Benjamin B. Noble

First principles modelling of free-radical polymerisation kinetics

Computational quantum chemistry can make valuable contributions to modelling and improving free radical polymerisation. At a microscopic level, it can assist in establishing reaction mechanisms and structure-reactivity trends; at a macroscopic level it can be used in the design and parameterisation of accurate kinetic models for process optimisation and control. This review outlines and critically evaluates various methodological approaches that have been employed in first principles prediction of rate coefficients in free radical polymerisation, examining in turn the choice of chemical model, electronic structure method, solvation modelling and the coupled issues of partition function evaluation and conformational analysis. It is shown that accurate and reliable predictions are possible but only if necessary precautions are taken into account. The practical value of accurate computational modelling of radical polymerisation kinetics is then illustrated through three representative case studies from the literature in which theory has been used to develop accurate kinetic models: free radical copolymerisation kinetics; defect structure formation in radical suspension polymerisation of vinyl chloride; and reversible addition fragmentation chain transfer polymerisation.

The effect of LiNTf2 on the propagation rate coefficient of methyl methacrylate

In the present work we use accurate Pulsed Laser Polymerization (PLP) to measure the influence of various concentrations of lithium bis(trifluromethane)sulfonamide (LiNTf2) on the propagation rate coefficient of methyl methacrylate (MMA). We also perform 1H-NMR analysis to evaluate the effect of LiNTf2 on poly(MMA) stereochemistry. Additionally, we perform high-level quantum-chemical calculations to model the interactions between Li+ and the MMA monomer and propagating radical. Across a broad range of concentrations, LiNTf2 only slightly increases the isotacticity of the resultant poly(MMA). However, a significant increase in the propagation rate coefficient was noted upon addition of LiNTf2. The magnitude of this increase was found to be dependent on the LiNTf2 concentration and temperature. Theoretical calculations reveal the complexities associated with Lewis acid-mediated stereocontrol. On the basis of this theoretical work, we suggest that the potential stereocontrol afforded by Lewis acids is being hindered by their action as propagation catalysts though non-stereoselective binding modes.

Wavelength Dependence of Light-Induced Cycloadditions

The wavelength-dependent conversion of two rapid photoinduced ligation reactions, i.e., the light activation of o-methylbenzaldehydes, leading to the formation of reactive o-quinodimethanes (photoenols), and the photolysis of 2,5-diphenyltetrazoles, affording highly reactive nitrile imines, is probed via a monochromatic wavelength scan at constant photon count. The transient species are trapped by cycloaddition with N-ethylmaleimide, and the reactions are traced by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The resulting action plots are assessed in the context of Beer-Lamberts law and provide combined with time-dependent density functional theory and multireference calculations an in-depth understanding of the underpinning mechanistic processes, including conical intersections. The π → π* transition of the carbonyl group of the o-methylbenzaldehyde correlates with a highly efficient conversion to the cycloadduct, showing no significant wavelength dependence, while conversion following the n → π* transition proceeds markedly less efficient at longer wavelengths. The influence of absorbance and reactivity has critical consequences for an effective reaction design: At high concentrations of o-methylbenzaldehydes (c = 8 mmol L-1), photoligations with N-ethylmaleimide (possible for λ ≤ 390 nm) are ideally performed at 330 nm, whereas at high light penetration regimes at lower concentrations (c = 0.3 mmol L-1), 315 nm irradiation leads to the highest conversion. Activation and trapping of 2,5-diphenyltetrazoles (possible for λ ≤ 322 nm) proceeds best at a wavelength shorter than 295 nm, irrespective of concentration.

The effects of Lewis acid complexation on type I radical photoinitiators and implications for pulsed laser polymerization

In the present work, we examine the effects of zinc chloride (ZnCl2) and aluminium chloride (AlCl3) complexation on the photochemistry of two well-known type I photoinitiators, methyl-4′-(methylthio)-2-morpholinopropiophenone (MMMP) and 2,2-dimethoxy-2-phenylacetophenone (DMPA). High-level ab initio calculations and experimental results demonstrate that Lewis acid complexation has a significant effect on the individual processes that comprise radical photoinitiation. Theoretical calculations predict that ZnCl2 coordinates to MMMP and DMPA to form thermodynamically stable bidentate ketone–amine and ketone–ether chelates, respectively. Meanwhile, the AlCl2+ cation coordinates to MMMP and DMPA to form a tridentate ether–amine–ketone chelate and a bidentate ketone–ether chelate, respectively. We found that addition of ZnCl2 and AlCl3 to solutions containing MMMP significantly increase its molar extinction coefficient (e) between 350–360 nm. In contrast, the complexation of either ZnCl2 or AlCl3 to DMPA slightly reduces the value of e in the 350–360 nm range. Time dependent density functional theory (TD-DFT) calculations demonstrate that Lewis acid complexation blue shifts the nπ* excitations of both DMPA and MMMP, while concurrently red shifting the ππ* transitions. Complexation also significantly alters the stability and reactivity of the photoinitiator fragment radicals. Lewis acid complexation localizes and destabilizes acyl radicals, resulting in significantly increased reactivity towards methyl methacrylate (MMA). In contrast, complexation of Lewis acids dramatically reduces the reactivity of the morpholine substituted isopropyl radical and the dimethoxyphenyl radical towards MMA. Alternative complexation at the methyl ester side-chain of MMA has a beneficial effect on the reactivity of all fragments, increasing addition rate coefficients by 2–4 orders of magnitude. We discuss some of the important implications of these findings for pulsed laser polymerization (PLP), and acetophenone photochemistry more generally.

Discrete and Stereospecific Oligomers Prepared by Sequential and Alternating Single Unit Monomer Insertion

Natural biopolymers, such as DNA and proteins, have uniform microstructures with defined molecular weight, precise monomer sequence, and stereoregularity along the polymer main chain that affords them unique biological functions. To reproduce such structurally perfect polymers and understand the mechanism of specific functions through chemical approaches, researchers have proposed using synthetic polymers as an alternative due to their broad chemical diversity and relatively simple manipulation. Herein, we report a new methodology to prepare sequence-controlled and stereospecific oligomers using alternating radical chain growth and sequential photoinduced RAFT single unit monomer insertion (photo-RAFT SUMI). Two families of cyclic monomers, the indenes and the N-substituted maleimides, can be alternatively inserted into RAFT agents, one unit at a time, allowing the monomer sequence to be controlled through sequential and alternating monomer addition. Importantly, the stereochemistry of cyclic monomer insertion into the RAFT agents is found to be trans-selective along the main chains due to steric hindrance from the repeating monomer units. All investigated cyclic monomers provide such trans-selectivity, but analogous acyclic monomers give a mixed cis- and trans-insertion.