Depropagation Kinetics of Sterically Demanding Monomers:  A Pulsed Laser Size Exclusion Chromatography Study

Abstract

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°C. The resulting Arrhenius plots (i.e., ln kp 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 (Ep ) 26.5 kJ mol -1 , ln Ap/L mol -1 s -1 ) 11.5), DBI (Ep ) 21.3 kJ mol -1 , ln Ap/L mol -1 s -1 ) 10.4), DMI (Ep ) 27.8 kJ mol -1 , ln Ap/L mol -1 s -1 ) 13.5). In addition, the kp eff data were analyzed in the depropagation regime for DCHI, resulting in estimates for the associated enthalpy and entropy (∆H ) -53.5 kJ mol -1 and ∆S ) -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 (∆H ) -55.0 kJ mol -1 ). ∆H of DBI and DMI were also determined via DSC or are available in the literature (∆H ) -42.0 and -60.5 kJ mol -1 ). These numbers were used to determine the respective entropies of polymerization for both monomers (∆S ) -110 and -156 J mol -1 K -1 ) by a fitting procedure of the kp 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.

Determination of the Propagation Rate Coefficient of Vinyl Pivalate Based on EPR Quantification of the Propagating Radical Concentration

“…More recently, PLP-SEC and modifications of this method have been employed for k p determination of a-substituted acrylates, [13][14][15] methacrylic acid, [16] acrylic acid [17,18] and other water soluble monomers, [19][20][21] itaconates, [22][23][24] and N-vinylcarbazole [25] as well as monomers in supercritical carbon dioxide. [26][27][28] Electron paramagnetic resonance spectroscopy (EPR) enables direct detection, identification and quantification of radical species involved in radical polymerization.…”

Radical Polymerization: Reversing the Irreversible?

“…[7] However, a further experiment using nBA (for which steric reasons for a reverse reaction can certainly be excluded) revealed a similar result. Further experiments suggest that the reversibility is not limited to primary radicals derived from the employed photo- initiator, but extends to benzoyl peroxide and also the frequently employed initiator azobisisobutyronitrile.…”

Radical Copolymerization Kinetics of Bio-Renewable Butyrolactone Monomer in Aqueous Solution

“…It is important to note that the parameter estimation fits the reactivity ratios based on the terminal model (i.e., no depropagation), assuming that the value of k p,SHMeMB remains constant with conversion. In the case of depropagation, the k p,11 value should be considered as an effective value, k p eff , dependent on k p , k dep and monomer concentration [M] as shown in Equation (2) [26], such that r SHMeMB would change with conversion.…”

“…Under these conditions, the propagation and depropagation mechanisms become a reversible reaction pair, with the relative rates (and hence overall rate of polymerization) a function of temperature and monomer concentration. Some monomers that are known to depropagate are BMA [25], itaconates [26], methyl ethacrylate [27], and α-methyl styrene [28], all studied in organic solution. In the presence of appreciable rates of depropagation, the polymerization does not reach full monomer conversion and the reaction can also influence copolymer composition as well as rate, as seen for the radical copolymerization of methyl ethacrylate and styrene (MEA/ST) [29] and BMA/ST at elevated temperature [25].…”