Novel polymers from atom transfer polymerisation mediated by copper(I) Schiff base complexes

The use of copper(I) Schiff base complex catalysed atom transfer polymerisation of methacrylates is described. The use of a range of functional and multi‐functional initiators enables the synthesis of a range of functional and star polymers to be prepared under undemanding synthetic conditions. End capping with silyl enol ethers allows for ω‐functional polymers. The combination of novel initiators, functional monomers and end capping allows an unprecedented array of macromolecular structures to be produced with limited need for protecting group chemistry.     

Shining a light on catalytic chain transfer

The catalytic chain transfer polymerization of styrene is only truly effective when the reaction mixture is exposed to (UV-)light. The apparent chain transfer constant depends inversely on radical concentration and can be increased up to 8000. These results can be explained by combining aspects of both catalytic chain transfer and the formation of cobalt-carbon bonds. For the catalytic chain transfer polymerization of n-butyl acrylate a chain transfer constant of 650 was found. The resulting transfer coefficient has the same order of magnitude as the one for n-butyl methacrylate. This means that the absence of an α-methyl group hardly influences the transfer step itself. Furthermore, the effect of possible impurities on the catalytic chain transfer polymerization of methyl methacrylate is investigated.     

Cobaloxime as a catalytic chain transfer agent

Cobaloxime, synthesized from Co^II acetate and dimethylglyoxime, was used as a chain transfer agent in the free radical polymerization of methyl methacrylate. It proved to be a catalytic chain transfer agent analogous to work recently reported on cobalt porphyrins. Chain transfer constants ranged from 10³–10⁴ and were chain length dependent. Catalytic behavior was demonstrated by calculating turnover numbers which showed that each cobaloxime molecule participated in many hundreds of chain transfer reactions.     

Kinetics of alpha‐methylstyrene oligomerization by catalytic chain transfer

Alpha‐methylstyrene (AMS) can be effectively dimerized by a free‐radical mechanism mediated by the catalytic chain transfer (CCT). Above the ceiling temperature of AMS, 61 °C, the dimer may become almost an exclusive product with only a small percentage of impurity of AMS trimer and tetramer. Kinetics of the AMS oligomerization has two characteristic features. First, the rate of the oligomerization increases with concentration of the CCT catalyst. Second, conversion reaches a plateau at 60–70%. A kinetic scheme explained both effects. Besides AMS, para‐ and methasubstituted AMS can also be dimerized. Orthosubstituted AMS's do not oligomerize. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1366–1376, 2002     

Effect of catalyst partitioning in Co(II) mediated catalytic chain transfer miniemulsion polymerization of methyl methacrylate

The effect of catalyst partitioning over the organic and water phases in the catalytic chain transfer mediated miniemulsion polymerization was investigated and a mathematical model developed to describe the instantaneous degree of polymerization of the formed polymer. Experimental and predicted instantaneous degrees of polymerization prove to be in excellent agreement. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5839–5849, 2008     

The 25th anniversary of catalytic chain transfer

The transfer of hydrogen from a free radical to an olefin is catalyzed by some cobalt chelates. This reaction, when used in free-radical polymerization, can be called catalytic chain transfer (CCT) to a monomer. It allows the manufacture of oligomers that have defined molecular weights and are useful for a variety of applications. Because all the oligomers are getting terminal double bonds, they can behave as macromers. These macromers give rise to the formation of diblock and graft copolymers, telechelic polymers that have the same or different functional groups depending on the conditions and origins of the comonomers. The catalyst structure–property relation is discussed. Redox properties affect the CCT and provide additional leverage in controlling polymerization processes. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1753–1766, 2000     

Well-controlled reversible addition-fragmentation chain transfer radical polymerisation under ultraviolet radiation at ambient temperature

Controlled reversible addition-fragmentation chain transfer radical polymerisation of methyl acrylate was carried out under long-wave (lambda > or = 365 nm) ultraviolet radiation using an acylphosphine oxide as a photoinitiator at ambient temperature; the polymerisation shows a "living" character at high conversions of polymerisation and leads to well-defined polymers with narrow polydispersities (Mw/Mn < 1.1).     

Catalytic chain transfer polymerisation (cctp) of methyl methacrylate: Effect of catalyst structure and reaction conditions on chain transfer coefficient

Catalytic chain transfer polymerisation of methyl methacrylate has been investigated by a range of cobalt(II) complexes. The effect of catalyst structure, reaction temperature and solvent has been examined. At 60°C in the absence of solvent, the chain transfer coefficient, Cs, for MMA and cobaloximes modified by bridging BF₂ groups e.g. 3 is found to be 40,900, this is constant over a wide mass range. Cs is lowered when catalysts of larger cross sectional area are used supporting a diffusion controlled process. Co-ordinating solvents suppress Cs by competing for coordination of the Co(II) effectively reducing the concentration of active species. No conclusions can be drawn regarding the effect of temperature and thus the activation energy of the process. CCTP lowers the observed rate of polymerisation.     

Synthesis of telechelic polymers initiated with selected functional groups by catalytic chain transfer

Catalytic chain transfer is found to be useful for making telechelic oligomers with a variety of initiating groups in a one‐step reaction procedure. Two olefinic components are required, the first being a normal free‐radical‐polymerizable monomer such as a methacrylate. The second is a vicinal or other olefin generally considered to be unreactive in free radical polymerizations. Under conditions of radical polymerization in the presence of a CCT catalyst, the copolymer that results incorporates predominantly one molecule of the second component at the initiation of each polymer chain. The terminal end group is a geminal double bond. This geminal‐disubstituted end group is radically polymerizable and would allow the preparation of functionalized arms on graft polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1911–1918, 2000     

Recent advances in the homogeneous polymerisation of olefins mediated by nickel complexes

A brief overview is presented on recent advances in the application of nickel coordination complexes as mediators of olefin polymerisation; in some cases involving stereo-regular polymer formation where appropriate. Emphasis is also given on recent findings concerning di- and tri-nuclear Ni clusters with details on ethylene, methyl methacrylate or styrene-based monomer polymerisations.     

First report of reversible addition-fragmentation chain transfer (RAFT) polymerisation in room temperature ionic liquids

The Reversible Addition-Fragmentation chain Transfer (RAFT) polymerisation of acrylates, methacrylates and styrene is reported for the first time in room temperature ionic liquids; the acrylate and methacrylate polymerisations show a living character and lead to well-characterised polymers, with narrow polydispersity (< 1.3); in the case of styrene, the insolubility of the polymer in the ionic liquids stops the polymerisation at an early stage.     

Group transfer polymerisation in hydrophobic ionic liquids

For the first time, group transfer polymerisation of methyl methacrylate (MMA) has been successfully carried out at ambient temperatures in an ionic liquid to produce living polymers of improved polydispersity.     

Substituent effects in the catalytic chain transfer polymerization of 2-hydroxyethyl methacrylate

The catalytic chain transfer behavior of 2-hydroxyethyl methacrylate (HEMA) was studied in bulk at 40 and 60 °C. The chain transfer constant of cobaloxime boron fluoride (COBF) was found to be 6×10² at 40 and 60 °C, which corresponds to chain transfer rate coefficients of about 1.2×10⁶ and 2.0×10⁶ l mol⁻¹ s⁻¹, respectively. These values are about 10-15 times lower than those found previously for methyl methacrylate (MMA) and this decrease can be conceivably ascribed to the combination of a monomer viscosity effect, which lowers the rate coefficient by a factor of 6-8, and the complexation of the hydroxyl group with the catalyst, which causes an additional lowering by a factor of about two. The latter effect was studied by UV/Vis spectroscopy and additional kinetic studies of the COBF-mediated polymerization of MMA in the presence of ethanol. Similar UV/Vis spectra as in the case of HEMA and a reduction in chain transfer constant by a factor of two were observed.     

Asymmetric catalytic cycloetherification mediated by bifunctional organocatalysts

Oxacyclic structures such as tetrahydrofuran (THF) rings are commonly found in many bioactive compounds, and this has led to several efforts toward their stereoselective syntheses. However, the process of catalytic asymmetric cycloetherification for their straightforward synthesis has remained a challenge. In this study, we demonstrate a novel asymmetric synthesis method for THF via the catalytic cycloetherification of ε-hydroxy-α,β-unsaturated ketones mediated by cinchona-alkaloid-thiourea-based bifunctional organocatalysts. This catalytic process represents a highly practical cycloetherification method that provides excellent enantioselectivities, even with low catalyst loadings at ambient temperature.     

Versatile pathways for in situ polyolefin functionalization with heteroatoms: catalytic chain transfer

Chain-transfer processes represent highly effective chemical means to achieve selective, in situ d- and f-block-metal catalyzed functionalization of polyolefins. A diverse variety of electron-poor and electron-rich chain-transfer agents, including silanes, boranes, alanes, phosphines, and amines, effect efficient chain termination with concomitant carbon-heteroelement bond formation during single-site olefin-polymerization processes. High polymerization activities, control of polyolefin molecular weight and microstructure, and selective chain functionalization are all possible, with distinctly different mechanisms operative for the electron-poor and electron-rich reagents. A variety of metal centers (early transition metals, lanthanides, late transition metals) and single-site ancillary ligand arrays (metallocene, half-metallocene, non-metallocene) are able to mediate these selective chain-termination/functionalization processes.     

The effect of Co(II)‐mediated catalytic chain transfer on the emulsion polymerization kinetics of methyl methacrylate

The effect the catalytic chain transfer agent, bis[(difluoroboryl) dimethylglyoximato] cobalt(II) (COBF), on the course of the ab initio emulsion polymerization of methyl methacrylate, and the product properties in terms of the molecular weight distribution were investigated. The emulsion polymerization kinetics have been studied with varying surfactant, initiator, and COBF concentrations. The experimentally determined average number of radicals per particle strongly depends on the concentration of COBF and proves to be in good agreement with the results of model calculations. The apparent chain transfer constant, determined up to high conversion, is in excellent agreement with the predicted value based on a mathematical model based on COBF partitioning and the Mayo equation. The results of this work enhance the fundamental understanding of the influence a catalytic chain transfer agent has on the course of the emulsion polymerization and the control of the molecular weight distribution. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5078–5089, 2009     

Synthesis of stimulus-responsive block copolymer gelators by atom transfer radical polymerisation

The synthesis of new stimulus-responsive block copolymer gelators using atom transfer radical polymerisation (ATRP) in either methanol or 2-propanol/water mixtures at 20 °C is described. Bifunctional and trifunctional initiators were used to prepare ABA triblock and I(BA)₃ three-arm star diblock copolymers, respectively, using a ‘one-pot’ ATRP protocol, in which the central block comprised poly(glycerol monomethacrylate) and the outer blocks comprised pH-responsive poly[2-(diethylamino)ethyl methacrylate] or poly[2-(diisopropylamino)ethyl methacrylate]. These copolymers dissolve molecularly in acidic solution but formed free-standing gels at around neutral pH on addition of base. Gel strength was judged by both tube inversion experiments and shear rheometry measurements and a comparison between the linear and star architectures was made.     

Functional graft poly(N-isopropyl acrylamide)s using reversible addition-fragmentation chain transfer (RAFT) polymerisation

A series of NIPAM/4-vinyl benzyl chloride copolymers were substituted with 4(5)-imidazole dithioic acid or N-pyrrole dithioic acid to form multi-functional linear dithioate-functional polymers, which can be used as macromolecular transfer agents in a controlled radical polymerisation (RAFT) process. The presence of imidazole dithioate or N-pyrrole dithioate units along the NIPAM copolymer was determined by (1)H NMR, which showed broad CH-imidazole or CH-N-pyrrole resonances. Subsequent reaction of these multi-branch point polymers to produce graft polymers was achieved by reaction with NIPAM in the presence of AIBN. The graft polymers are produced as mixtures containing the desired product and linear polymer. The linear polymer is produced following transfer to the pendant dithioate group. Some of the branched polymers formed from the imidazole dithioate polymers were insoluble in water whilst others were found to be water soluble only in the presence of copper(II) ions. The use of N-pyrrole dithioate groups was found to substantially increase the solubility of the branched polymers in conventional solvents.     

Evaluation of ruthenium‐based catalytic systems for the controlled atom transfer radical polymerisation of vinyl monomers

Only [RuCl₂(p‐cymene)(PR₃)] complexes where the phosphine ligand, PR₃, is both strongly basic and bulky proved to be effective catalysts for the controlled atom transfer radical polymerisation (ATRP) of methyl methacrylate and styrene. The best phosphine ligands were typically P(i‐Pr)₃, P(cyclohexyl)₂Ph, P(cyclohexyl)₃, and P(cyclopentyl)₃. Less basic and/or bulky phosphines led to ineffective systems for ATRP. Tricyclohexylarsine gave rise to a highly efficient catalyst system. However, related complexes in which the phosphine ligand was replaced by tricyclohexylstibine, nitrogen (piperidine and 4‐cyanopyridine) and carbon ligands (alkyl isocyanides) proved to be inefficient. The observation of a direct relationship between the p‐cymene lability (measured by TGA) and catalyst activity suggests that p‐cymene release is a prerequisite for the polymerisation process.