1-disubstituted monomers dicyclohexyl itaconate (DCHI), dibutyl itaconate (DBI), and dimethyl itaconate (DMI). The effective rate coefficient of propagation, k(p)(eff) was determined in bulk and solution of cyclohexanone (DCHI) and N-methylformamide (DMI) for monomer concentrations between 1.5 < c(M)(0) < 7.1 mol L-1 in the temperature range 0 < T < 90 degrees C. The resulting Arrhenius plots (i.e., In k(p)(eff) vs 1/RT) displayed a significant curvature in the higher temperature regimes and were analyzed in their respective linear parts to yield the activation parameters of the forward reaction. In the temperature region where no depropagation was observed, the following set of Arrhenius parameters for k(p) were obtained for the bulk systems: DCHI (E-p 26.5 M mol(-1), In A(p)/L mol(-1) s(-1) = 11.5), DBI (E-p = 21.3 kJ mol(-1), In A(p)/L mol(-1) s(-1) = 10.4), DMI (E-p 27.8 kJ mol(-1), In A(p)/L mol(-1) s(-1) = 13.5). In addition, the k(p)(eff) data were analyzed in the depropagation regime for DCHI, resulting in estimates for the associated enthalpy and entropy (Delta H = -53.5 M mol(-1) and AS = -142.3 J mol(-1) K-1) of polymerization. The value for the heat of polymerization was independently measured via on-line differential scanning calorimetry (DSC) as well (Delta H = -55.0 kJ mol(-1)). Delta H of DBI and DMI were also determined via DSC or are available in the literature (Delta H = -42.0 and -60.5 M mol(-1)). These numbers were used to determine the respective entropies of polymerization for both monomers (Delta S = -110 and -156 J mol(-1) K-1) by a fitting procedure of the k(p)(eff) data. DBI polymerization displays significantly different activation parameters as well as thermodynamic properties in comparison with the corresponding DCHI and DMI polymerizations. With decreasing monomer concentration, it is increasingly more difficult to obtain well-structured molecular weight distributions. The DCHI system displayed a significant reduction in k(p)(eff) with increasing cyclohexanone concentration

Laser induced marking of polymer chains with radical spin traps

A pathway for marking of polymer chains with radical spin traps during pulsed laser polymerization in free radical polymerization is presented. By introducing a so-called marker that forms a non-propagating radical at (or shortly after) the incidence of a laser pulse, a polymer subdistribution is generated by specifically terminating propagating radicals via combination with such a marker radical. The generated polymer subdistribution can subsequently be imaged by modern softionization mass spectrometry. Herein, the general methodology of the method in which such marker is generated via reaction of an initiating radical with a nitrone is demonstrated on the examples of BA and VAc. © 2008 WILEY-VCH Verlag GmbH &amp; Co. KGaA

Depolymerization kinetics of di(4-tert-butyl cyclohexyl) itaconate and Mark-Houwink-Kuhn-Sakurada parameters of di(4-tert-butyl cyclohexyl) itaconate and di-n-butyl itaconate

Multipulse pulsed laser polymerization coupled with size exclusion chromatography (MP-PLP-SEC) has been employed to study the depropagation kinetics of the sterically demanding 1,1-disubstituted monomer di(4-tert-butylcyclohexyl) itaconate (DBCHI). The effective rate coefficient of propagation, kp eff, was determined for a solution of monomer in anisole at concentrations, cM 0, 0.72 and 0.88 mol L-1 in the temperature range 0 ≤ T ≤ 70°C. The resulting Arrhenius plot (i.e., ln kp eff vs. 1/RT) displayed a subtle curvature in the higher temperature regime and was analyzed in the linear part to yield the activation parameters of the forward reaction. In the temperature region where no depropagation was observed (0 ≤ T ≤ 50°C), the following Arrhenius parameters for kp were obtained (DBCHI, Ep = 35.5 ± 1.2 kJ mol-1, ln Ap = 14.8 ± 0.5 L mol-1 s-1). In addition, the k p eff data was analyzed in the depropagatation regime for DBCHI, resulting in estimates for the associated entropy (-ΔS = 150 J mol-1 K-1) of polymerization. With decreasing monomer concentration and increasing temperature, it is increasingly more difficult to obtain well structured molecular weight distributions. The Mark-Houwink-Kuhn- Sakurada (MHKS) parameters for di-n-butyl itaconate (DBI) and DBCHI were determined using a triple detection GPC system incorporating online viscometry and multi-angle laser light scattering in THF at 40°C. The MHKS for poly-DBI and poly-DBCHI in the molecular weight range 35-256 kDa and 36.5-250 kDa, respectively, were determined to be KDBI = 24.9 (103 mL g-1), αDBI = 0.58, KDBCHI = 12.8 (10 3 mL g-1), and αDBCHI = 0.63. © 2006 Wiley Periodicals, Inc