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.     

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.     

Mechanism and kinetics of the free radical ring-opening polymerization of cyclic allylic sulfide lactones

The polymerization of 7-, 8- and 11-membered lactones, 6-methylene-1,4-oxathiepan-7-one, 3-methylene-1,5-oxathiocan-2-one and 3-methylene-1-oxa-5-thiacycloundecan-2-one in benzene at 70, 40–70 and 40–65 °C, respectively, is presented. All polymerizations proceeded with complete ring-opening up to approximately 25% conversion, where insoluble polymer was formed. Evidence is given attributing polymer double bond loss to crosslinking, although redistribution of the molecular weights via addition to polymer double bonds followed by β-fragmentation also appears to occur for polymerizations of the 8- and 11-membered lactones. Michael adducts of lactones with 2-methyl-2-propanethiol were prepared as models for chain-transfer products of hydrogen abstraction by carbon-centred radicals. Polymerization rates were found to increase marginally with ring size. Arrhenius parameters obtained for the polymerizations of the 8- and 11-membered lactones indicated that the addition step was more important than fragmentation in determining the rate of propagation.     

自由基聚合反应动力学常数测定技术

自由基聚合是一类重要的聚合反应,其反应动力学常数的获取对于聚合机理研究、聚合物链结构定制、反应过程调控都有着重要的意义。综述了自由基聚合中最重要的链增长和链终止反应速率常数的测定方法,着重介绍连续脉冲激光引发结合凝胶渗透色谱测定链增长反应速率常数、单脉冲-连续脉冲激光耦合结合近红外光谱与电子顺磁共振波谱测定链终止反应速率常数,涵盖了各方法的检测原理、要求和适用条件,讨论了增长自由基链长对链增长和链终止反应速率常数的影响。此外,对链增长和链终止反应速率常数检测方法的提升和新方法的开发进行了展望。     

Exceptional chiroptical behavior near ceiling temperature in free radical polymerization of menthyl 2-acetamidoacrylates

Effect of polymerization conditions on chiroptical properties of polymer has been studied in the polymerization of (−)- and (+)-menthyl 2-acetamidoacrylates using radical initiators under the conditions with various temperatures, monomer concentrations, and reaction times. Specific rotation and circular dichroism of the resulting polymers indicated that a ceiling temperature (T_c) affected the chiroptical properties of the polymers and the polymerizations would give preferentially a helical polymer through a radical vinyl polymerization near T_c. In addition, the helical structure of the polymer was maintained intact even heating at 120 °C in anisole.     

Forgotten monomers: free radical polymerization behavior of norbornadiene derivatives in comparison with methyl methacrylate

The free radical polymerization behavior of three norbornadiene derivatives was studied as a function of their structure. The compounds examined were 3-ethoxycarbonyl-tricyclo[3.2.1.0^2,4]oct-6-ene 1 (liquid), 2-carbethoxybicyclo[2.2.1]-2,5-heptadiene 2 (liquid) and 2-carboxybicyclo[2.2.1]-2,5-heptadiene 3 (solid). They were polymerized in bulk and in benzene solutions. The tricyclic compound 1 was the least reactive monomer whereas the bicyclic derivatives 2 and 3 readily polymerized radically to high conversions. The corresponding polymers were isolated and their structure were determined by means of IR- and NMR spectroscopy. The polymerization of 3-ethoxycarbonyl-tricyclo[3.2.1.0^2,4]oct-6-ene 1 is supposed to proceed through a simultaneous ring opening-ring closing mechanism. The corresponding polymer poly-1 shows low molecular weights. The polymerization results were compared with those obtained for a conventional solution polymerization of methyl methacrylate.     

脉冲激光轰击法连续制备纳米铜研究

采用脉冲激光轰击法连续制备了纳米铜及表面活性剂原位修饰的油溶性纳米铜,用UV-Vis研究了不同表面活性剂不同浓度对纳米铜／乙醇溶胶的紫外-可见光谱的影响,从而确定各表面活性剂的最佳浓度,用TEM研究了不同表面活性剂对纳米铜溶胶分散稳定性的影响,确定了最佳表面活性剂为平平加O,傅立叶红外光谱发现纳米铜／乙醇溶胶中乙醇分子基团振动波长受纳米铜颗粒的影响而出现红移现象,分散性实验表明：平平加O表面修饰纳米铜具有良好的油溶性。     

Behavior of free radical transfer between aqueous phase and latex particles in emulsion polymerization

The free radical transfer between aqueous phase and latex particles was investigated in the seeded emulsion polymerization using potassium persulfate (KPS) as an initiator. The effects of seed particle size, initiator concentration and polymerization temperature on the radical entry into and/or exit from the particles were emphatically examined. The results suggest that the aqueous-phase radical entry into the particles should be a competitive process, i.e. the competition between radical diffusion from the bulk aqueous phase to the interface of particles and its reactions in the aqueous phase determines whether it may be adsorbed into the particles or not. This implies that the reactions and properties of radicals in the aqueous phase play an essential role in particle nucleation in the emulsion polymerization.     

Photomediated controlled radical polymerization

Photomediated controlled radical polymerization is a versatile method to prepare, under mild conditions, various well-defined polymers with complex architecture, such as block and graft copolymers, sequence-controlled polymers, or hybrid materials via surface-initiated polymerization. It also provides opportunity to manipulate the reaction through spatiotemporal control. This review presents a comprehensive account of the fundamentals and applications of various photomediated CRP techniques, including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), nitroxide mediated polymerization (NMP) and other procedures. In addition, mechanistic aspects of other photomediated methods are discussed.     

m－DVB和p－DVB原子转移自由基聚合动力学

以氯端基聚苯乙烯为大分子引发剂,氯化亚铜／２,２’－联二吡喧为催化剂,邻二甲苯为溶剂,用气相色谱分别研究了对二乙烯基苯（ｐ－ＤＶＢ）和间二乙烯在苯（ｍ－ＤＶＢ）在１００￣１３０℃的原子转移自由基聚合动力学。结果表明,聚合符合表观一级反应动力学。ｐ－ＤＶＢ在１００,１１０,１２０、１３０℃的聚合速率常数分别为０．０７２,０．１２８,０．３７９,０．５４６ｈ＾－１,ｍ－ＤＶＢ在上述不同温度的聚合速率常     

Radical copolymerization of 2-acetoxyacrylates with nitrile monomers and properties of the copolymers

Radical copolymerization of captodatively (cd) substituted methyl and ethyl 2-acetoxyacrylates (MAA and EAA) with electron-withdrawing olefins including vinylidene cyanide and acrylonitrile is studied using azobisisobutylonitrile as a radical initiator at 60 °C, and the structure of copolymers is examined by ¹³C NMR spectra. It is found that the copolymerization of MAA and EAA with vinylidene cyanide provides a 1:1 alternating copolymer with ε-tacticity of 0.540 and 0.517, respectively, but that with acrylonitrile gives a random copolymer containing a larger amount of acetoxyacrylate unit. Addition of zinc chloride to the copolymerization of MAA and acrylonitrile, however, leads to the increase of acrylonitrile unit contents in the copolymer. Dielectric constant, gas permeability, and deformation by elongation of the copolymer film of MAA and vinylidene cyanide are also examined.     

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.     

Structural control in radical polymerization with 1,1 diphenylethylene: 2. Behavior of MMA-DPE copolymer in radical polymerization

Copolymers of 1,1-diphenylethylene (DPE) behave in a very special way in radical polymerization. Particularly, the behavior of MMA-DPE copolymers in radical polymerization is investigated. The results reveal that the semiquinoid structure of the precursor polymer identified in a previous contribution is activated by the attack of free radicals and thus, in a second stage polymerization with a second monomer, block copolymers are formed. The block copolymer yield depends strongly on the ratio between the amount of DPE-containing precursor polymer and the initiator and monomer concentration used in the second stage. The mechanism proposed is able to explain at least qualitatively all experimental results including the restriction of this mode of control of radical polymerization to the formation of diblock copolymers only.     

Iron-mediated reversible deactivation controlled radical polymerization

Metal-mediated reversible deactivation radical polymerization (RDRP) is now a cornerstone of functional polymer synthesis, dominated by copper complexes and the Atom Transfer Radical Polymerization (ATRP) moniker. A limitation of this approach is the contamination of the resultant polymers by the coloured copper complexes, thus requiring further purification, although protocols to reduce the amount of copper catalyst have been developed. Iron is an interesting alternative because of its low cost, low toxicity and reduced intensity of its optical absorption spectrum. Use of this metal in RDRP began in the late 90s and has continuously intensified. This review comprehensively covers all the work reported so far on RDRP mediated by iron complexes, organized according to ligand type, and discusses the specificities of this metal in terms of the multitude of accessible spin states and the interplay of different equilibria: atom transfer vs. direct radical trapping, associative vs. dissociative exchange, chain transfer by direct β-H atom transfer vs. β-H elimination from the dormant alkyl species.     

Effect of annealing on SiC thin films prepared by pulsed laser deposition

Crystalline cubic SiC thin films were successfully fabricated on Si(100) substrates by using laser deposition combined with a vacuum annealing process. The effect of annealing conditions on the structure of the thin films was investigated by X-ray diffraction and Fourier transform infrared spectroscopy. It was demonstrated that amorphous SiC films deposited at 800°C could be transformed into crystalline phase after being annealed in a vacuum and that the annealing temperature played an important role in this transformation, with an optimum annealing temperature of 980°C. Results of X-ray photoelectron spectroscopy revealed the approximate stoichiometry of the SiC films. The characteristic microstructure displayed in a scanning electron microscope image of the films was indicative of epitaxial growth along the (100) plane.     

Living free radical polymerization using cyclic trifunctional initiator

Free radical polymerization is a very robust route to produce polymers, however, it does not promote a control on the microstructure of the polymer: usually polymers with wide molecular weight distribution and polydispersity (PDI) greater than 1.5 are obtained. Ionic polymerization can be used to produce polymers with low PDI, however, this kind of route needs to be held with a high degree of purity. Living free radical polymerization (LFRP) is a promising technique to produce controlled polymers. In the literature, monofunctional conventional initiators have been used in LFRP process. In this study, the use of a cyclic trifunctional peroxide on controlled nitroxide mediated radical polymerization was investigated. The initiator used was Trigonox 301 (3,6,9‐triethyl‐3,6,9‐trimethyl‐1,4,7‐triperoxonane—41% solution in isoparaffinic hydrocarbons) and TEMPO (2,2,6,6‐tetramethyl‐1‐piperidin‐1‐oxyl) was used as controller agent. The results obtained show that it is possible to produce controlled polymers at certain operating conditions, when Trigonox 301 is used. It could be observed that the concentration of initiator and the ratio [TEMPO]/[Trigonox 301] have a huge effect on the polymer properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of laser power density on the electrochromic properties of WO3 films obtained by pulsed laser deposition

Pulsed laser deposition (PLD) was used to prepare tungsten trioxide (WO₃) films on ITO substrates with a varying laser power density of 4.0–5.5 W/cm². XPS indicated that when the laser power density decreased, the peak positions of the W 4f and O 1s orbits shifted slightly to low energy due to the difference in oxygen vacancies. As the laser power density decreased, W⁶⁺ gradually replaced the lattice position of O^2?, increasing oxygen vacancies in the lattice. The transmittance modulated values (ΔT) were over 44% at 830 nm, indicating strong absorption by the WO₃ thin films in the near-infrared ray. The switching time of the WO₃ thin films between bleached states and coloured states decreased as the laser power density increased due to the amorphous structure, morphology, and lower oxygen deficiency at a high power density. The high ΔT and very fast switching time of t_b (1.09 s) and t_c (6.01 s) demonstrated the excellent electrochromic (EC) properties of the WO₃ films prepared by PLD.     

Photoinitiated free radical polymerization of methylmethacrylate by using of quinoxalines in the presence of aldehydes

A series of quinoxalines at different concentrations were used in the presence of benzaldehyde (BA) and phthaldialdehyde (PDA) to initiate the polymerization of methylmethacrylate (MMA) formulation containing 2-ethyl-2-(hydroxymethyl)-1,3-propanediol-trimethylmethacrylate (TMPTMA) as a multifunctional acrylate. Highest conversion percentage of polymerization of MMA were obtained in the presence of benzaldehyde when diphenylquinoxaline, phenanthroquinoxaline and acenaphthoquinoxaline were used as an initiator. In the case of benzaldehyde, benzoyl radical were possibly responsible for the initiation of polymerization where quinoxalines do not initiate polymerization of acrylates in the presence of oxygen.     

Solvent effect on TEMPO-mediated living free radical dispersion polymerization of styrene

Living free radical dispersion polymerization of styrene in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 2,2-azobisisobutyronitrile (AIBN) was conducted in various glycol media having different solubility parameters. Genuine heterogeneous polymerization nature was observed from the initial stage of the reaction. The solubility parameter of the media was found to significantly influence the living characteristics of the polymerization including the polymerization kinetics and the molecular weight evolution at a fixed concentration of TEMPO. A grafting of stabilizer molecules onto monomer and a possible partitioning of TEMPO between the continuous medium and particle phase were postulated for the deviation of the experimentally obtained molecular weight from the calculated value. For a poor medium for styrene, such as diethylene glycol, the living nature was achieved by increasing the amount of TEMPO. The polystyrene (PS) microsphere was well obtained in all tested glycol media and the average size was increased with enhanced affinity of the media to styrene and with decreasing concentration of TEMPO.     

Kinetic study of radical polymerization. II. Solid‐state bulk polymerization of sodium methacrylate by differential scanning calorimetry

The solid‐state radical polymerization of sodium methacrylate was investigated. It was initiated by azobisisobutyronitrile, which was used as a radical initiator. Differential scanning calorimetry (DSC) was used to observe the endothermic and exothermic transitions during the polymerization reaction. Structural studies were performed with the DSC thermograms and Fourier transform infrared and ultraviolet–visible spectra, and all of the results confirmed the progress of the reaction. The obtained data revealed that the polymerization reaction proceeded completely with a 100% conversion. ΔH of this reaction was calculated with various amounts of the initiator, and the peak temperatures were determined at different heating rates. The activation energy (19.7 kcal mol^?1) was also obtained by the Kissinger method for this type of solid polymerization reaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1648–1654, 2003