Cobalt(II) phenoxy-imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization

A series of cobalt(II) phenoxy-imine complexes (Co^II(FI)₂) have been synthesized to mediate the radical polymerization of vinyl acetate (VAc) and methyl acrylate (MA) to evaluate the influence of chelating atoms and configuration to the control of polymerization. The VAc polymerizations showed the properties of controlled/living radical polymerization (C/LRP) with complexes 1a and 3a , but the catalytic chain transfer (CCT) behaviors with complexes 2a , 1b , 2b , and 3b . The control of VAc polymerization mediated by complex 1a could be improved by decreasing the reaction temperature to approach the molecular weights that not only linearly increased with conversions but also matched the theoretical values and relatively narrow molecular weight distributions. The catalytic chain transfer polymerizations (CCTP) mediated by complexes 2a , 1b , 2b , and 3b were characterized by Mayo plots and the polymer chain end double bonds were observed by ¹H NMR spectra. The tendency toward C/LRP or CCTP in VAc polymerization mediated by Co^II(FI)₂ could be determined by the ligand structure. Cobalt complex coordinated by the ligand with more steric hindered and less electron-donating substituents favored the controlled/living radical polymerization. In contrast, the efficiency of CCT process could be enhanced by less steric hindered, more electron-donating ligands. The controlled/living radical polymerization of MA, however, could not be achieved by the mediation of these cobalt(II) phenoxy-imine complexes. Associated with the results of polymerization mediated by other cobalt complexes, this study implied that the configuration and spin state of cobalt complexes were more critical than the chelating atoms to the control behavior of radical polymerization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 101–113     

Viscosity effects in cobaloxime‐mediated catalytic chain‐transfer polymerization of methacrylates

The effect of bulk viscosity on the cobaloxime‐mediated catalytic chain‐transfer polymerization of methacrylates at 60 °C was investigated by both the addition of high molecular weight poly(methyl methacrylate) to methyl methacrylate polymerization and the dilution of benzyl methacrylate polymerization by toluene. The results indicate that the bulk viscosity is not directly linked to the chain‐transfer activity. The previously measured relationship between chain‐transfer‐rate coefficient and monomer viscosity therefore probably reflects changes at the molecular level. However, the results in this article do not necessarily disprove a diffusion‐controlled reaction rate because cobaloxime diffusion is expected to scale with the monomer friction coefficient rather than bulk viscosity. Considering the published data, to date we are not able to distinguish between a diffusion‐controlled reaction rate or a mechanism directly affected by the methacrylate substituent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 782–792, 2002; DOI 10.1002/pola.10152     

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     

Modeling the Synthesis of Butyl Methacrylate Macromonomer by Sequential ATRP‐CCTP

The atom transfer radical polymerization of butyl methacrylate mediated by Cu(I)Br/N,N,N′,N″,N″‐pentamethyldiethylenetriamine in anisole at 70 °C with the subsequent addition of bis(difluoroboryldiphenylglyoximato)cobalt(II) after 2 h is modeled using Predici software, to gain additional insight to the system used experimentally to produce macromonomer chains with narrow dispersity. The mechanistic model, using kinetic coefficients from the literature and activation and deactivation rate coefficients estimated from this work, provides a good representation of experimental results. The simulations demonstrate that the time (conversion) at which cobalt chain transfer agent is added to the system is critical to control the number‐average molar mass of the final product and also confirm that chains of higher length in the final product are more likely to be nonfunctionalized, in agreement with experimental observations. The model predicts the production of a significant fraction of macromonomer oligomers with lengths of 1–3 units, also consistent with experiments.     

Catalytic‐chain‐transfer polymerization of styrene revisited: The importance of monomer purification and polymerization conditions

The cobaloxime‐mediated catalytic‐chain‐transfer polymerization of styrene at 60 °C was studied with an emphasis on the effects of monomer purification and polymerization conditions. Commonly used purification methods, such as column chromatography and simple vacuum distillation, were not adequate for obtaining kinetic data to be used in mechanistic modeling. A purification regime involving inhibitor removal with basic alumina, followed by polymerization of the styrene in the presence of the cobaloxime and subsequent vacuum distillation, was found to be essential to this end. It was then possible to quantitatively investigate effects such as the initiator concentration and conversion dependencies of the apparent chain‐transfer constant that resulted from the occurrence of cobalt–carbon bond formation. A value of about 9 × 10³ was found for the true chain‐transfer constant to cobaloxime boron fluoride, that is, its value in the absence of cobalt–carbon bond formation. Furthermore, previous predictions were confirmed: the measured chain‐transfer constant decreased with increasing initiator concentration and conversion. Finally, it was confirmed that the presence of light increased the amount of free Co(II) catalyst in agreement with other studies. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 752–765, 2003     

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 poly(tert‐butyl methacrylate)‐graft‐poly(dimethylsiloxane) graft copolymers via reversible addition‐fragmentation chain transfer polymerization

Well‐defined poly(tert‐butyl methacrylate)‐graft‐poly (dimethylsiloxane) (PtBuMA‐g‐PDMS) graft copolymers were synthesized via reversible addition‐fragmentation chain transfer (RAFT) copolymerization of methacryloyl‐terminated poly (dimethylsiloxane) (PDMS‐MA) with tert‐butyl methacrylate (tBuMA) in ethyl acetate, using 2,2′‐azobis(isobutyronitrile) (AIBN) as the initiator and 2‐cyanoprop‐2‐yl dithiobenzoate as the RAFT agent. The RAFT statistical copolymerization of PDMS‐MA with tBuMA is shown to be azeotropic and the obtained PtBuMA‐g‐PDMS graft copolymers have homogeneously distributed branches because of the similar reactivity of monomers (r_tBuMA ≈ r_PDMS‐MA ≈ 1). By the RAFT block copolymerization of PDMS‐MA with tBuMA, moreover, narrow molecular weight distribution (M_w/M_n < 1.3) PtBuMA‐g‐PDMS graft copolymers with gradient or blocky branch spacing were synthesized. The graft copolymers exhibit the glass transitions corresponding to the PDMS and PtBuMA phase, respectively. However, the arrangement of monomer units in copolymer chains and the length of PtBuMA moieties have important effects on the thermal behavior of PtBuMA‐g‐PDMS graft copolymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Cobalt‐mediated radical polymerization routes to poly(vinyl ester) block copolymers

Cobalt‐mediated radical polymerizations (CMRPs) utilizing redox initiation are demonstrated to produce poly(vinyl ester) homopolymers derived from vinyl pivalate (VPv) and vinyl benzoate (VBz), and their block copolymers with vinyl acetate (VAc). Combining anhydrous Co(acac)₂, lauroyl peroxide, citric acid trisodium salt, and VPv at 30 °C results in controlled polymerizations that yield homopolymers with M_n = 2.5–27 kg/mol with M_w/M_n = 1.20–1.30. Homopolymerizations of scrupulously purified VBz proceed with lower levels of control as evidenced by broader polydispersities over a range of molecular weights (M_n = 4–16 kg/mol; M_w/M_n = 1.34–1.65), which may be interpreted in terms of the decreased nucleophilicity of these less electron donating propagating polymer chain ends. Based on these results, we demonstrate that sequential CMRP reactions present a viable route to microphase separated poly(vinyl ester) block copolymers as shown by small‐angle X‐ray scattering analyses. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Single‐electron‐transfer/degenerative‐chain‐transfer mediated living radical polymerization of vinyl chloride catalyzed by thiourea dioxide/octyl viologen in water/tetrahydrofuran at 25 °C

The single‐electron‐transfer/degenerative‐chain‐transfer mediated living radical polymerization (SET–DTLRP) of vinyl chloride (VC) in H₂O/tetrahydrofuran at 25 °C catalyzed by thiourea dioxide [(NH₂)₂C?SO₂] is reported. This polymerization occurs only in the presence of a basic sodium bicarbonate (NaHCO₃) buffer and the electron‐transfer cocatalyst octyl viologen. The resulting poly(vinyl chloride) (PVC) has a number‐average molecular weight of 1500–7000 and a weight‐average molecular weight/number‐average molecular weight ratio of 1.5. This PVC does not contain detectable amounts of structural defects and has both active chloroiodomethyl and inactive chloromethyl chain ends. Because of possible side reactions caused by the primary sulfoxylate anion (SO), the catalytic activity of (NH₂)₂C?SO₂ in the SET–DTLRP of VC is lower than that of the single‐electron‐transfer agent sodium dithionite. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 287–295, 2005     

Exploiting catalytic chain transfer polymerization for the synthesis of carboxylated latexes via sulfur‐free RAFT

We present a systematic study of incorporating carboxyl groups into latex particles to enhance colloidal stability and the physical properties of the latex. Statistical copolymers of methacrylic acid and methyl methacrylate) were synthesized via catalytic chain transfer polymerization (CCTP) in emulsion. The vinyl‐terminated oligomers were in turn successfully utilized as chain transfer agents for the formation of diblock and pseudo triblock copolymers via sulfur‐free reversible addition–fragmentation chain transfer polymerization (SF‐RAFT). These copolymers were characterized using ¹H NMR, size exclusion chromatography (SEC), dynamic light scattering (DLS), dynamic mechanical analysis (DMA), contact angle measurements and matrix‐assisted laser desorption/ionization time of flight mass spectroscopy (MALDI‐TOF‐MS) techniques. © 2019 The Authors. Journal of Polymer Science Part A: Polymer Chemistry published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, E1–E9     

Mitigating chain‐transfer and enhancing the thermal stability of co‐based olefin polymerization catalysts through sterically demanding ligands

Sterically demanding Fe‐ and Co‐based olefin polymerization catalysts 2‐Fe and 2‐Co bearing 2,6‐bis(biphenylmethyl)‐4‐methylaniline substituted bis(imino)pyridine ligands were synthesized and evaluated for ethylene polymerization. The late‐transition metal complexes were characterized by X‐ray diffraction, NMR spectroscopy, and HRMS, while their resultant polymers were characterized by size‐exclusion chromatography and ¹H NMR spectroscopy. While catalyst 2‐Fe was inactive, catalyst 2‐Co was found to polymerize ethylene and avoid any detectable chain‐transfer to aluminum events that are known to plague other Fe‐ and Co‐based catalyst systems and to limit molecular weight. Furthermore, 2‐Co displays virtually perfect thermal stability up to 80 °C and shows greatly enhanced thermal stability at 90 °C as compared to previously reported analogues. These observations are attributed to the extreme steric demand imposed by the ligand which mitigates catalyst transfer, deactivation, and decomposition reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3990–3995     

Evolution of molecular weight distributions in the catalytic chain transfer polymerization of methyl methacrylate up to high monomer conversions

The evolution of molecular weight distributions (MWDs) with monomer conversion in the catalytic chain transfer (CCT) polymerization of methyl methacrylate at 60 °C is investigated by simulation (via the program package PREDICI®) and experiment. A Co(III)‐based complex is used as the precursor for the CCT agent, which is formed in situ by initiator‐derived (2,2′‐azobisisobutyronitrile) radicals to yield the catalytically active Co(II) species. The small shifts seen in the MWD toward lower molecular weights with increasing monomer conversion are shown to be of the same order of magnitude as the associated changes in the MWD in non‐CCT controlled free‐radical polymerization, indicating that no significant change in the MWD with monomer conversion is associated with the CCT process. These results are compared to the evolution of MWDs in conventional chain transfer polymerizations with thiols as transfer agents. A clear shift toward higher molecular weights is seen with increasing monomer conversion, indicating disparate rates of thiol and monomer consumption. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3303–3312, 2000     

ABA triblock copolymers with a ring‐opening metathesis polymerization/macromolecular chain‐transfer agent approach

Block copolymers containing polystyrene and polycyclooctene were synthesized with a ring‐opening metathesis polymerization/chain‐transfer approach. Polystyrene, containing appropriately placed olefins, was prepared by anionic polymerization and served as a macromolecular chain‐transfer agent for the ring‐opening metathesis polymerization of cyclooctene. These unsaturated polymers were subsequently converted to the corresponding saturated triblock copolymers with a simple heterogeneous catalytic hydrogenation step. The molecular and morphological characterization of the block copolymers was consistent with the absence of significant branching in the central polycyclooctene and polyethylene blocks [high melting temperatures (114–127 °C) and levels of crystallinity (17–42%)]. A dramatic improvement in both the long‐range order and the mechanical properties of a microphase‐separated, symmetric polystyrene–polycyclooctene–polystyrene block copolymer sample was observed after fractionation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 361–373, 2007     

Synthesis of poly(vinyl acetate)‐graft‐polystyrene by a combination of cobalt‐mediated radical polymerization and atom transfer radical polymerization

Vinyl acetate and vinyl chloroacetate were copolymerized in the presence of a bis(trifluoro‐2,4‐pentanedionato)cobalt(II) complex and 2,2′‐azobis(4‐methoxy‐2,4‐dimethylvaleronitrile) at 30 °C, forming a cobalt‐capped poly(vinyl acetate‐co‐vinyl chloroacetate). The addition of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy after a certain degree of copolymerization was reached afforded 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐terminated poly(vinyl acetate‐co‐vinyl chloroacetate) (PVOAc–MI; number‐average molecular weight = 31,000, weight‐average molecular weight/number‐average molecular weight = 1.24). A ¹H NMR study of the resulting PVOAc–MI revealed quantitative terminal 2,2,6,6‐tetramethyl‐1‐piperidinyloxy functionality and the presence of 5.5 mol % vinyl chloroacetate in the copolymer. The atom transfer radical polymerization (ATRP) of styrene (St) was studied with ethyl chloroacetate as a model initiator and five different Cu‐based catalysts. Catalysts with bis(2‐pyridylmethyl)octadecylamine (BPMODA) or tris(2‐pyridylmethyl)amine (TPMA) ligands provided the highest initiation efficiency and best control over the polymerization of St. The grafting‐from ATRP of St from PVOAc–MI catalyzed by copper complexes with BPMODA or TPMA ligands provided poly(vinyl acetate)‐graft‐polystyrene copolymers with relatively high polydispersity (>1.5) because of intermolecular coupling between growing polystyrene (PSt) grafts. After the hydrolysis of the graft copolymers, the cleaved PSt side chains had a monomodal molecular weight distribution with some tailing toward the lower number‐average molecular weight region because of termination. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 447–459, 2007     

Micellar polymerization of amphiphilic poly(vinyl alcohol) macromonomer having a methacrylate end group prepared by aldol‐type group‐transfer polymerization

Amphiphilic and heterotactic‐rich poly(vinyl alcohol) (PVA) macromonomer, that is, PVA having a phenyl or phenoxyethyl methacrylate unit as the polymerizable end group, was synthesized via the aldol‐type group‐transfer polymerization (aldol‐GTP) technique. Aldol‐GTPs of vinyloxytriethylsilane (VOTES) were carried out in dichloromethane with 4‐methacryloylbenzaldehyde and 4‐(2‐methacryloylethoxy)benzaldehyde as the initiators with various Lewis acids. The polymerizations proceeded smoothly to give silylated PVA macromonomers (number‐average molecular weights: 1.3 × 10³–1.96 × 10⁴). Poly(VOTES) was easily desilylated to give heterotactic‐rich PVA macromonomer in good yield. The critical micelle concentration of the PVA macromonomer was determined by surface‐tension measurement. Micellar polymerization of the amphiphilic macromonomer gave comb‐shaped (graft) polymer having PVA side chains effectively (conversion: 80–82%), whereas polymerization in dimethyl sulfoxide (homogeneous state) did not. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4477–4484, 2002     

Kinetics of the styrene emulsion polymerization using n‐dodecyl mercaptan as chain‐transfer agent

The kinetics of the styrene emulsion polymerization using n‐dodecyl mercaptan as chain‐transfer agent was studied. It was found that the chain‐transfer agent (CTA) had no effect on polymerization rate but substantially affected the molecular weight distribution (MWD). The efficiency of the CTA in reducing the MWD was lowered by the mass‐transfer limitations. The process variables affecting CTA mass transfer were investigated. A mathematical model for the process was developed. The outputs of the model include monomer conversion, particle diameter, number of polymer particles, and number‐average and weight‐average molecular weights. The model was validated by fitting the experimental data. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4490–4505, 2000     

Homopolymerization of methyl methacrylate and styrene: Determination of the chain‐transfer constant from the Mayo equation and the number distribution for n‐dodecanethiol

Chain‐transfer constants were evaluated for n‐dodecanethiol in the homopolymerization of styrene (S) and methyl methacrylate (MMA). The polymerizations were carried out in benzene at 50 °C with different amounts of 2,2′‐azobisisobutyronitrile as the initiator. The new chain length distribution (CLD) analytical method was used and compared to the traditional Mayo method. The chain‐transfer‐constant values were independent of the initiator concentration and slightly higher (by a factor of 1.1 for MMA and 1.2 for S) when obtained according to the CLD method compared to the Mayo method. The chain‐transfer constant for S was 20 times higher than for MMA. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 170–178, 2000     

Long‐lived intermediates in reversible addition–fragmentation chain‐transfer (RAFT) polymerization generated by γ radiation

A novel experimental procedure is presented that allowed probing of reversible addition–fragmentation chain‐transfer (RAFT) free‐radical polymerizations for long‐lived species. The new experimental sequence consisted of gamma irradiation of a mixture of initial RAFT agent (cumyl dithiobenzoate) and monomer at ambient temperature, a subsequent predetermined waiting period without initiation source also at ambient temperature, and then heating of the reaction mixture to a significantly higher temperature. After each sequence step, the monomer conversion and molecular weight distribution were determined, indicating that controlled polymer formation occurs only during the heating period. The results indicated that stable intermediates (either radical or nonradical in nature) are present in such experiments because thermal self‐initiation of the monomer can be excluded as the reason for polymer formation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1058–1063, 2002