Stable Free Radical Polymerization Research Articles

History of nitroxide mediated polymerization in Canada

AbstractNitroxide mediated polymerization (NMP), sometimes known as stable free radical polymerization (SFRP), is considered one of the main types of reversible deactivation radical polymerization (RDRP) that have emerged as a major advance in polymer synthesis in the past 30 years. This review examines NMP's development from a uniquely Canadian perspective. Inspired by reports of a reversible equilibrium between the TEMPO persistent radical and a growing polymeric radical chain, researchers at the Xerox Research Centre of Canada (XRCC) reported radical polymerizations of styrene with hallmarks typically associated with living polymerizations: linear degree of polymerization of with conversion, narrow molecular weight distributions, and ability to re‐initiate the chain end with additional monomer. This effort expanded to polymerizations in dispersed aqueous media (eg, miniemulsion) and to polymerizations directed for various architectures (block, graft, star), but NMP was ultimately generally limited to styrenic monomers. Consequently, NMP trailed other RDRP methods, such as atom transfer radical polymerization (ATRP) and reversible addition fragmentation transfer polymerization (RAFT), due to its limited monomer choice. With the advent of second‐generation alkoxyamine initiators, however, the range of monomers polymerizable in a controlled manner greatly expanded to include acrylates, acrylamides, and eventually methacrylates (under certain conditions), providing renewed impetus towards using NMP. Consequently, Canadian researchers applied these new alkoxyamines for more versatile polymers, resulting in products such as stimuli‐responsive polymers, CO2‐switchable latexes, bio‐hybrid composites, organic photovoltaic materials, and photolithographic materials. The chronological progression of the developments in NMP from various Canadian laboratories highlights these achievements in new polymeric materials.

Benzotriazinyl-mediated controlled radical polymerization of styrene

The stable free radical polymerization (SFRP) process based on (1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl), the so-called ‘Blatter radical’, and several C-7 substituted derivatives is introduced for the first time for the polymerization of styrene. Polystyrenes characterized by polydispersity indices in the 1.05 − 1.27 range were obtained in the presence of the Blatter radical and its derivatives containing CF3, Ph, Fur-2-yl and 4-PhC6H4 substituents, while polymerization proceeded either in a non-controlled manner or in very low polymerization yields in the presence of derivatives containing halogen (Cl, Br, I) substituents. This preliminary investigation, demonstrating the potential use of the Blatter radical and its derivatives in mediated SFRP, creates new opportunities to design and develop radicals to optimize performance in such polymerization processes. © 2013 Society of Chemical Industry

Star Polymers of Sodium Styrenesulfonate Prepared by One-Pot TEMPO-Controlled SFRP

Star polymers of sodium styrenesulfonate with controlled arm length were prepared in a one-pot polymerization. Sodium styrenesulfonate was first polymerized with controlled molecular weight and narrow polydispersity by stable free radical polymerization. Poly(sodium styrenesulfonate) was terminated with divinyl benzene and star polymers prepared via stable free radical coupling of vinylic terminal groups. Star polymers based on arms of 20 and 32 repeat units possessed ~33 and ~41 arms per star respectively. Formation of star polymers with much longer arms was limited by poor coupling kinetics.

SFRP Synthesis of Acenaphthylene Oligomers and Block Copolymers. Potential Light Harvesting Structures

Azo-acenaphthylene oligomers with repeating acenaphthylene units “n” up to 4, 5, 7, 17 and 19 have been prepared successfully using nitroxide mediated Stable Free Radical Polymerization (SFRP). Azoacenaphthylene oligomers, reversibly end-capped by the stable nitroxide 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO), were further reacted via radical addition to 4-(naphthalenemethoxy)styrene monomer for diblock co-polymer formation. Characterization of the oligomers and diblock co-polymers was accomplished using MALDI-MS supported by GPC (Gel Permeation Chromatography) and 1 H NMR spectrometry. MALDI-MS afforded definitive results by providing an inter-peak interval of 152 (m/z), corresponding to acenaphthylene monomer, and inter-peak interval of 260 (m/z) for the naphthalenemethoxystyrene monomer unit in block copolymers. Our study opens the way to control the number of repeat units in the oligomers. Further these oligomers can be tailored with various monomers for the formation of block copolymers.

Preparation and Characterization of Well-defined and Polystyrene-grafted Nanocomposites via Nitroxide-mediated Radical Polymerization

A stable free radical (2,2,6,6-tertamethylpiperidinyl-1-oxy, TEMPO) polymerization technique was applied to graft polymerization of styrene (St) onto palygorskite (PT) surface. The introduction of peroxide groups onto the PT surface was achieved by the reaction of hydrogen peroxide with chlorosilyl groups, which were introduced by the treatment of PT with thionyl chloride. The observed narrow molecular weight distributions (Mw/Mn), linear kinetic plots and linear plots of molecular weight (Mn) vs. conversion for the polymer formed indicate that the chain growth from the PT surface is a controlled process with a “living” characteristic. The prepared PT-g-PSt hybrid nanoparticles have been extensively characterized by FT-IR, GPC, TGA, TEM, respectively.

Synthesis of Star-Shaped Polymers

Star-shaped polymers, consisting of a core and a number of arms radiating from the core, have attracted more and more attention because of their highly branched structures and unique rheological properties. Various synthesis methods, like reversible addition–fragmentation chain transfer polymerization (AFCTP), stable free radical polymerization (SFRP), atom transfer radical polymerization (ATRP), etc., are used to synthesize star-shaped polymers. Star-shaped polymers are formed from various different chemicals compounds, like divinyl compounds, polyisoprene, polylactides and even amphiphilic compounds. In this review various synthesis schemes and chemical compounds utilized are explained in brief to give the readers an insight in the field of star-shaped polymers.

Exfoliation of layered silicates through in situ controlled free radical polymerization mediated by a silicate-anchored initiator

In this study, a new type of controlled free radical polymerization (CFRP) initiators from azetidine-2,4-dione functional groups were synthesized and then embedded within the interlayer spaces of layered silicates to prepare a series of polymer-layered silicate nanohybrids. Kinetics analyses and determination of the molecular weights and distributions of the polymerization products revealed that these CFRP initiators were viable species for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated stable free radical polymerization. Anchoring the CFRP initiator within the galleries of montmorillonite (MMT) enlarged the d-spacing from 15 to 38 A. The incorporated cationic CFRP initiator (a silicate-anchored TEMPO derivative) provides the initiation functionality for the controlled free radical polymerization within the interlayers. The molecular weights of the resulting polystyrenes varied from 6800 to 85 200 g mol−1, depending on the initiator–MMT/styrene weight ratio, with polydispersities of 1.14–1.45 for the lower-molecular-weight samples and 1.96–2.43 for the higher-molecular-weight samples. X-Ray diffraction patterns and transmission electron microscopy revealed that the growing polymer chains separated the layered silicates, thereby leading to the formation of exfoliated individual nanoplatelets. Thus, in situ controlled polymerization within the two-dimensional MMT platform, mediated by silicate-anchored initiators, is an effective dispersion/exfoliation route toward polymer-layered silicate nanohybrids.

Synthesis and characterization of low-molecular-weight azo-acetoxystyrene and azo-naphthalene oligomers via stable free radical polymerization (SFRP)

As an approach toward controlled molecular architecture through stable free radical polymerization (SFRP), we have prepared a series of oligomers of controlled molecular weights (Mn) and low polydispersities (structures 2 and 3, with n values ranging from 2 to 52). Definitive evidence of structure was obtained through MALDI/MS (inter-peak interval of 162 m/z in azo-acetoxystyrene oligomers 2 and 260 m/z in azo-naphthalene oligomers 3, which correspond to the acetoxystyrene (AS) and naphthalenic (Np) repeating units, with corroborative evidence from NMR and GPC. Two synthetic pathways were explored. Pathway 1 yields azo-acetoxystyrene oligomer 2 via SFR addition of a TEMPO-capped unimer 1 to acetoxystyrene. Subsequent reactions would convert 2 into 3. In an alternative pathway 2, SFR addition of 1 to 4-(1-methoxynaphthyl)styrene gives azo-naphthalene oligomer 3 directly. Thus, the present reported methodology for controlled architecture has achieved synthesis of oligomers from low Mn (chlorobenzene, PhCl, solvent) to relatively high Mn (bulk), with incorporation of naphthyl (donor) and azobenzene (acceptor) moieties, as well as spacer moieties, in a controlled manner.

Modeling Controlled/Living Radical Polymerization Kinetics: Bulk and Miniemulsion

AbstractA first‐principle model for the controlled/living radical polymerization (CLRP) is discussed. The polymerization rate of CLRP is conveniently represented by a simple relationship, Rp ∝ [Intermediate]/[Trapping Agent], which highlights the important characteristics of various types of CLRPs. In stable free radical polymerization and atom‐transfer radical polymerization, the relationship, [Trap] ≪ [Interm] holds, and the polymerization rate is controlled by [Trap]. When the polymerization is conducted in nanosized particles, even a single trapping agent in a particle may lead to a larger [Trap] than for bulk polymerization. This single‐molecule‐concentration (SMC) effect theory leads to determine the particle diameter, Dp,SMC below which Rp starts to decrease significantly compared with the corresponding bulk polymerization, and $R_{{\rm p}} \,{\propto} \,D_{{\rm p}}^{3} $ for Dp < Dp,SMC. For the particle sizes somewhat larger than Dp,SMC, the statistical variation in the number of trapping agents can make Rp larger. A simple equation to estimate the Dp,Fluct‐value, below which the acceleration due to the fluctuation effect is predicted to occur, is presented. In conjunction with the SMC effect, an acceleration window, in which Rp is larger than for bulk polymerization, may be observed for Dp,SMC < Dp < Dp,Fluct. On the other hand, many reversible‐addition‐fragmentation chain transfer polymerizations conform to the condition [Interm] ≪ [Trap], and Rp is controlled by [Interm]. If [Interm] in a particle under the zero‐one condition is larger than for bulk polymerization, Rp can be increased significantly by reducing the particle size due to the zero‐one intermediate molecule (ZIM) effect. The ZIM effect theory leads to determine the particle diameter, Dp,ZIM below which Rp increases significantly compared with the bulk polymerization, and $R_{{\rm p}} \,{\propto} \,D_{{\rm p}}^{{-} 3} $ for Dp < Dp,ZIM. magnified image

Synthesis of Complementary Host- and Guest-Functionalized Polymeric Building Blocks and Their Self-Assembling Behavior

New polymers that incorporate paraquat or crown ether moieties as chain ends or central units were synthesized by stable free radical polymerization (SFRP) and atom transfer radical polymerization (ATRP). For SFRP, TEMPO derivatives that contain a functional unit were used as initiators. For ATRP, a copper−amine or copper−bipyridine complex was used as a catalyst with radical initiators from halide derivatives of paraquat and crown ethers. These end- or center-functionalized polymers formed pseudorotaxane complexes with complementary small molecules. They also formed reversible pseudorotaxane polymers: chain-extended, star-shaped homopolymers, and block copolymers. Complexation studies to determine stoichiometry and association constants with thermodynamic parameters were performed by NMR spectroscopy and isothermal microcalorimetric (ITC) titration. The association constants of crown ether derivatives and paraquat compounds in chloroform were measured for the first time: Ka = 2.97 × 103 M−1 for paraquat polystyrene and bis(m-phenylene)-32-crown-10 and Ka = 4.38 × 103 M−1 for paraquat polystyrene and crown ether polystyrene. These are 4−6-fold higher than the association constant of the analogous small molecule host−guest system in acetone.

Stable free radical polymerization of styrene with 4-sulphonate-2,2,6,6-tetramethylpiperidine-N-oxyl as mediators

The stable free radical polymerizations of styrene were investigated with five 4-sulphonate-2,2,6,6-tetramethylpiperidine-N-oxyl stable radicals as mediators and benzoyl peroxide (BPO) as initiators at 125 °C. The results indicated that the polymerizations proceeded in a “living”/controlled manner, i.e., the polymerization rates were first-order with respect to the monomer concentrations, molecular weights increased linearly with conversions and the molecular weight distributions were relatively low (Mw/Mn = 1.2–1.4), 1H NMR analysis of the polymer chain-ends and successful chain extensions. The polymerization rates were faster than that of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl (OH-TEMPO) mediated ones. The effects of steric interference of different substitute groups at four-position of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), the molar ratios of stable radical to BPO and the temperature on the polymerizations were investigated.

Thermal Self-Initiation in Stable Free-Radical Polymerization of Styrene

Stable free radical polymerization (SFRP) of styrene was carried out using bis(4-bromomethylbenzoyl)peroxide (BBMBPO)/4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy (HTEMPO) or 4,4′-azobis(4-cyanopentyl)-α-bromoisobutyrate (ABCBIB)/HTEMPO as bimolecular initiating system. It was found that the molecular weights of obtained polystyrenes were controlled by the concentration of HTEMPO. Besides those generated by the initiators, some polystyrene chains were formed via thermal initiation. Polystyrene chains generated by the initiator had initiating groups for atom transfer radical polymerization (ATRP) at their ends, so they grew to higher molecular weights in ATRP of styrene. While the molecular weights of polystyrene chains generated by thermal initiation kept constant in ATRP conditions because of their inert terminals for ATRP. The amount of polystyrenes generated by thermal initiation was investigated quantitatively via gel permeation chromatography technique.

Stable free radical polymerization of n-butyl acrylate in the presence of high-temperature initiators

Living radical polymerizations of acrylate are known to be difficult to achieve using TEMPO as a mediator. The stable free radical polymerization (SFRP) of acrylate tends to stop at low monomer conversion due to the accumulation of TEMPO in the medium as a result of unavoidable bimolecular termination. Rather than solving this problem by destroying the excess nitroxide using ascorbic acid or glyceraldehyde associated with pyridine as reported recently, high temperature initiators were used to slowly and continuously generate new radicals throughout the polymerization to consume the excess TEMPO molecules. Polymerizations of n-butyl acrylate initiated by the alkoxyamine unimer (1-benzoyloxy)-2-phenyl-2-(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)ethane (BST) were performed between 130 °C and 134 °C in the presence of a series of high temperature peroxide and azo initiators. The best results in this study were obtained by the continuous addition of small amounts of di- tert-amyl peroxide throughout the polymerization. Under these conditions, the acrylate polymerizations fulfilled the criteria of a controlled polymerization process although the molecular weight distributions were slightly broad ( M w/ M n ∼ 1.5).

Stable Free Radical Polymerization in Emulsion: Modeling the Thermodynamics of Monomer Transfer between Droplets and Particles

AbstractA mathematical model is developed from polymer solution thermodynamics and stable free radical polymerization kinetics to predict the monomer swelling behavior of large droplets and small particles that coexist during the seeded polymerization of styrene in emulsion. This model is used to predict the sensitivity of polymerized latex stability (based on the persistence of large particles) to changes in seed particle size and final latex target molecular weight. Simulation results show that the use of small seed particles (diameter ≤ 50 nm) in recipe formulation will lead to polymerization occurring preferentially in the large droplets, and offers theoretical evidence that TEMPO‐mediated ab initio emulsion polymerizations will not be feasible. Our predictions are consistent with experimental evidence suggesting that the presence of large particles leads to the formation of undesirable coagulum in the final product.magnified image

Synthesis and Evaluation of Sterically Hindered 1,1-Diadamantyl Nitroxide as a Low-Temperature Mediator for the Stable Free Radical Polymerization Process

An efficient procedure for the oxidation of 1,1-diadamantylamine with m-chloroperbenzoic acid (m-CPBA) to the corresponding sterically hindered nitroxide in high yield (72%) is reported. The corresponding styrene-based alkoxyamine was synthesized by an atom transfer radical addition (ATRA) reaction. Because of the bulkiness of the adamantyl substituents, the cleavage of the C−ON bond of the alkoxyamine occurred readily at temperatures below 100 °C. However, in spite of the low bond dissociation temperature of the alkoxyamine, the (co)polymerizations of styrene and n-butyl acrylate proceeded poorly. The results in this paper support the argument that the most important determinant in a nitroxide-mediated polymerization of styrene and acrylate is the ability to control the excess concentration of nitroxide in solution either by an inherent instability of the nitroxide or by the use of additives that destroy the excess nitroxide. Thus, for the 1,1-diadamantyl nitroxide case, and maybe more generally, while a lower C−O bond dissociation energy of an alkoxyamine moiety at the end of the chain may allow lower polymerization temperatures and faster reactions rates, it does not enable polymerizations to proceed to high conversions if the excess nitroxide is not controlled in some manner.

Synthesis of AB 2 miktoarm star-shaped copolymers by combining stable free radical polymerization and atom transfer radical polymerization

A stable nitroxyl radical functionalized with two initiating groups for atom transfer radical polymerization (ATRP), 4-(2,2-bis-(methyl 2-bromo isobutyrate)-propionyloxy)-2,2,6,6-tetramethyl-1-piperidinyloxy (Br 2-TEMPO), was synthesized by reacting 4-hydroxyl-2,2,6,6-tetramethyl-1-piperidinyloxy with 2,2-bis-(methyl 2-bromo isobutyrate) propanoic acid. Stable free radical polymerization of styrene was then carried out using a conventional thermal initiator, dibenzoyl peroxide, along with Br 2-TEMPO. The obtained polystyrene had two active bromine atoms for ATRP at the ω-end of the chain and was further used as the macroinitiator for ATRP of methyl acrylate and ethyl acrylate to prepare AB 2-type miktoarm star-shaped copolymers. The molecular weights of the resulting miktoarm star-shaped copolymers at different monomer conversions shifted to higher molecular weights without any trace of the macroinitiator, and increased with monomer conversion.

Rapid additive-free TEMPO-mediated stable free radical polymerizations of styrene

The stable free radical polymerization (SFRP) of styrene, initiated with benzoyl peroxide in the presence of TEMPO, under bulk conditions, is demonstrated to proceed rapidly without the need for any rate enhancing additives such as camphorsulfonic acid, 2-fluoro-1-methyl pyridinium p-toluenesulfonate, or acetic anhydride. Monomer conversions as high as 70% can be achieved in 5 h or less while maintaining polydispersity indexes of 1.15. These results stand in stark contrast to earlier reactions that required 70 h to achieve similar conversions. This study demonstrates that the single largest factor governing the rates of polymerization is the molar concentration of excess TEMPO remaining in solution after initiation. A reduction in the TEMPO to BPO ratio is required when large amounts of BPO are used to target low molecular weight polystyrenes. However, when a lower molar amount of BPO is used to obtain high molecular weight polystyrenes, a higher TEMPO to BPO ratio is required. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5487–5493, 2007

A Quantitative 1H NMR Method for the Determination of Alkoxyamine Dissociation Rate Constants in Stable Free Radical Polymerization. Application to Styrene Dimer Alkoxyamines

A high-resolution 1H NMR method was developed and applied to the determination of the dissociation rate constants (kd values) of seven stable free radical polymerization (SFRP) model compounds, including three pairs of styrene dimer alkoxyamines, using oxygen as the alkyl radical scavenger. The efficiency and completeness of oxygen irreversible trapping were tested by taking advantage of the diastereoisomerism of the dimer alkoxyamines. The effects of alkoxyamine stereochemistry on the C−ON bond dissociation kinetics were also demonstrated. The 1H NMR method was validated by comparing kd values obtained by this method with ESR and HPLC results previously reported for 2,2,6,6-tetramethyl-1-(1-phenylethoxy)piperidine, a commonly studied alkoxyamine. In contrast to ESR methods, the 1H NMR procedure allows for the simultaneous determination of multiple kd values in a single experiment.

Synthesis of Poly(styrene-b-styrenesulfonic acid) and its Blend with PPO for Proton Exchange Membrane

Poly(styrene-b-styrenesulfonic acid sodium salts) (PS-b-PSSNa) were synthesized via stable free radical polymerization (SFRP) using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical and the block copolymers were blended with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) to prepare polymer electrolyte membranes (PEM). The ratio of block length and total molecular weight of PS-b-PSSA were varied, and weight ratio of PPO/PS-b-PSSA was also controlled. The blend membranes showed good proton conductivity, low methanol permeability, and good thermal property.

Stable Free Radical Polymerization of Acrylates Promoted by α-Hydroxycarbonyl Compounds

The living-radical polymerization of n-butyl acrylate, moderated with TEMPO, is shown to proceed in a controlled fashion in the presence of a series of α-hydroxycarbonyl compounds with different organic bases. The best results were obtained with glyceraldehyde dimer in the presence of pyridine. These results further support the suggestion that the difficulty of polymerizing acrylate monomers in the presence of TEMPO is related to the excess buildup of TEMPO due to a small amount of polymer chain termination and to a lesser extent on the higher bond dissociation energy of the TEMPO acrylate bond relative to the TEMPO styrene bond at the chain's terminus.