Aminolysis of polystyrene and poly(methyl methacrylate) (PMMA) prepared by reversible addition−fragmentation chain transfer (RAFT) polymerization was investigated. The product of the former contains predominantly double molecular weight species by the formation of disulfide bond, whereas the latter formed coupled species which consequently cleaved to unimolecular weight species. MALDI−TOF MS (matrix-assisted laser desorption ionization time-of-flight mass spectrometry), elemental analysis and 1H NMR indicated that thiolactone terminus was formed after aminolysis of PMMA. We propose that the thiol end groups generated during the aminolysis of PMMA tend to cyclize through “backbiting” to form thiolactone structure. A similar reaction was observed in the case of poly(N, N-dimethylaminoethyl methacrylate) and poly(laury methacrylate). Despite this, the preparation of thiol-end functionalized PMMA was achieved by introducing a short block of polystyrene after the RAFT polymerization of MMA.

Styrene radical polymerization was carried out in the presence of a polymerizable dithioester, benzyl 4-vinyldithiobenzoate, which possesses a dithioester group and a polymerizable double bond. Branched polystyrene was formed during the polymerization, as indicated by multimodal GPC curves of the products. The branched polystyrene contains a dithiobenzoate C(dS)S moiety at each branch point and thus can be analyzed by cleavage with amine. After cleavage, the GPC profiles became narrow. The molecular weight of the cleaved product increased linearly with monomer conversion, illustrating a living fashion of the polymerization. Solution property obtained by simultaneous online measurements of viscosity and light scattering indicates that the viscosity of the branched product decreased remarkably as compared to the linear polystyrene of equivalent molecular weight. The copolymerization behavior of styrene and benzyl 4-vinyldithiobenzoate was investigated by FT-IR monitoring during the polymerization. The results show that the latter was incorporated homogeneously into polystyrene chain. Therefore, branched polystyrene was synthesized with controlled architecture in the light of the length and narrow distribution of primary chains as well as the degree and the distribution of branching along the polymer chain.

The kinetics and chain length distributions occurring in living free-radical polymerizations are simulated using a hybrid Monte Carlo algorithm. The new algorithm is much faster than the conventional one because the activation/deactivation exchange reactions, which are CPU intensive, are treated by a biased-sampling method with an analytical expression for the exchange equilibrium, while the reactions of chain propagation, irreversible chain termination, etc. are treated by exact stochastic Monte Carlo simulation. Two models of living radical polymerizations, i.e., the polymerization initiated by alkoxyamines and the nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxy, mediated radical polymerization, are simulated to study the effects of experimental variables, such as the concentration ratio of stable free radicals to initiators, initiation rate constants, etc., on the kinetics and molecular weight distributions. A comparison between simulated and analytical results in the literature is made. Taking thermal initiation into consideration, the algorithm reproduces the experimental results very well. Therefore, its feasibility and usefulness in studying living free-radical polymerization are demonstrated.

Thermal decomposition of dithioesters, e.g., cumyl dithiobenzoate (CDB), poly(methyl methacrylate) (PMMA) end-capped with dithioester, and 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate (EPDB), and the consequent effect on reversible addition-fragmentation chain transfer polymerization were investigated. The former two dithioesters underwent thermal decomposition at 120 °C. The thermal decomposition yielded unsaturated compound and dithiobenzoic acid, leading to some loss of living character of the polymerization, such as retarded reaction rate and broadened molecular weight distribution. Nevertheless, thermal decomposition of EPDB, a model compound for PMMA dithioester, does not yield unsaturated product despite the resemblance of the chemical structures. Thermogravimetric analysis shows that PMMA dithioesters are more thermally unstable than the other two.

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Nanodiamond (ND) particles were functionalized with V-shaped polymer brushes of polystyrene and poly(t-butyl methacrylate) (PtBMA) through the reaction of surface carboxylic groups of NDs toward epoxy functionalities located in the middle of the polymer precursor, an ABC-type triblock copolymer. The block copolymer was prepared through sequential radical polymerizations of tBMA, glycidyl methacrylate (GMA), and styrene mediated by a reversible addition−fragmentation chain transfer process, in which the lengths of different segments are well-controlled by virtue of the living nature of the reaction. The polymerization product, PtBMA-b-PGMA-b-PS, carried a short block of PGMA in the middle, which was used for subsequent reaction with -COOH on the convex surface of the NDs. This grafting-onto approach through a center-linking process functionalized nanoparticles with V-shaped polymer brushes possessing an exact 1:1 molar ratio of different arms. Furthermore, ND particles with amphiphilic functionalities were prepared after hydrolysis of the PtBMA segment. The obtained polymer grafted ND was characterized by electron microscopy (TEM and SEM), NMR and IR spectroscopy, and TGA. The product not only formed stable dispersions in organic solvents such as tetrahydrofuran, toluene, and chloroform but also self-assembled at oil−water interfaces to form flat films or large droplets of water-in-oil and oil-in-water. The mechanism of self-assembly at liquid−liquid interfaces is discussed.

SUMMARYIn this paper, the basic principle and a Monte Carlo method are described for numerically simulating the chain-length distribution in radical polymerization with transfer reaction to monomer. The agreement between the simulated and analytical results shows that our algorithm is suitable for systems with transfer reaction. With the simulation algorithm, we confirm that transfer reaction has a similar effect as disproportionation on the molecular weight distribution in radical polymerization with continuous initiation. In the pulsed laser (PL) initiated radical polymerization with transfer reaction, the 'waves' on the chain-length distribution profile become weaker as the ratio of transfer reaction rate constant, k,, , to the propagation rate constant, k,, is increased in the case with either combination-type or disproportionation-type termination. Moreover, it seems that the combination termination has a broadening effect on the waves. Therefore, k , can also be determined by precisely locating the inflection point Lo on the chain-length distribution profile for radical polymerization with transfer reaction, unless k,, is large enough to smear out the waves on the chain-length distribution.

The thermal decomposition of different classes of RAFT/MADIX agents, namely dithioesters, trithiocarbonates, xanthates, and dithiocarbamates, were investigated through heating in solution. It was found that the decomposition behavior is complicated interplay of the effects of stabilizing Z-group and leaving R-group. The mechanism of the decomposition is mainly through three pathways, i.e., β-elimination, α-elimination, and homolysis of dithiocarbamate (particularly for universal RAFT agent). The most important pathway is the β-elimination of thiocarbonylthio compounds possessing β-hydrogen, leading to the formation unsaturated species. For the leaving group containing solely α-hydrogen, such as benzyl, α-elimination takes place, resulting in the formation of (E)-stilbene through a carbene intermediate. Homolysis occurs specifically in the case of a universal RAFT agent, in which a thiocarbonyl radical and an alkylthio radical are generated, finally forming thiolactone through a radical process. The stabilities of the RAFT/MADIX agents are investigated by measuring the apparent kinetics and activation energy of the thermal decomposition reactions. Both Z-group and R-group influence the stability of the agents through electronic and steric effects. Lone pair electron donating heteroatoms of Z-group show a remarkable stabilizing effect while electron withdrawing substituents, either in Z-or R-group, tends to destabilize the agent. In addition, bulkier or more β-hydrogens result in faster decomposition rate or lower decomposition temperature. Thus, the stability of the RAFT/MAIDX agents decreases in the order where R is (with identical Z = phenyl) ÀCH 2 Ph (5) > ÀPS (PS-RAFT 15) > ÀC(Me)HPh (2) > ÀC(Me) 2 C(dO)OC 2 H 5 (7) > ÀC(Me) 2 Ph(1) > ÀPMMA (PMMA-RAFT 16) > ÀC(Me) 2 CN (6). For those possessing identical leaving group such as 1-phenylethyl, the stability decreases in the order of O-ethyl (11) > ÀN(CH 2 CH 3 ) 2 (13) > ÀSCH(CH 3 )Ph (8) > ÀPh (2) > ÀCH 2 Ph (4) > ÀPhNO 2 (3). These results consort with the chain transfer acitivities measured by the CSIRO group and agree well with the ab initio theoretical results by Coote. In addition, the difference between thermal stabilities of the universal RAFT agents at neutral and protonated states has also been demonstrated.