Photochemically masked benzophenone: Photoinitiated free radical polymerization by using benzodioxinone

A new photoinitiating system for free radical polymerization of methyl methacrylate (MMA) is reported. This system consists of benzodioxinone and hydrogen donors such as triethylamine (TEA), N,N-dimethyl ethanol amine (DMEA) and tetrahydrofuran (THF). A feasible mechanism involves photoinduced formation of benzophenone from benzodioxinone and subsequent hydrogen abstraction of photoexcited benzophenone from hydrogen donors to yield radicals capable of initiating polymerization of MMA.     

Unexpected effect of the monomer concentration on the rate of a radical polymerization

The radical polymerization of vinyl monomers is usually initiated by physical and chemical means. After an increasing polymerization rate, R_p, at low monomer concentrations, some reactive systems show an unexpected minimum for R_p at high enough monomer concentrations. The radical polymerization of methyl methacrylate (MMA) initiated by the redox system D -glucose–ceric ion at varying MMA concentration is discussed. The peculiar behaviour of R_p is explained by the presence of two circumstances: the initiation rate from D -glucose radicals does not depend on MMA concentration when most of the D -glucose radicals formed react by adding to monomer, and the radical chains initiated by D -glucose radicals undergo mutual termination with a portion of the radical chains initiated by monomer radicals. Some information about the nature of the polymer end-groups is reached from the mechanistic approach.     

Atom transfer radical polymerization of methyl methacrylate with low concentration of initiating system under microwave irradiation

Homogeneous atom transfer radical polymerization of methyl methacrylate (MMA) under microwave irradiation (MI) with low concentration of initiating system [ethyl 2-bromobutyrate (EBB)/CuCl/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA)] was successfully carried out in N,N-dimethylformamide (DMF) at 69 °C. Plots of ln ([M]₀/[M]) vs. time and molecular weight evolution vs. conversion showed a linear dependence. A 27.3% conversion for a polymer with number-average molecular weight (M_n) of 57,280 and a polydispersity index (PDI) of 1.19, was obtained under MI (360 W) with the ratio of [MMA]₀/[EBB]₀/[CuCl]₀/[PMDETA]₀=2400/1/2/2 in only 150 min; but 963 min was needed under conventional heating (CH) process to reach a 26.0 % conversion (M_n=63,990 and PDI=1.14) under identical polymerization conditions, indicating a significant enhancement of the polymerization rate under MI.     

Photopolymerization of methyl methacrylate using benzoin isopropyl ether as photoinitiator: Effect of thiophenol compounds

Effects of a series of thiophenols R? ArSH with substituting groups R in the para-position and 2-mercaptobenzoic acid on the kinetics of polymerization of methyl methacrylate (MMA) photoinitiated by benzoin isopropyl ether (BIPE) were investigated using an autorecording dilatometer. Thiophenols were found to have a dual effect on polymerization: reducing induction time and accelerating rate of polymerization. A mechanism was proposed suggesting that this increased rate of polymerization and reduced induction time with addition of a thiophenol is due to the fact that, instead of consuming radicals, the dissolved oxygen in the MMA/BIPE system can be converted into active radicals through effective photooxidation of the thiophenol. Although the maximum increase in rate of polymerization is of a minor difference between various thiophenol compounds, reduction in induction time is strongly dependent on the nature of substituting groups in the following order: ? CH₃ > ? CH(CH₃)₂ > ? OCH₃ or ? Cl > ? H. 2-Mercaptobenzoic acid, on the other hand, increases induction time and decreases rate of polymerization. © 1993 John Wiley & Sons, Inc.     

Block Copolymers by Using Combined Controlled Radical and Radical Promoted Cationic Polymerization Methods

Summary A novel synthetic procedure for the preparation of block copolymers based on the combination of diphenylethene (DPE) mediated controlled radical polymerization and radical promoted cationic polymerization is described. In the first step, the controlled polymerization of styrene (St) and methyl methacrylate (MMA) in presence of DPE yields polymers with various structures of diphenylethyl units in the main chain. Upon heating these prepolymers undergo scission to give polymeric radicals. Oxidation of these radicals by onium salts such as diphenyliodonium heaxfluorophosphate (Ph₂I⁺PF₆ ⁻) and N-ethoxy-2-methylpyridinium hexafluorophosphate (EMP⁺PF₆ ⁻) yields reactive cations capable of initiating cationic polymerization of cyclohexene oxide (CHO). Block copolymer structure was confirmed by IR and NMR spectral measurements and GPC analysis. Received: 13 December 2002/Revised version: 25 March 2003/ Accepted: 11 March 2003 Correspondence to Yusuf Yagci     

Vinyl polymerization: 414. Polymerization of vinyl monomer initiated by poly(N,N,N-trimethyl-N-2-methacryloxyethyl)ammonium chloride

The polymerization of vinyl monomer initiated by an aqueous solution of poly(N,N,N-trimethyl-N-2-methacryloxyethyl)ammonium chloride (poly(Q-DMAEM-CI) has been carried out at 85°C. The effects of the amounts of vinyl monomer, poly(Q-DMAEM-CI) and water on the conversion of vinyl monomer have been studied. The overall activation energy in the polymerization of MMA is estimated as 41.9 kJ mol^?1. The polymerization proceeds through a radical mechanism. The location in which the polymerization occurs is discussed. The selectivity for vinyl monomer is explained by ‘the concept of hard and soft hydrophobic areas and monomers’.     

Atom transfer radical polymerizations of methyl methacrylate and styrene with an iniferter reagent as the initiator

Three novel iniferter reagents were synthesized and used as initiators for the polymerizations of methyl methacrylate (MMA) and styrene (St) in the presence of copper(I) bromide and N,N,N′,N″,N″‐pentamethyldiethylenetriamine at 90 and 115°C, respectively. All the polymerizations were well controlled, with a linear increase in the number‐average molecular weights during increased monomer conversions and relatively narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight ≤ 1.36) throughout the polymerization processes. The polymerization rate of MMA was faster in bulk than that in solution and was influenced by the different polarities of the solvents. A slight change in the chemical structures of the initiators had no obvious effect on the polymerization rates of MMA and St. The initiator efficiency toward MMA was lower than that toward St. The results of ¹H‐NMR, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrum analysis, and chain‐extension experiments demonstrated that well‐defined poly(methyl methacrylate) and polystyrene bearing photolabile groups could be obtained via atom transfer radical polymerization (ATRP) with three iniferter reagents as initiators. The polymerization mechanism for this novel initiation system was a common ATRP process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Photomediated atom transfer radical polymerization of MMA under long-wavelength light irradiation

In this study, a novel photocatalyst, pentarylenebis(dicarboximide) dye: (1,6,13,18-tetra(4-(2,3,3-trimethylbut-2-yl)phenoxy)-N,N’-(2,6-diisopropylphenyl)-pentarylene-3,4,15,16-tetracarboxidiimide) (TTPDPT), was first used in metal-free photoinduced atom transfer radical polymerization (ATRP) of methyl methacrylates (MMA). The initiator was methyl α-bromoisobutyrate (MBI) and the light source was mild near-infrared (NIR) light irradiation (λ_max?≈?870 nm). The TTPDPT-mediated ATRP relies on in situ photoreduction of a MBI through an electron transfer process to generate the desired alkyl radical, which could induce polymerization of the monomer. The photoinduced metal-free ATRP of MMA shows typical characteristics of controlled free radical polymerization, showing the linear evolution of number-average molecular weight (M_n,GPC) with monomer conversion, where polymers with predetermined degree of polymerization have well-controlled molecular weights and narrow molecular weight distribution (M_w/M_n). The photoinduced metal-free ATRP of MMA can be carried out with just ppm level of TTPDPT. The polymerization initiation and propagation can be operated by the aid of pulsed light sequences while NIR light source was used to promote carbon–carbon bond formation and to produce poly(methyl methacrylate) (PMMA) with M_w/M_n as low as 1.5. The synthesized PMMA was characterized by ¹H nuclear magnetic resonance (¹H NMR). The resultant PMMA contained a bromide end group that can be employed to reinitiate styrene polymerization to produce block copolymers through chain extension experiments.     

2‐Mercapto thioxanthone as a chain transfer agent in free‐radical polymerization: A versatile route to incorporate thioxanthone moieties into polymer chain‐ends

2‐Mercapto thioxanthone (TX‐SH) was used as a chain transfer agent in free‐radical polymerization of methyl methacrylate (MMA) and styrene (St), by using 2,2′‐azobisisobutyronitrile (AIBN) as an initiator at 70°C. Chain transfer constants were found to be 1.41 and 0.12 for St and MMA, respectively. The use of TX‐SH as a chain transfer agent leads to the formation of polymers with thioxanthone (TX) end groups. The incorporation TX moiety was confirmed by spectral measurements. Polymers obtained this way were used as triplet photosensitizer in free‐radical polymerization of MMA in the presence of a hydrogen donor such as N‐methyldiethanolamine (MDEA). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3766–3770, 2007     

Light-induced crosslinking polymerization of a novel N-substituted bis-maleimide monomer

The photoinduced polymerization of a N-alkyl substituted bis-maleimide (MI) has been studied by real-time infrared spectroscopy. The polymerization of the neat monomer proceeds rapidly and extensively because of both its great absorbance in the UV-range, and the presence on the alkyl chain of easily abstractable hydrogen atoms, which are needed to produce the initiating radicals. Such photoinitiator-free resin was found to be less sensitive to oxygen inhibition than typical UV-curable acrylate resins, the peroxyl radicals formed being capable to propagate the chain reaction. This N-substituted bis-maleimide was also successfully used as monomeric photoinitiator to induce the crosslinking polymerization of acrylate monomers through the free radicals formed upon its photolysis.     

Tentative study on kinetics of bulk polymerization of methyl methacrylate in presence of montmorillonite

The kinetics of the bulk polymerization of methyl methacrylate (MMA) in the presence of montmorillonite (MMT) were studied. The effect of MMT on the radical polymerization of MMA was researched by determining the polymerization rate dilatometrically. It was assumed that there were both bimolecular and monomolecular termination processes involved in the termination of the radicals in the polymerization. It was found that a lower benzoyl peroxide (BPO) concentration promotes a higher fraction of monomolecular mode in chain termination. The results show that there is an optimal ratio of MMT to initiator that increases the bulk polymerization rate of MMA. The X‐ray results show that the layer structure of the formed PMMA–MMT composites was also affected by the BPO concentration. With lower initiator concentration, less pronounced layer structure will be observed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3690–3695, 2003     

Long-lived polymer radicals: 5. Synthesis of block copolymer by the reaction of living poly(N-phenylmethacrylamide) radical with vinyl monomers

When N-phenylmethacrylamide (NPMAm) was photopolymerized at room temperature in benzene by using di-tert-butyl peroxide as sensitizer, the resulting heterogeneous polymerization mixture was found to contain very stable propagating poly (NPMAm) radical. An e.s.r. study revealed that the living polymer radical reacted easily at room temperature with methyl methacrylate (MMA) yielding persistent poly (MMA) radical. Block copolymer was prepared by the reaction of living poly (NPMAm) radical with MMA and methyl acrylate. Block copolymer constituted 30–60% of the resultant total polymer. Block copolymer was characterized by g.p.c., i.r. spectroscopy and electron microscopy.     

Synthesis and characterization of trifluoromethyl substituted styrene polymers and copolymers with methacrylates: Effects of trifluoromethyl substituent on styrene

2-Trifluoromethyl styrene (2TFMS), 2,5-bis(trifluoromethyl) styrene (25BTFMS), and 3,5-bis(trifluoromethyl) styrene (35BTFMS) were synthesized. These styrenes were readily polymerized in bulk and also in solution using AIBN as a free radical initiator. The polymerization rate of these trifluoromethyl substituted styrenes and other monomers such as styrene (St), pentafluorostyrene (PFS) and 4-trifluoromethyl-tetrafluorostyrene (TFMTFS) were measured in benzene and dioxane by monitoring the ¹H NMR spectra of the double bond hydrogen. The order of polymerization rates was TFMTFS > 35BTFMS > 25BTFMS > PFS > 2TFMS > St. T_gs of styrene polymers with CF₃ substituted on the ortho position of the phenyl ring were much higher than those of the meta and para substituted styrenes due to the steric hindrance of the bulky CF₃ group close to the polymer main chain, which resulted in a decrease in the segment mobility of the polymer chains and an increasing T_g of the polymers. The copolymers of 2TFMS with methyl methacrylate (MMA) and also 25BTFMS with trifluoroethyl methacrylate (TFEMA) were prepared. T_gs of the copolymers were in the range of 120-145 °C and the copolymers were transparent and thermally stable. The copolymer films were flexible and exhibited high transmittance as the homopolymers of MMA and TFEMA. Thus, these copolymers may be utilized as novel optical materials.     

Activators regenerated by electron transfer in ATRP of methyl methacrylate with alcohol as reducing agent in the presence of a base

Poly(methyl methacrylate) (PMMA) was synthesized via activators regenerated by electron transfer (ARGET) in atom transfer radical polymerization (ATRP) (ARGET ATRP) of methyl methacrylate in N,N-dimethylformamide and using ethyl 2-bromoisobutyrate as initiator, CuCl₂ as catalyst, N,N,N′,N′-tetramethylethylene-diamine as ligand and ethanol as a reducing agent. The polymerization temperature was kept at 70 °C. A well-defined PMMA with predetermined molecular weight and narrow molecular weight distribution was obtained. A linear relationship between ln([M]₀/[M]) and polymerization time was found to show the living and controllable radical polymerization. The molecular weights of the obtained polymers increased linearly with monomer conversion and the data are in good agreement with the theoretical values with narrow molecular weight distribution (M _w/M _n). That is to say, alcohols were found to be a kind of highly efficient agents in the presence of Na₂CO₃ in this system. The effects of temperature, different types of alcohols and the amount of n-propanol on the polymerization were investigated. With increasing temperature (changed from 70 to 90 °C) and the amount of n-propanol (changed from 500:1:1:2:1:2 to 500:1:1:2:10:2), the conversion increased from 15.6 to 34.7 % and from 8.6 to 26.8 %, respectively. However, the value of M _w/M _n became broader when the molar ratio of [MMA]₀/[EBiB]₀/[CuCl₂]₀/[TMEDA]₀/[n-propanol]₀/[Na₂CO₃]₀ was 500:1:1:2:1:2, indicating that the amount of n-propanol played an important role in ARGET ATRP of MMA. The activation energy was calculated to be 51.96 kJ/mol. The different types of alcohol were demonstrated to be an efficient reducing agent in this system except for tert-butyl alcohol. The “living” feature of the obtained polymer was further verified by a chain extension experiment. The obtained polymer was characterized by ¹H NMR and GPC.     

Investigation of AGET-ATRP of methyl methacrylate in surface-active ionic liquid microemulsions

The first example of N-tetradecyl-N-methyl-2-pyrrolidonium was synthesized by using bromide as both an ionic liquid and a surfactant in ionic liquid-based microemulsion polymerization of methyl methacrylate under atom transfer radical polymerization at activator generated electron transfer conditions. The polymerization was carried out at room temperature using copper (II) chloride (CuCl₂)/hexamethylene tetramine (HMTA) as a catalyst and CCl₄ as an initiator in the absence of surfactant. Ascorbic acid was used as reducing agent. A pseudoternary phase diagram was constructed at 25 °C using the water titration method in the presence and absence of hexadecyltrimethylammonium bromide (CTAB). Kinetics experimental results showed that the polymerization proceeded in a controlled/‘living’ process as evidenced by a linear increase of molecular weights of polymers with monomer conversion with a relatively narrow polydispersity (<1.35) in all cases and M _n,GPC values of the resulting polymer were in excellent agreement with the theoretical values M _n,th. The polymerization rate increased with the amount of surfactant. However, the polydispersity became broader. The polymerization rate increased with the amount of ligand and decreased with the amount of monomer. In this system, particles of nanoscale (33–51 nm) were prepared. The average particle diameter was found to be affected by the amount of surfactant. The AGET-ATRP of MMA retained the characteristic of living polymerization when the ionic liquid C₁₄MPnBr and catalyst complex were recovered and reused. Living nature of the polymerization was confirmed by the successful homo chain extension experiment. The resultant PMMA was characterized by ¹H NMR spectroscopy and gel permeation chromatography techniques.     

Reverse ATRP of acrylonitrile with diethyl 2,3-dicyano-2,3-diphenyl succinate/FeCl3/iminodiacetic acid

The reverse atom transfer radical polymerization (RATRP) technique using FeCl₃/iminodiacetic acid (IMA) complex as a catalyst was applied to the living radical polymerization of acrylonitrile (AN). A hexa-substituted ethane thermal initiator, diethyl 2,3-dicyano-2,3-diphenylsuccinate (DCDPS), was firstly used as the initiator in this iron-based RATRP 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]:[DCDPS]:[FeCl₃]:[IMA] at 500:1:2:4. The rate of polymerization increases with increasing the polymerization temperature and the apparent activation energy was calculated to be 49.9 kJ mol⁻¹. 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/IMA catalyst system via a conventional ATRP process. The resultant polyacrylonitrile fibers were obtained with the fineness at 1.16 dtex and the tenacity at 6.01cN dtex⁻¹.     

Controlled/“living” radical polymerization of methyl methacrylate catalyzed by CpCo(I) complexes conveniently generated from cobaltocene in situ

Atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) catalyzed by a series of exo-substituted η⁴-cyclopentadiene CpCo(I) complexes generated conveniently from cobaltocene in situ was studied. It is found that the steric and electronic effects of the exo-substituents at the η⁴-cyclopentadiene ligand have significant effects on the polymerization. More bulky or weaker electron-withdrawing substituents lead to higher polymerization rate and better initiation efficiency. Polymerization of styrene, and the block copolymerization of MMA with styrene were also studied, using the selected catalyst system.     

Non-classical free-radical polymerization: Degradative addition to monomer in the polymerization of 1-vinylimidazole

A study of the free-radical polymerization of 1-vinylimidazole at 70°C is described; most attention has been devoted to the (homogeneous) reaction in ethanol, but polymerizations in N,N-dimethylformamide (DMF), water and bulk monomer are also considered. The polymerization in ethanol is unusual in that the rate becomes effectively zero-order in monomer M at moderately high [M]. Kinetic results indicate the occurrence of a degradative reaction between propagating radicals and monomer (see below); the available molecular weight data suggest that bimolecular termination takes place by radical combination. Polymerization in DMF is generally similar although the kinetic treatment is rather less satisfactory. At high [M] there are indications of occlusion phenomena in DMF and these latter are marked in the (heterogeneous) bulk polymerization. In water, significant interactions between solvent and monomer, evidenced by viscosity-composition behaviour, affect the kinetics of polymerization, which, however, resemble those with the other solvents. Of considerable interest is the influence of pH on rates and degrees of polymerization, which change in the same sense and to nearly the same extent. We believe these findings imply suppression of the degradative reaction by protonation of the monomer. The degradative reaction between propagating chains and monomer is thought to be formation of a relatively unreactive radical by addition to position 2 of the monomer, rather than by chain transfer. Evidence for this is adduced from molecular weight data, which are not consistent with transfer, the existence of occlusion phenomena and the observations on the pH dependence in aqueous solution. Molecular weight distributions expected in polymerizations of this type are calculated in the Appendix. Reinitiation by the adduct radical, which probably occurs only to a limited extent under non-occlusion conditions, becomes important when occlusion is significant. A kinetic treatment of these phenomena is presented.     

Rôle of semi-pinacol radicals in the benzophenone-photoinitiated polymerization of methyl methacrylate

Semi-pinacol radicals (Ph₂?OH), generated by photolysis of benzophenone in the presence of benzhydrol, are shown to act as terminating species in free radical polymerization of methyl methacrylate (MMA) at 30° and 70°C; initiation by Ph₂?OH radicals is significant only at the higher temperature. Polymerizations of MMA photoinitiated by substituted benzophenones (Ar₂CO) in hydrogen donor solvents are affected by these reactions and, contrary to previous assumption, the present results demonstrate that combination of Ar₂?OH with solvent derived radicals is a process which competes with initiation. Results from the photochemical experiments are discussed in relation to those of other workers, obtained by thermal generation of semi-pinacol radicals.