Stable Free Radical Polymerization Research Articles

Block Copolymer Synthesis by a Miniemulsion Stable Free Radical Polymerization Process

The ultimate success of living-radical polymerizations will depend on whether they can be used to make materials with unique properties in an economical and acceptable manufacturing process. We report a major step in this direction for the SFRP process by demonstrating that a miniemulsion process can be used to polymerize monomers to high conversion with a high degree of livingness and that this process can be used to synthesize block copolymers. The key to the polymerizations is the use of a nitroxide-terminated oligomer to initiate the miniemulsions. Miniemulsions are performed at 135 °C to greater than 99% conversion to give polystyrenes with polydispersities between 1.15 and 1.25. Chain extension studies show these materials to have a high degree of livingness. Reaction of these polystyrene latexes with n-butyl acrylate lead to the first block copolymers completely made under SFRP miniemulsion conditions.

Synthesis of block copolymers consisting of liquid crystalline and amorphous segments by living/controlled radical polymerization

Living polymerization is most often observed in systems where the growing species are ions. In such systems the chain ends do not react to each other due to elestrostactic repulsion, but only to monomers allowing, this way, the control in structure of the formed polymer. Free radicals, which are the growing species in the radical polymerization, easely undergo combination and prevent a living radical polymerization. Thus, a great challenge to polymer science was in meeting a system that offered to the radical polymerization a radical stabillization alike in ionic polymerizations. At the same time, the radicals should undergo rapid propagation and should not be able to initiate new chains, in a controlled reaction. Some succesfull techniques of living/controlled radical polymerization, such as stable free radical polymerization (SFRP), mediated by nitroxide, INIFERTER and atom transfer polymerization (ATRP) will be overviewed here, as well as their application to the synthesis of liquid crystalline polymers.

Well-Defined Polystyrene Grafted to Polypropylene Backbone by “Living” Radical Polymerization with TEMPO

A stable free radical (2,2,6,6-tertamethylpiperidinyl-1-oxy, TEMPO) polymerization technique was applied to a graft polymerization of styrene (ST) to polypropylene (PP). PP peroxides produced by γ-irradiation (7.2, 12, and 35 kGy) in air were used as macromolecular initiators for grafting. Grafted polystyrene (PSgraft) was cleaved from the main PP chain by reaction with trifluoroacetic acid. The molecular weight (Mn) and polydispersity of the PSgraft and the free (nongrafted) polystyrene (PSfree) were determined by GPC. These PS's have almost the same Mn (nearly theoretical Mn) and narrow polydispersity. Nitroxide-terminated PS grafted to the PP was chain extended by sequential activation of the dormant chain ends in the presence of additional ST monomer. The polydispersity of the extended PSgraft remained narrow. A controlled random graft copolymer of PP with ST and n-butyl methacrylate was also synthesized using this technique.

Controlled radical polymerization in the presence of N-oxyls

Recent development in controlled radical polymerization has provided a tool to combine a relatively robust radical polymerization technique with structural control. This contribution focuses on stable free radical polymerization in the presence of nitroxides. The influence of 2,2,6,6-tetramethyl-piperidine-N-oxyl (TEMPO) and temperature on the copolymerization of styrene and acrylonitrile will be discussed. In the second part a new class of nitroxide stable free radicals will be presented that shows enhanced performance in styrene polymerizations.

Multifunctional ATRP initiators: Synthesis of four-arm star homopolymers of methyl methacrylate and graft copolymers of polystyrene and poly(t-butyl methacrylate)

Tetrakis bromomethyl benzene was used as a tetrafunctional initiator in the synthesis of four-armed star polymers of methyl methacrylate via atom transfer radical polymerization (ATRP) with a CuBr/2,2 bipyridine catalytic system and benzene as a solvent. Relatively low polydispersities were achieved, and the experimental molecular weights were in agreement with the theoretical ones. A combination of 2,2,6,6-tetramethyl piperidine-N-oxyl-mediated free-radical polymerization and ATRP was used to synthesize various graft copolymers with polystyrene backbones and poly(t-butyl methacrylate) grafts. In this case, the backbone was produced with a 2,2,6,6-tetramethyl piperidine-N-oxyl-mediated stable free-radical polymerization process from the copolymerization of styrene and p-(chloromethyl) styrene. This polychloromethylated polymer was used as an ATRP multifunctional initiator for t-butyl methacrylate polymerization, giving the desired graft copolymers.

Synthesis of block copolymer by “living” radical polymerization of styrene with nitroxyl-functionalized poly(ethylene oxide)

A series of well-defined block copolymers of poly(ethylene oxide-b-styrene)s (PEO-b-PS)s with narrow polydispersity were synthesized by the following two-step approach: living anionic polymerization of ethylene oxide (EO) with sodium 4-oxy-2,2,6,6-tetramethy-1-piperidinyloxy (TEMPONa) as initiator to yield PEOn with a TEMPO moiety at chain end, was followed by a stable free radical polymerization (SFRP) of styrene (St) to give a block copolymer. In the process of anionic polymerization, TEMPONa could induce living anionic opening-polymerization of EO at low temperature under which the stable niroxyl radical at the end of the PEO chain could be not destroyed at all. The TEMPO moiety in the resulting PEOn acts as a radical trapper in quantitative efficiency and St can be designed for polymerization for good control of the SFRP. Such a polymerization proceeded with a living radical mechanism. However, PEOn should be more sterically hindered and possess lower diffusion ability and thus be less efficient at trapping propagating chain radicals, resulting in enhanced polymerization rate.

Synthesis of Block Copolymer from Dissimilar Vinyl Monomer by Stable Free Radical Polymerization

Controlled polystyrenes with different molar mass values were synthesized starting from benzoyl peroxide and TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy). The polystyrene homopolymers served as macroinitiators for the block copolymerizations of the dissimilar vinyl monomers butyl methacrylate (BMA), ethyl methacrylate (EMA), methyl methacrylate (MMA), octyl methacrylate (OMA), vinyl acetate (VAc), N,N-dimethylacrylamide (DMA), and 2-(dimethylamino)ethyl acrylate (DAEA). Polystyrene−polymethacrylate diblock copolymers with well-defined structures as well as controlled and narrow molar mass distribution were obtained from the lower-mass polystyrene macroinitiator. Contrary to methacrylates, VAc and DAEA are not readily initiated by the polystyrene macroinitiator. Block copolymer formation was confirmed by 1H NMR and GPC measurements. On the basis of the 1H NMR investigation, there is strong evidence for the presence of TEMPO-terminated copolymers. The TEMPO-mediated polymerization is suitable for the synthe...

Synthesis of star-shaped polystyrenes via nitroxide-mediated stable free-radical polymerization

High molecular weight star-shaped polystyrenes were prepared via the coupling of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) terminated polystyrene oligomers with divinylbenzene (DVB) in m-xylene at 138 °C. The optimum ratio of the coupling solvent (m-xylene) to divinylbenzene was determined to be 9 to 1 based on volume. Linear polystyrene oligomers (M n = 19,300 g/mol, M w /M n = 1.10) were prepared in bulk styrene using benzoyl peroxide in the presence of TEMPO at approximately 130 °C under an inert atmosphere. Coupling of the TEMPO-terminated oligomers under optimum conditions resulted in a product with a number average molecular weight exceeding 300,000 g/mol (M w /M n = 3.03) after 24 h, suggesting the formation of relatively well-defined star-shaped polymers. Additionally, the intrinsic viscosities of the star-shaped products were lower than calculated values for linear analogs of equivalent molecular weight, which further supported the formation of a star-shaped architecture.

Mesogen-jacketed liquid crystalline polymers via stable free radical polymerization

Stable free radical polymerization has been used in the controlled synthesis of poly(2,5-bis[(4-butylbenzoyl)oxy]styrene), PBBOS. This “mesogen-jacketed liquid crystalline polymer”, which has mesogenic units attached directly to the backbone in a side-on mode, has been found to exhibit thermotropic liquid crystallinity similar to more conventional main-chain architectures. Stable free radical polymerization of PBBOS consistently produced molecular weight distributions below the theoretical limiting polydispersity of 1.5 calculated for a conventional free radical polymerization process. Surprisingly, a comparison of the synthesis of polystyrene to the polymerization of PBBOS under nearly identical conditions showed that the PBBOS polymerized with a significantly higher reaction rate and monomer conversion efficiency. The nematic phase of these polymers was determined to be stable over the temperature range spanning the polymer glass transition temperature up to the temperature for thermal decomposition. The molecular arrangement of the PBBOS polymers was examined by wide-angle X-ray diffraction and is described here.

Modeling stable free-radical polymerization

A kinetic model has been developed for stable free-radical polymerization (SFRP) processes by using the method of moments. This model predicts monomer conversion, number-average molecular weight, and polydispersity of molecular weight distribution. The effects of the concentrations of initiator, stable radical, and monomer, as well as the rate constants of initiation, propagation, termination, transfer, and the equilibrium constant between active and dormant species, are systematically investigated by using this model. It is shown that the ideal living-radical polymerization having a linear relationship between number-average molecular weight and conversion and a polydispersity close to unity is the result of fast initiation, slow propagation, absence of radical termination, and a high level of dormant species. Increasing stable radical concentration helps to reduce polydispersity but also decreases polymerization rate. Thermal initiation significantly broadens molecular weight distribution. Without the formation of dormant species, the model predicts a conventional free-radical polymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2692–2704, 1999

Synthesis of block and graft copolymers containing liquid-crystalline segments

Block and graft copolymers bearing amorphous or semi-crystalline and liquid-crystalline moieties were obtained by means of non-traditional synthetic routes. Chemical modification of block and graft copolymer precursors by molecular liquid crystals and controlled radical block copolymerization between liquid-crystalline monomers and styrene were used for this purpose. Controlled radical polymerization was accomplished by means of stable free radical polymerization (SRFP) and by INIFERTER methodologies. In the former methodology, 2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPO) was used to mediate the propagation. The INIFERTER technique comprised the polymerization reaction between a liquid-crystalline monomer and a sample of thiuram-terminated polystyrene promoted by UV radiation. The performance of these techniques was evaluated taking into account the chemical composition of the materials produced. The thermal properties were determined by DSC and were correlated to the chemical composition of the polymers obtained. © 1999 Society of Chemical Industry

Synthesis of block copolymers by transformation of photosensitized cationic polymerization to stable free radical polymerization

Anthracene-photosensitized cationic polymerization of cyclohexene oxide at λ=350 nm in conjuction with onium salts, namely N-ethoxy-2-methyl pyridinium hexafluorophosphate (EMP +PF 6 −) or diphenyliodonium hexafluorophosphate (Ph 2I +PF 6 −), in the presence of a stable radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), yielded polymers with alkoxyamine terminal groups. These polymers were found to be efficient initiators for stable free radical polymerization (SFRP) of styrene. IR, 1H-NMR spectral analysis and GPC studies of the obtained polymers show that block copolymers are readily formed as a result of combination of photosensitized cationic and stable free radical polymerization mechanisms.

Stable Free Radical Polymerization Process: Kinetic and Mechanistic Study of the Thermal Decomposition of MB-TMP Monitored by NMR and ESR Spectroscopy

A detailed experimental investigation was conducted to study the thermal decomposition of the nitroxide adduct N-(1‘-methylbenzyloxy)-2,2,6,6-tetramethylpiperidine (MB-TMP) at elevated temperatures...

Polyacrylates and Polydienes to High Conversion by a Stable Free Radical Polymerization Process: Use of Reducing Agents

Polyacrylates and Polydienes to High Conversion by a Stable Free Radical Polymerization Process: Use of Reducing Agents

Simple and Efficient Synthesis of Various Alkoxyamines for Stable Free Radical Polymerization

Simple and Efficient Synthesis of Various Alkoxyamines for Stable Free Radical Polymerization

Living tandem free radical polymerization of a liquid crystalline monomer

In order to synthesize liquid crystalline polymers with controlled structure, the stable free radical polymerization (SFRP) technique was employed to obtain homopolymers and block copolymers based on mesogenic acrylates. By the use of the stable free radical 2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPO) we have obtained resins with polydispersity index (PDI) below the limiting theoretical value of 1.5, established for conventional free radical polymerization of acrylates.

Synthesis of Poly(N,N-dimethylacrylamide) via Nitroxide-Mediated Radical Polymerization

Poly(N,N-dimethylacrylamide) (PDMA) with a low polydispersity was prepared via stable free radical polymerization (SFRP) using 2,2,6,6-tetramethyl-1-piperdinyloxy nitroxide (TEMPO). PDMA with a polydispersity of 1.55 (Mn = 10 600) was obtained for [TEMPO]/[AIBN] = 1.0 with quantitative monomer conversion; higher [TEMPO]/[AIBN] ratios decreased conversion. The polydispersity of PDMA was higher and polymerization times were shorter compared to SFRP of styrene or n-butyl acrylate. Attempts to prepare higher molecular weight PDMA with low polydispersity were unsuccessful. When polystyrene-TEMPO was used as a macroinitiator, N,N-dimethylacrylamide could be polymerized, but the conversion was low. Unlike styrene or acrylate polymerization, SFRP of N,N-dimethylacrylamide is not a living process, as evidenced by an unsuccessful incremental-monomer-addition experiment.

Electron Spin Resonance Studies of the Stable Free-Radical Polymerization of Styrene

Electron Spin Resonance Studies of the Stable Free-Radical Polymerization of Styrene

Synthesis of Polystyrene−Poly(2-(dimethylamino)ethyl methacrylate) Block Copolymers by Stable Free-Radical Polymerization

Synthesis of Polystyrene−Poly(2-(dimethylamino)ethyl methacrylate) Block Copolymers by Stable Free-Radical Polymerization

Controlled radical polymerization initiated by stable radical terminated polytetrahydrofuran

Block copolymers of tetrahydrofuran and styrene were prepared by using stable free radical polymerization (SFRP) of styrene with 2,2,6,6-tetramethyl-1-piperidinyl-1-oxy (TEMPO) terminated polytetrahydrofuran (PTHF). For this purpose azo containing PTHFs were prepared and reacted with TEMPO.