Philipp Vana

Kinetic Analysis of Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerizations: Conditions for Inhibition, Retardation, and Optimum Living Polymerization

Careful simulations of conversion vs. time plots and full molecular weight distributions have been performed using the PREDICI(R) program package in conjunction with the kinetic scheme suggested by the CSIRO group for the reversible addition fragmentation chain transfer (RAFT) process to probe RAFT agent mediated polymerizations. In particular, conditions leading to inhibition and rate retardation have been examined to act as a guide to optimum living polymerization behavior. It is demonstrated that an inhibition period of considerable length is induced by either slow fragmentation of the intermediate RAFT radicals appearing in the pre-equilibrium or by slow re-initiation of the leaving group radical of the initial RAFT agent. The absolute values of the rate coefficients governing the core equilibrium of the RAFT process - at a fixed value of the equilibrium constant - are confirmed to be crucial in controlling the polydispersity of the resulting molecular weight distributions. A higher interchange frequency effects narrower distributions. It is further demonstrated that the size of the rate coefficient controlling the addition reaction of propagating radicals to polyRAFT agen, k(beta), is mainly responsible for optimizing the control of the polymerization. The fragmentation rate coefficient k(-beta), of the macroRAFT intermediate radical, on the other hand, may be varied over orders of magnitude without affecting the amount of control exerted over the polymerization. On the basis of the basic RAFT mechanism, its value mainly governs the extent of rate retardation in RAFT polymerizations.

Reversible addition-fragmentation chain transfer polymerization initiated with γ-radiation at ambient temperature: an overview

Using gamma-radiation as initiation source at ambient temperatures (i.e. T approximate to 20 degreesC) for reversible addition-fragmentation chain transfer (RAFT) polymerizations allows for the generation of narrowly distributed polymeric material with living characteristics. It is shown that the living characteristics effected by RAFT agent mediated bulk polymerizations using gamma-irradiation are associated with a RAFT mechanism rather than with reversible termination processes. Furthermore, gamma-radiation as initiation source for an appropriate RAFT agent/monomer system allows for effective radical storage and the generation of long-lived reaction intermediates at ambient temperatures. The current overview further demonstrates how the RAFT process together with gamma-radiation as source of initiation can be employed to graft various monomers onto polypropylene surfaces in a controlled manner

Easy Access to Chain-Length-Dependent Termination Rate Coefficients Using RAFT Polymerization

Chain-length-dependent termination rate coefficients of the bulk free-radical polymerization of styrene at 80degreesC are determined by combining online polymerization rate measurements (DSC) with living RAFT polymerizations. Full k(t) versus chain-length plots were obtained indicating a high k(t) value for short chains (2 x 10(9) L . mol(-1) . s(-1)) and a weak chain-length dependence between 10 and 100 monomer units, quantified by an exponent of -0.14 in the corresponding power law [k(t)(i,i)] = k(t)(0) . P(-b).

Is the rate constant of chain propagation kp in radical polymerization really chain-length independent ?

A careful investigation of the k p data obtained from pulsed-laser polymerization at different pulse separations to in a lot of systems has revealed that k p exhibits a slight but significant decrease when t 0 is increased, corresponding to an about 20% decrease of k p extending over several hundreds in degree of polymerization. Transformation of this integral effect to individual chain-lengths reduces this range, of course, but still shows more than one hundred propagation steps to be concerned. This is interpreted in terms of a decrease of the monomer concentration at the site of propagation caused by the segments already added to the growing chain.

Recent Advances in the Kinetics of Reversible Addition Fragmentation Chain-Transfer Polymerization

The formation of complex molecular architectures with low polydispersities by reversible addition fragmentation chain-transer (RAFT) polymerization renders this process of high synthetic utility. Mechanistic and kinetic studies, however, are lacking. The authors here probed the process for long-lived intermediates (see Fig.), the results of which suggested that a stable polymeric radical sink exists.

End-Group Analysis of Polymers by Electrospray Ionization Mass Spectrometry: 2-Methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one Initiated Free-Radical Photopolymerization

End-group analysis of poly(methyl acrylate) and poly(dicyclohexyl itaconate) initiated by 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one (MMMP) has been performed by electrospray ionization mass spectrometry (ESI-MS). MMMP has been found to fragment into two radicals that are near-equally capable of initiating free-radical polymerization processes. The present study indicates the occurrence of significant termination by disproportionation in the polymerization of methyl acrylate. Additionally, it has been demonstrated that dicyclohexyl itaconate undergoes termination by disproportionation almost exclusively.

Planet–Satellite Nanostructures Made To Order by RAFT Star Polymers

The investigation and application of complex nanostructures requires the hierarchical arrangement of distinct domains on a small scale. Herein, we report a method to prepare planet-satellite arrangements using RAFT polymers. Our approach is based on star polymers decorated with trithiocarbonate groups on their outer periphery that attach to gold surfaces and thus provide the polymer with the ability to connect (larger) gold nanoparticle planets with (smaller) gold nanoparticle satellites. By adjusting the molecular weight of the polymeric linker, nanostructures with tailored planet-satellite distances, as evidenced by transmission electron microscopy, are obtained. This strategy offers a straightforward way to prepare gold nanoparticle scaffolds with multiple reactive functionalities at defined distances from the central core.

Shielding effects in polymer-polymer reactions. II. Reactions between linear and star-branched chains with up to six arms.

The shielding effect of surrounding arms and chains on the encounter probability of reactive sites located both at the end of a linear chain and at several positions along the arms of star-branched chains with up to six arms is calculated by means of exact enumeration of samples prepared by Monte Carlo simulation. The changes of parameters that characterize the size and the shape of chain configurations during the approach of reactive centers located at the end of the linear chain and at the center of the star are evaluated. In addition to this specific case, which represents the central reaction step in reversible addition-fragmentation chain transfer star polymerization following the Z-group approach, a general discussion is given on the chain-length dependence of shielding factors associated with distinct segment positions.

Chain‐length dependent termination in pulsed‐laser polymerization, 7. The evaluation of the power‐law exponent b from the chain‐length distribution in the low frequency (single‐pulse) limit for the reference systems styrene and methyl methacrylate in bulk at 25°C

The chain-length distribution (CLD) was examined for polymers prepared by low frequency pulsed laser polymerization (LF-PLP), i.e. for very long pulse separations in the so-called low frequency or single pulse limit. The data were fitted to the theoretical CLD which could be derived in a closed form for a chain-length dependent rate coefficient k t and the parameter b that characterizes this chain-length dependence was determined by this fitting procedure, b values close to 0.2 were obtained for styrene as well as for MMA, indicating a moderate chain-length dependence of k t at low conversions. This result, which is in good agreement with data evaluated by other methods in our laboratory, points to the fact that under these conditions end-segment diffusion is the rate-determining step in bimolecular termination. Factors like moderate chain transfer to monomer and uncertainties with respect to the mechanism of termination (combination or disproportionation) appear to have very little influence on this result.

Mechanism of CPDB-Mediated RAFT Polymerization of Methyl Methacrylate: Influence of Pressure and RAFT Agent Concentration

Reversible addition–fragmentation chain transfer (RAFT) polymerizations of methyl methacrylate (MMA) in bulk at 60°C were performed at five pressures up to 200 MPa using 2-(2′-cyanopropyl)dithiobenzoate (CPDB) as RAFT agent at concentrations between 1.5 × 10–3 and 2.0 × 10–2 mol L–1. Applying high pressure during polymerization increases the rate of polymerization, but no effect on polydispersity was observed. Molecular weight distributions and average molecular weights of the final polymer indicated the successful control of MMA polymerization even at low CPDB concentrations. The slight retardation observed is adequately described by the dependence the termination rate coefficient, kt, on the chain-length.