Tacticity control by conformational isomerization in free radical polymerization of acrylate

Contribution of conformational isomerization to polymer tacticity has been studied in free radical polymerization of (−)-menthyl 2-acetamidoacrylate with azo initiators. It has been demonstrated that the chain end of the growing polymer radical, which is generated from the s-trans and s-cis conformers of monomer, produces stereoselectively new R and S configurational quaternary carbons, respectively, for the attack of monomer. In addition, polymerization reactivity of both conformers is indistinguishable under the present conditions, and the polymerization is considered to proceed through a chain-end controlled mechanism, which excludes a penultimate unit effect on tacticity in the polymerization. The results obtained would give a clue to understand an origin of tacticity in conventional free radical polymerization of acrylates.     

Free-radical propagation and termination kinetics of the butyl acrylate dimer studied by pulsed laser polymerization techniques

The propagation and termination rate coefficients for bulk polymerization of the butyl acrylate dimer (BA dimer) are determined by pulsed laser techniques. The rate coefficient for propagation, k_p, is deduced for temperatures from 20 to 90 °C via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) method at pulse repetition rates between 1 and 10 Hz. The Arrhenius parameters were found to be: E_A(k_p) = (34.2 ± 1.0) kJ mol⁻¹ and A(k_p)/L mol⁻¹ s⁻¹ = (1.08 ± 0.49) × 10⁷ L mol⁻¹ s⁻¹. The termination rate coefficient, k_t, has been measured via SP-PLP-ESR, single pulse-pulsed laser polymerization in conjunction with time-resolved electron spin resonance detection of radical concentration. The resulting Arrhenius parameters as deduced from the temperature range −15 to +30 °C are: E_A(〈k_t〉) = (22.8 ± 3.7) kJ mol⁻¹ and log(A/L mol⁻¹ s⁻¹) = 10.6 ± 1. The chain-length dependence of k_t was studied at 30 °C. For short chains a significant dependence was found which may be represented by an exponent α = 0.79 in the power-law expression k_t(i) = k_t⁰i^−α.     

Well-defined dibenzocyclooctyne end functionalized polymers from atom transfer radical polymerization

The dibenzocyclooctyne end functionalized agent 1 was designed as atom transfer radical polymerization (ATRP) initiator. The ATRP was then explored on three types of monomers widely used in free radical polymerization: methyl methacrylate, styrene, and acrylates (n-butyl acrylate and tert-butyl acrylate). The living polymerization behaviors were obtained for the methyl methacrylate and styrene monomers. The SPAAC click reactivity of dibenzocyclooctyne end group were demonstrated by successfully reacting with azide functionalized small chemical agents and polymers. Various topological polymers such as block and brush polymers were produced from strain-promoted azide-alkyne cycloaddition reaction (SPAAC) using the resultant dibenzocyclooctyne end functionalized poly(methyl methacrylate)/polystyrene as building blocks. For the acrylates, however, the polymerization did not hold the living characteristics with the dibenzocyclooctyne end functionalized ATRP initiator 1.     

Stereochemistry of the radical polymerization of vinyl pentafluorobenzoate

The free-radical polymerization of vinyl pentafluorobenzoate (VPBz) was carried out under various conditions in order to compare its stereochemistry to that of the vinyl benzoate (VBz) polymerization using similar conditions. Contrary to the stereochemistry of the radical polymerization of VBz, VPBz favors syndiotactic propagation. The poly(VPBz) obtained in hexafluoroisopropanol (HFIP) with nBu₃B-air at −30 °C has a triad syndiotacticity (rr) of 52% which achieved the highest syndiotacticity reported for the radical polymerization of vinyl esters. The stereochemistry difference for the VPBz polymerization was ascribed to the electron-withdrawing effect of the fluorine on the aromatic ring. The solvent effect of enhancing the rr specificity in HFIP may be related to the hydrogen-bonding between HFIP and VPBz or the growing species. It was also found that the glass transition temperatures (T_g) of the VPBz polymers apparently increased with an increase in their diad syndiotacticities (r): the T_g of poly(VPBz) with r=72% was 79 °C, which was 25 °C higher than that of poly(VPBz) with r=56% obtained in toluene.     

载体硅胶表面氟改性对Phillips铬系催化剂诱导期内引发乙烯聚合行为的影响

通过密度泛函理论考察了载体表面氟改性对吸附不同数目甲醛分子的Phillips催化剂诱导期内引发乙烯聚合反应的影响。结果表明,当乙烯还原六价铬酸酯形成的二价铬前驱体模型上吸附两分子甲醛时,其位阻效应阻碍了任何反应的进一步发生;当二价铬前驱体模型上吸附一分子甲醛时,只能通过先形成铬金属五元环进而发生乙烯二聚反应和易位反应,但是氟改性对两者的影响很小;当二价铬前驱体模型上吸附的甲醛分子完全脱附后,则可以进一步环增长生成铬金属七元环,并且氟改性对这一步反应有促进作用;而氟改性对铬金属七元环进一步开环生成1-己烯则是不利的。研究还表明,氟改性对于三价铬-烷基聚合活性中心模型上的链增长是有利的。     

Magnetic field effects on the free radical solution polymerization of acrylamide

Systematic polymerization experiments quantified magnetic field (MF) influences on the free radical solution polymerization of acrylamide for various polymerization conditions. The type of initiation, the initiator concentration, the monomer concentration, and the viscosity of the aqueous medium were the subject of variation. The initiator efficiency (Φ) increased up to 60% and the initiator exponent of the overall rate expression raised from 0.1 to 0.28 when a weak MF of 0.1 T was applied to photochemical initiation. However, no appropriate effect was observed for thermal initiation. Kinetic analysis proved a reduction of the termination rate coefficient (k_t) up to 40%. Photochemically initiated polymerizations in media of enhanced viscosity revealed the highest increment of the initiator efficiency ratio Φ^MF/Φ and the most pronounced reduction of k_t^MF/k_t. On the contrary, the propagation rate coefficient (k_p) and the monomer exponent were not influenced by MF. Despite considerable increase of the polymerization rate in MF, no reduction of the molar mass was found. Compensation of increased Φ^MF and decreased k_t^MF are suggested as explanation. The singlet-triplet intersystem crossing mechanism for radical pairs served to explain the MF effects.     

An iron‐based reverse ATRP process for the living radical polymerization of acrylonitrile

A hexa‐substituted ethane thermal iniferter, diethyl‐2,3‐dicyano‐2,3‐di(p‐tolyl) succinate (DCDTS), was firstly used as the initiator in the reverse atom transfer radical polymerization (RATRP) of acrylonitrile. FeCl₃ coordinated by isophthalic acid (IA) was used as the catalyst in this system. The polymerization in N,N‐dimethylformamide not only shows the best control of molecular weight and its distribution but also provides rather rapid reaction rate with the ratio of [AN] : [DCDTS] : [FeCl₃] : [IA] at 500 : 1 : 2 : 4. The polymers obtained were end‐functionalized by chlorine atom, and they were used as macroinitiators to proceed the chain extension polymerization in the presence of FeCl₂/IA catalyst system via a conventional ATRP process and polyacrylonitrile obtained was with M_n = 39,260, PDI = 1.25. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Reverse atom transfer radical polymerization of acrylonitrile

FeCl₃ coordinated by succinic acid was used as the catalyst for the first time in azobisisobutyronitrile‐initiated reverse atom transfer radical polymerization of acrylonitrile (AN). N,N‐dimethylformamide (DMF) was used as a solvent to improve the solubility of the ligand. A FeCl₃ to succinic acid ratio of 0.5 not only gives the best control of molecular weight and its distribution but also provides rather rapid reaction rate. Effects of different solvents on polymerization of AN were also investigated. The rate of the polymerization in DMF is faster than that in propylene carbonate and toluene. The molecular weight of polyacrylonitrile agrees reasonably well with the theoretical molecular weight in DMF. The rate of polymerization increases with increasing the polymerization temperature, and the apparent activation energy was calculated to be 64.8 kJ mol^?1. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 32–36, 2006     

Study of microwave irradiation polymerization mechanism in the presence of carriers

Microwave irradiation polymerization was studied in the presence of a carrier. The influence of different carriers, such as Al₂O₃, SiO₂, and MgO on the microwave irradiation polymerization mechanism of acrylamide and 2‐ethylhexyl acrylate was studied by adopting polymerization inhibitors such as an anionic inhibitor, cationic inhibitor, and radical inhibitor hydroquinone. When the system contains the carriers Al₂O₃ and SiO₂, the polymerization mechanism occurs according to a radical mechanism, but when the system contains MgO, the polymerization mechanism occurs according to not only a radical mechanism but also an anionic polymerization mechanism with superficial grafting occurring. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1519–1524, 2004     

Phenyltrialkylborates as co-initiators with cyanine dyes in visible light polymerization of acrylates

Photoredox pairs consisting of selenocarbocyanine dye cations and phenyltrialkylborate anions were employed as the novel, effective visible-wavelength initiators of the radical polymerization of acrylic monomer. The influence of the sensitizers and electron donor structure on the photopolymerization kinetics of multiacrylate monomer was investigated by photo-DSC. It was found that the polymerization rate and the final conversion degree were dependent on both dye and borate structure. The kinetic studies of the free radical polymerization revealed an increase in the polymerization rate with a decrease of the borate oxidation potentials which was additionally reflected by the linear relationship between the Hammett constant and rate of polymerization. The efficiency of these initiators was discussed on the basis of the free energy change for electron transfer from an excited cyanine dye cation to a borate anion. The ΔG_el values were estimated for photoredox pairs containing a series of phenyltrialkylborate anions and one selenocarbocyanine dye cation. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. The photoreduction of cyanine phenyltrialkylborate complex was studied using nanosecond laser flash photolysis. The dye triplet was found to be quenched by the electron donors via an electron transfer process. Rate constants (k_q) for the quenching of the excited states were high and approached diffusion-controlled limits and were found to depend on the borate structure.     

Synthesis of polyacrylonitrile via reverse atom transfer radial polymerization (ATRP) initiated by diethyl 2,3‐dicyano‐2,3‐diphenylsuccinate,FeCl3, and triphenylphosphine

The reverse atom transfer radical polymerization (RATRP) technique using FeCl₃/triphenyl‐phosphine (PPh₃) complex as a catalyst was applied to the living radical polymerization of acrylonitrile (AN). A hexa‐substituted ethane thermal iniferter, diethyl 2,3‐dicyano‐2,3‐diphenylsuccinate (DCDPS), was first used as the initiator in this iron‐based RATRP initiation system. A FeCl₃ to PPh₃ ratio of 1:3 not only gives the best control of molecular weight and its distribution but also provides a rather rapid reaction rate. The rate of polymerization increases with increasing the polymerization temperature and the apparent activation energy was calculated to be 54.9 kJ mol^?1. Because the polymers obtained were end‐functionalized by chlorine atoms, they were used as macro‐initiators to proceed the chain extension polymerization in the presence of an FeCl₂/PPh₃ catalyst system via a conventional ATRP process. Copyright © 2005 Society of Chemical Industry     

Controlled synthesis of azobenzene functionalized homo and copolymers via direct acyclic diene metathesis polymerization

A powerful and efficient strategy for obtaining a series of azobenzene functionalized linear unsaturated polyolefins is described, which involves the first design and synthesis of three different α,ω-diene monomers with different reactivity of acrylates and terminal double bonds and their subsequent acyclic diene metathesis (ADMET) polymerization under mild reaction conditions. Different ruthenium based metathesis catalysts and conditions were tested to optimize the ADMET polymerization of these monomers. Besides, this polycondensation method is also reported allowing for the synthesis of alternating and diblock azobenzene-containing copolymers with molecular weight control by using the selectivity of olefin cross-metathesis between acrylates and terminal olefins. The resulting polymers showed well-defined molecular weights, reasonable polydispersity index, and reversible photoresponsive behavior. This special combination of the benefits of metathesis polymerization and azobenzene is a highly versatile system for materials in many applications.     

Emulsifier-free, organotellurium-mediated living radical emulsion polymerization of Styrene: Initial stage of polymerization

Behavior of the particle formation based on self-assembling in emulsifier-free, organotellurium-mediated living radical emulsion polymerization (emulsion TERP) of styrene was studied from the molecular weight distributions (MWDs) of polystyrene (PS) formed in an initial stage of the polymerization at different temperatures from 50 °C to 70 °C. As the polymerization temperature was decreased, the larger number of poly(methacrylic acid) (PMAA; degree of polymerization, 30)-methyltellanyl (TeMe) (PMAA₃₀-TeMe) participated in the polymerization, resulting in amphiphilic PMAA₃₀-b-PS-TeMe oligomers. Almost all control agents were consumed and a self-assembly nucleation occurred in the initial stage of the polymerization at 50 °C, which lead to depress of particle formation of a homogeneous nucleation. The consumption rate of PMAA₃₀-TeMe affected directly the particle formation. From these results, it is concluded that it is important for the emulsion TERP of styrene with excellent control/livingness that the self-assembly nucleation proceeds without the homogeneous nucleation in the initial stage of the polymerization.     

Synthesis of 4-arms hydroxy-functionalized PMMA-b-PE through combining free radical polymerization with coordination polymerization

The copolymerization of MMA with ethylene was promoted by metallocene complex in the presence of initiator tetra(2,3-epoxy propoxy)silane (I_s), reducing agent Zn and cocatalyst MAO, combining free radical polymerization with coordination polymerization via sequential monomer addition strategy in one-pot to produce 4-arms hydroxy-functionalized PMMA-b-PE. The effects of polymerization conditions such as temperature, time, ethylene pressure and Al/Ti molar ratio on the polymerization performance were investigated. 4-Arms hydroxy-functionalized PMMA-b-PE was obtained by solvent extraction and determined by GPC, MALLS, DSC, FT-IR, WAXD and ¹H(¹³C) NMR. The DSC result indicated that the 4-arms hydroxy-functionalized PMMA-b-PE had one T_g at 87.0 °C and one T_m at 117.0 °C which attributed to T_g of PMMA segment and T_m of PE segment, respectively. The microstructure of 4-arms hydroxy-functionalized PMMA-b-PE was further confirmed by WAXD, FT-IR, and ¹³C NMR analysis. These results demonstrated that the obtained 4-arms block copolymer consisted of PMMA segment and crystalline PE segment.     

Highly efficient ring-opening polymerization of tetrahydrofuran by anhydrous ferric chloride

Poly(tetramethylene ether glycol) (PTMG), widely used as a precursor in the fabrication of commodity polymers, is typically produced byacid-catalyzed polymerization of tetrahydrofuran (THF). Herein, we report a detailed investigation of the cationic ring-opening polymerization of THF catalyzed by ferric chloride (FeCl₃), which can be considered as a strong Lewisacid. The polymerization reactions were performed in the presence of acetic anhydride at 20°C with FeCl₃, which readily produced poly(tetramethylene ether glycol diester) (PTMG_DE). A maximum yield of 79.2% was obtained within 30 min using a FeCl₃ to acetic anhydride molar ratio of 5:4. The resulting polymers generally exhibited low molecular weights and a narrow polydispersity index and could be easily converted into PTMG using aqueous NaOH. In contrast with the FeCl₃-acetic anhydride system, other iron-based catalysts such as FeCl₂ and FeCl₃·6H₂O did not show any noticeable activity in the polymerization of THF. The proposed mechanism involves initiation of the acetyl cation generated by FeCl₃, propagation by nucleophilic addition of THF, and termination by the acetate anion, accounting for the high activity of the FeCl₃ catalyst for THF polymerization. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47999.     

Quantification of spontaneous initiation in radical polymerization of styrene in aqueous miniemulsion at high temperature

The spontaneous (thermal) initiation rate (R_i,th; no added initiator) in radical polymerization of styrene in aqueous miniemulsion at 110 and 125 °C with sodium dodecylbenzenesulfonate or poly(vinyl alcohol) as surfactants (colloidal stabilizers) has been estimated using a novel approach based on the total number of chains. In qualitative agreement with previous work at lower temperatures, R_i,th was found to be 3.1-15.1 times greater than that in bulk. According to the activation energy and conversion dependence of R_i,th, the radical generation mechanism differs from that in bulk. The experimental evidence is consistent with the enhanced R_i,th in miniemulsion being related to the oil-water interface, with radical generation in the aqueous phase playing a negligible role. The implications with regards to nitroxide-mediated radical polymerization in aqueous dispersed systems are discussed.     

Adsorption of block copolymers at latex surface and their utilization in emulsion polymerization

Amphiphilic block copolymers have been investigated for their utilization in emulsion polymerization of butyl methacrylate. Special attention has been paid to the adsorption mechanism of the block copolymers from systematic measurements of equilibrium adsorption isotherms. A series of well-defined water-soluble amphiphilic block copolymers, composed of poly(butyl methacrylate) and poly(sodium methacrylate) blocks, were synthesized by anionic polymerization of butyl methacrylate and tert-butyl methacrylate followed by the thermal deprotection of the tert-butyl ester groups and final hydrolysis. The number density of emulsion polymer particles N_P varied as [copolymer]^α, α lying between 0.44 and 0.73 according to the hydrophilic content of the copolymers. In contrast with SDS taken as a reference emulsifier, the adsorption of the copolymers was very strong and this provided quite an efficient stabilization of the polymer particles during emulsion polymerization, even at low concentrations (<10⁻⁴ mol L⁻¹) and low coverages (<10% of the interfacial area).     

Termination kinetics of free-radical polymerization in ionic liquids

Termination rate coefficients, k_t, for free-radical polymerization of 15 vol.-% methyl methacrylate (MMA) dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide and in 1-butyl-3-methylimidazolium tetrafluoroborate were measured at 10 °C via the single pulse − pulsed laser polymerization − electron paramagnetic resonance (SP-PLP-EPR) technique. Whereas absolute k_t in ionic liquid solution is by about one order of magnitude below the associated bulk MMA value, the chain-length dependence of k_t is very similar in both liquid environments.     

Effect of ferrocene moieties on the copper‐based atom transfer radical polymerization of methyl methacrylate

The atom transfer radical polymerization (ATRP) of methyl methacrylate catalyzed by copper–tripodal complexes with ferrocene moieties (CuX/TRENFcImine, where X is Br or Cl, and TRENFcImine is tris‐[2‐(ferrocenylmethyleneimino)ethyl]amine) was investigated to understand the effect of redox active moieties on the performance of ATRP catalysts. The CuBr/TRENFcImine system was highly active, with 82% conversion in 2 h. However, the polymerization became slower at higher molar ratios of monomer to catalyst. The polydispersity index was broad, and the initiation efficiency was relatively low. On the basis of the conformational analysis, the highly active and less controlled polymerization was probably caused by the electronic effect rather than the steric effect on the ferrocene moieties, which led to the higher and lower values in the activation and deactivation steps, respectively. The polydispersity index was improved by the addition of CuBr₂, but this led to slower rates of polymerization. The effect of halide groups on ATRP caused a faster rate in the CuBr/TRENFcImine polymerization system than in the CuCl/TRENFcImine system. The higher molar ratio of monomer to catalyst had no significant effect on the CuCl/TRENFcImine system. Nonetheless, the trace of water in the CuCl₂·2H₂O system accelerated the rate of propagation, which led to a higher molecular weight. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis of polyvinyl alcohol stereoblock copolymer via the combination of living cationic polymerization and RAFT/MADIX polymerization using xanthate with vinyl ether moiety

Different types of novel xanthates containing a vinyl ether moiety, S-benzyl O-2-(vinyloxy)ethyl carbonodithioate (Xanthate 1) and S-1-(ethoxycarbonyl)ethyl O-2-(vinyloxy)ethyl carbonodithioate (Xanthate 2) were synthesized. In particular, the Xanthate 2 enabled to design polyvinyl alcohol (PVA) stereoblock copolymer via the combination of living cationic vinyl polymerization and RAFT/MADIX polymerization. For cationic polymerization of isobutyl vinyl ether (IBVE) and tert-butyl vinyl ether (TBVE), the polymerizations were conducted under Xanthate 1-HCl adduct/SnCl₄ and Xanthate 1 or 2-CF₃COOH adduct/EtAlCl₂ initiating system in the presence of ethyl acetate. Both systems proceeded in living polymerization fashion because the calculated M_n of both poly(IBVE) and poly(TBVE) matches with the M_n polymerized assuming that one polymer chain is formed per one molecule of the Xanthate 1 or 2. The resulting poly(TBVE) had a high number average α-end functionality as determined by MALDI-TOF-MS spectrometry. Xanthate 2 is more efficient for the following RAFT/MADIX polymerization of vinyl acetate (VAc). The RAFT/MADIX polymerization of vinyl acetate (VAc) using azobis(isobutyronitrile) (AIBN) at 60 °C was conducted using either poly(IBVE) or poly(TBVE) macro-CTA. The poly(TBVE) macro-CTAs synthesized from the Xanthate 2 were able to polymerize VAc smoothly via RAFT/MADIX polymerization, to prepare well-defined diblock copolymer, poly(TBVE)-b-poly(VAc). The resulting block copolymer was then hydrolyzed using KOH in methanol and followed by acid hydrolysis using HBr gas bubbling. The resulting polymer is inherently stereoblock like copolymer, isotactic rich PVA-b-atactic PVA (iPVA-b-aPVA). From the DSC measurement, the iPVA-b-aPVA has one glass transition at 69.5 °C and two melting points according to iPVA and aPVA at 237.9 and 198.1 °C, respectively. Thus, it can be suggested that the obtained PVA has two different geometries by the combination of living cationic polymerization and RAFT/MADIX polymerization.