Polymer Radical Concentration Research Articles

Carbon Dots as a Promising Green Photocatalyst for Free Radical and ATRP-Based Radical Photopolymerization with Blue LEDs.

Carbon dots (CDs) have been used for the first time as a sensitizer to initiate and activate free radical and controlled radical polymerization, respectively, based on an ATRP protocol with blue LEDs. Consideration of diverse heteroatom‐doped CDs indicated that N‐doped CDs could serve as an effective photocatalyst and photosensitizer in combination with LEDs emitting either at 405 nm or 470 nm. Free radical polymerization was initiated by combining the CDs with an iodonium or sulfonium salt in tri(propylene glycol) diacrylate. Polymerization of methyl methacrylate (MMA) by photo‐induced ATRP was achieved with CDs and ethyl α‐bromophenylacetate using CuII as catalyst in the ppm range. The polymers obtained showed temporal control, narrower dispersity ≲1.5, and chain‐end fidelity. The first‐order kinetics and ON/OFF experiments additionally gave evidence of the constant concentration of polymer radicals. No remarkable cytotoxic activity was observed for the CDs, underlining their biocompatibility.

A Study of the Effects of Irradiation on the Polymerization of Dual-cured Self-etching Bonding System Using Electron Spin Resonance (ESR) Spectroscopy

The purpose of this study was to investigate the effect of irradiation on the polymerization behavior of a bonding agent of a dual-cured self-etching bonding system. By means of electron spin resonance spectroscopy, it was shown that the concentration of polymer radicals in samples cured chemically without irradiation was closely similar to that in samples dual-cured under irradiation. There was no significant difference in the time required to reach the maximum spin concentration between these two sample groups, thereby showing that the radical generation rates were similar. Findings of this study revealed that the dual-cured self-etching bonding system tested in this study was effective in polymerization in regions where irradiated light could hardly reach.

Synthesis and Characterization of Amphiphilic Polyphosphates with Hydrophilic Graft Chains and Cholesteryl Groups as Nanocarriers

Amphiphilic polyphosphate graft copolymers with varied densities of cholesteryl esters and hydrophilic graft chains were prepared, and the solution properties of the graft copolymers were evaluated. Polyphosphates were synthesized as backbones by ring-opening polymerization of 2-isopropyl-2-oxo-1,3,2-dioxaphospholane (IPP), 2-(2-oxo-1,3,2-dioxaphosphoroyloxyethyl-2-bromoisobutyrate) (OPBB), and 2-choresteryl-2-oxo-1,3,2-dioxaphospholane (ChOP) using triisobutylaluminum as an initiator. Three types of polyphosphates (PIBr(x)Ch(y), x = number of OPBB units in a polymer; y = number of ChOP units in a polymer) such as PIBr4, PIBr6Ch1, and PIBr3Ch2 were obtained. The molecular weights of these polymers were 2.4 x 10(4), 2.4 x 10(4), and 2.6 x 10(4) g/mol, respectively. 2-Methacryloyloxyethyl phosphorylcholine (MPC) was grafted from the OPBB sites in PIBr(x)Ch(y) via atom transfer radical polymerization (ATRP) in EtOH. In each polymer system, the molecular weight of the graft polymer was linear with conversion. Furthermore, the polymer radical concentration remained constant during polymerization; that is, the molecular weights of the graft chains were easily controllable with polymerization time. The solution properties of amphiphilic PIBr(x)Ch(y)-g-PMPCs were investigated by the methods of surface tension measurement, light scattering, and fluorescence probe. The transition point (cmc) of the surface tension of the PIBr(x)Ch(y)-g-PMPCs aqueous solution decreased with an increase in the number of ChOP units in a graft polymer. Particularly, PIBr3Ch2-g-PMPC14.9K formed nanosized associates (R(h) = 7.5 nm) with 2.2 molecules above 0.1 wt %. v79 cells were used to evaluate the cytotoxicity of the graft polymers, but no cytotoxicity was observed. The graft polymers containing cholesteryl groups effectively enhanced the solubility of paclitaxel in an aqueous solution.

Homolysis of N‐alkoxyamines: A Computational Study

During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (Tc), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.

A Theoretical Study of the Structure and Bonding of UOX4(X=F, Cl, Br, I) Molecules: The Importance of InverseTransInfluence

During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (T(c)), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.

Synthesis and radical polymerization of methacrylic monomers with crown ethers in the ester residue: 1,4,7,10-tetraoxacyclododecan-2-ylmethyl methacrylate

The synthesis and radical polymerization of 1,4,7,10-tetraoxacyclododecan-2-ylmethylmethacrylate (CR4MA) is described. The polymerization reactions of CR4MA were carried out at different temperatures and the kinetic curves of monomer depletion against time were obtained by direct measurements of the instantaneous monomer concentrations by using nuclear magnetic resonance (NMR) spectroscopy. At the same time electron paramagnetic resonance (EPR) spectroscopy was used to determine the actual polymer radical concentration during all the reaction time. The conjunction of both techniques (NMR and EPR) allowed the determination of the polymerization rate parameter (2fkp/〈kt〉1/2) and separately of kp and 〈kt〉/f, where f, kp and 〈kt〉 are, respectively, the initiator efficiency factor and the overall averages of propagation (kp is considered to be practically independent of the chain length) and termination rate constants. The values found for this ratio and for kp were comparatively higher than those recently reported in the literature for its lateral open chain counterpart, the methacrylic monomer with equal number of oxyethylene units in the residue ester (TTEMA). However, the 〈kt〉 values were similar for the polymerization of both monomers CR4MA and TTEMA. The polymer, PCR4MA, is soluble in water as its open chain homologous, and exhibits a glass transition temperature in the vicinity of the ambient temperature (about 35 °C), much higher than the value found for the homologous polymethacrylate derived from the TTEMA.

Synthesis and radical polymerization of methacrylic monomers with crown ethers in the ester residue: 1,4,7,10-tetraoxacyclododecan-2-ylmethyl methacrylate

The synthesis and radical polymerization of 1,4,7,10-tetraoxacyclododecan-2-ylmethylmethacrylate (CR4MA) is described. The polymerization reactions of CR4MA were carried out at different temperatures and the kinetic curves of monomer depletion against time were obtained by direct measurements of the instantaneous monomer concentrations by using nuclear magnetic resonance (NMR) spectroscopy. At the same time electron paramagnetic resonance (EPR) spectroscopy was used to determine the actual polymer radical concentration during all the reaction time. The conjunction of both techniques (NMR and EPR) allowed the determination of the polymerization rate parameter (2fkp/〈kt〉1/2) and separately of kp and 〈kt〉/f, where f, kp and 〈kt〉 are, respectively, the initiator efficiency factor and the overall averages of propagation (kp is considered to be practically independent of the chain length) and termination rate constants. The values found for this ratio and for kp were comparatively higher than those recently reported in the literature for its lateral open chain counterpart, the methacrylic monomer with equal number of oxyethylene units in the residue ester (TTEMA). However, the 〈kt〉 values were similar for the polymerization of both monomers CR4MA and TTEMA. The polymer, PCR4MA, is soluble in water as its open chain homologous, and exhibits a glass transition temperature in the vicinity of the ambient temperature (about 35 °C), much higher than the value found for the homologous polymethacrylate derived from the TTEMA.

Limitations in the Synthesis of High Molecular Weight Polymers via Nitroxide-Mediated Controlled Radical Polymerization: Experimental Studies

Limitations associated with preparing high molecular weight polystyrene (PS) by nitroxide-mediated controlled radical polymerization have been tested by considering the role of unimolecular initiator concentration on active polymer radical concentration and thus degree of polymerization. Recent theories ignoring autopolymerization effects lead to the conclusion that, at low monomer conversion, the number-average molecular weight, Mn, scales with the −2/3 power of unimolecular initiator concentration. Bulk polymerizations were done using either α-methylstyryl di-tert-butyl nitroxide (A−T) as unimolecular initiator or PS macroinitiator made from A−T. These initiators allow relatively low reaction temperature (77, 87, or 97 °C) and moderate, but not eliminate, the contribution of autopolymerization or thermal initiation of polymerization. By varying unimolecular initiator concentration over nearly 4 orders of magnitude, well-controlled PS, with polydispersity index ≤ 1.4, can be made with Mn values in the range 114 000−238 000 g/mol using either A−T as initiator or a PS macroinitiator. For conditions yielding controlled PS, in general the experimental Mn−initiator concentration data afforded good agreement with the −2/3 power-law expression and allowed estimation of the equilibrium constant for the capping−uncapping reaction. However, attempts to make controlled, higher molecular weight PS by further reducing initiator concentration resulted in loss of control due to autopolymerization effects. The impact of autopolymerization in producing well-controlled PS was evident from studies yielding a nearly constant conversion as a function of macroinitiator concentration.

Interfacial Mass Transfer in Nitroxide-Mediated Miniemulsion Polymerization

Nitroxide-mediated polymerization (NMP) represents a viable way to produce polymers with predictable molecular weights (MWs) and well-defined architectures. Such polymers have potential application in the fabrication of commercially valuable block copolymers, drug delivery systems, nanotechnology, adhesives, surfactants, and polymer compatibilizers. The distinguishing feature of NMP is the use of a nitroxide, such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), to reversibly deactivate active polymer radicals to give a dormant alkoxyamine molecule (see Scheme 1). In Scheme 1, ka is the alkoxyamine dissociation rate coefficient, and kd is the radical deactivation rate coefficient. At 125 8C, the equilibrium coefficient for the reversible deactivation of polystyrene radicals by TEMPO has been measured to be Keq1⁄4 ka/kd1⁄4 2.1 10 11 M. Thus, the chemical equilibrium of the radical deactivation reaction favors the formation of the dormant alkoxyamine. As a result, the concentration of active polymer radicals in NMP systems is lower than in conventional free-radical polymerization systems, which reduces the likelihood Full Paper: A mathematical model has been developed to describe the interfacial mass transfer of TEMPO in a nitroxide-mediated miniemulsion polymerization (NMMP) system in the absence of chemical reactions. The model is used to examine how the diffusivity of TEMPO in the aqueous and organic droplet phases, the average droplet diameter and the nitroxide partition coefficient influences the time required for the nitroxide to reach phase equilibrium under non-steady state conditions. Our model predicts that phase equilibrium is achieved quickly (< 1 10 4 s) in NMMP systems under typical polymerization conditions and even at high monomer conversions when there is significant resistance to molecular diffusion. The characteristic time for reversible radical deactivation by TEMPO was found to be more than ten times greater than the predicted equilibration times, indicating that phase equilibrium will be achieved before TEMPO has an opportunity to react with active polymer radicals. However, significantly longer equilibration times are predicted, when average droplet diameters are as large as those typically found in emulsion and suspension polymerization systems, indicating that the aqueous and organic phase concentrations of nitroxide may not always be at phase equilibrium during polymerization in these systems. Influence of droplet phase TEMPO diffusivity, DTEMPO,drop, on the predicted organic phase concentration of TEMPO. Macromol. Theory Simul. 2002, 11, 953–960 953

MATHEMATICAL MODELING OF PARTICLE MORPHOLOGY DEVELOPMENT INDUCED BY RADICAL CONCENTRATION GRADIENTS IN SEEDED STYRENE HOMOPOLYMERIZATION

The development of concentration gradients in the seeded emulsion homopolymerization of styrene has been studied using a mathematical model. Under ideal conditions, emulsion polymer particles are expected to be relatively uniform across their radius. When this is true, kinetic models can be reliably used to predict the reaction rates, molecular weight, and composition (for copolymerizations). However for large particles, large radial gradients in the polymer radical concentration can develop during polymerization. While gradients in the monomer concentration are not observed, the radical concentration gradients result in non-uniform reaction rates across the particle radius, rendering conventional models unreliable. This study examines the effect of seed particle diameter and various kinetic parameters on the development of particle morphology.

Solid Phase Modification of Poly(propylene). The Role of the Polymer Texture in the Peroxide Distribution

Different poly(propylene)s were reacted with tert-butyl peroxybenzoate (TBPB) in solid phase and the time dependence of the concentration of tertiary polymer radicals ([A∗︁]) formed was measured by ESR spectroscopy at 145°C. The distribution (or solubility) of peroxide was found uneven (or limited), the segregation of solid dilauroyl peroxide (DLPO) was proved by DSC. It was found that not the crystalline/amorphous ratio was the decisive factor in poly(propylene)/peroxide solid phase reactions, but much more the crystallite size of the polymer texture. The latter was determined by X-ray measurements. Poly(propylene)s with average crystallite diameters of about 250 Å showed a characteristic dependence on peroxide concentration in the solid phase reaction of tertiary carbon radical forming. The log [A∗︁] vs. time curves show a good linearity, the relationships correspond to a unimolecular overall kinetics. When the crystallite diameters were 17–40% less than 250 Å, the polymer radical concentration ([A∗︁]) did not show a characteristic dependence on TBPB concentration. Our findings are interpretable in terms of capillary condensation of peroxides.

Effect of 1,1-diphenylethylene on the radical polymerization of di- n-butyl itaconate in benzene

Effect of 1,1-diphenylethylene (DPE) on the polymerization of di- n-butyl itaconate (DBI) with dimethyl 2,2 ′-azobisisobutyrate (MAIB) was studied at 50°C and 60°C in benzene kinetically and by means of ESR. DPE in the low concentration range (up to 0.03 mol/l) was found to serve as a retarder in the polymerization at 50°C and 60°C. As a result, the polymerization rate and the molecular weight of resulting polymer decreased with increasing DPE concentration. The rate constant of the addition of poly(DBI) radical to DPE was estimated to be 74 l/mol s at 50°C and 97 l/mol s at 60°C, respectively, compared to 9.5 l/mol s at 50°C and 11 l/mol s at 60°C as propagation rate constant of DBI. ESR spectrum of the propagating polymer radicals was observed for the actual polymerization mixtures containing DBI, DPE and MAIB in benzene. ESR-determined apparent k p value decreases with increasing DPE concentration, while the apparent k t value increases with the DPE concentration. This is because the relative concentration of DBI-ended polymer radical to DPE-ended one decreases with increasing DPE concentration.

Synthesis of Well-Defined, Semibranched, Hydrophilic−Hydrophobic Block Copolymers Using Atom Transfer Radical Polymerization

A series of well-defined, semibranched hydrophilic−hydrophobic diblock and triblock copolymers have been synthesized in high yield under environmentally friendly conditions via atom transfer radical polymerization (ATRP). The hydrophilic block is based on methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA), a branched analogue of PEO, and the hydrophobic component is a linear PPO block. In view of the commercial importance of linear, nonionic PEO−PPO type surfactants, we were interested in the aqueous solution properties of these related, semibranched POEGMA−PPO diblock and triblock copolymers. High conversions of POEGMA (>95%) and good control (high initiator efficiencies and low polydispersities) were routinely achieved within 16 h at 20 °C. The polymerization is first order with respect to monomer up to 95% conversion, which implies that the polymer radical concentration remains constant for the duration of the polymerization. As the PPO block length was fixed at 33 units, the hydrophilic−hydrophobic balance of the block copolymers was adjusted by varying the Dp of the POEGMA block; this was achieved simply by changing the monomer-to-macroinitiator ratio from 14 to 97. With linear PEO−PPO block copolymers the limiting surface tension is governed by the PPO block length. With the semibranched POEGMA−PPO copolymers a similar trend was also evident, since, for the fixed Dp of 33 for the PPO block, the surface tension curves are almost independent of both the length of the POEGMA block and the diblock/triblock copolymer architecture. Combined 1H NMR and DLS studies indicated that the POEGMA−PPO copolymers are thermoresponsive, forming large, highly hydrated micelles reversibly at 20−60 °C. This is understandable since coronal branching should reduce micellar packing efficiency, leading to bigger, looser micellar aggregates.

Radical polymerization of methyl methacrylate and styrene in the presence of ferrocene

The influence of ferrocene on the kinetic parameters of methyl methacrylate (MMA) or styrene polymerization is investigated. It is shown, that in the presence of ferrocene the growth of the initial rate of polymerization is accompanied with significant decrease of the polymerization degree of polymer obtained when peroxides are used as the polymerization initiators. The polymerization kinetics parameters are evaluated. Its demonstrated that the process of polymerization in the presence of a ferrocene has a low activation energy, that allows to carry out the polymerization at lower room temperatures. The influence of the ferrocene on the molecular-mass characteristics of poly(methyl methacrylate) (PMMA) was studied and it was found that the influence of ferrocene on PMMA molecular mass is determined by the nature of the initiator used. The decomposition temperature of PMMA increases with the increase of ferrocene concentration. Monte-Carlo technique was used for numerical simulation of MMA radical polymerization kinetics in the presence and in the absence of ferrocene and to follow the change of molecular-weight distribution during the polymerization. The peculiarities of polymer radical concentration changes at the initial stage of polymerization are shown. Satisfactory correspondence between calculated and experimental data was obtained.

Kinetic and ESR Studies of Ring-Opening Radical Polymerization of 1,1-Bis(ethoxycarbonyl)-2-vinylcyclopropane

Ring-opening radical polymerization of 1,1-bis(ethoxycarbonyl)-2-vinylcyclopropane (BEVCP) with dimethyl 2,2‘-azobisisobutyrate (MAIB) was investigated kinetically in benzene. The overall activation energy (Ea = 94 ± 4 kJ/mol) was calculated from the polymerization rate (Rp) obtained at 30−60 °C. Rp (40 °C) is given by Rp = k[MAIB]0.6±0.1[BEVCP]1.0±0.1, which is similar to that of conventional radical polymerizations involving bimolecular termination. The present polymerization system involves ESR-observable polymer radicals under the actual polymerization conditions. The total polymer radical concentration ([P•]) was determined at different temperatures by ESR. Using Rp, [P•], and the initiation rate obtained separately, the apparent rate constants of propagation (kp) and termination (kt) were estimated at different temperatures. The kp (71 L/mol·s, 60 °C) and kt (5.8 × l04 L/mol·s, 60 °C) values are much lower than those of typical vinyl monomers such as methyl methacrylate (MMA) and styrene. Owing to t...

Mechanism and Kinetics of RAFT-Based Living Radical Polymerizations of Styrene and Methyl Methacrylate

The bulk polymerizations of styrene and methyl methacrylate in the presence of model polymer−dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied. The polymerization rate, and hence the concentration of polymer radical P•, was proportional to [BPO]1/2. The pseudo-first-order activation rate constants kact were determined by the GPC peak-resolution and the polydispersity-analysis methods. The results showed that kact is directly proportional to [P•], indicating that reversible addition−fragmentation chain transfer (RAFT) is the only important mechanism of activation. The magnitude of the exchange rate constant kex (= kact/[P•]) was strongly dependent on both the structures of the dithiocarbonate group and the polymer. The kex values for the three RAFT systems examined in this work were all very large, which explains why these systems can provide low-polydispersity polymers from an early stage of polymerization. The activation energy of kex for a polystyryl dithioacetate (PSt−SCSCH3) was 21.0 kJ mol-1, which is reasonable for a fast addition reaction.

Polymerization control through the free-radical retrograde-precipitation polymerization process

In this article, we present results of our work in a novel polymerization process [called the free-radical retrograde-precipitation polymerization (or FRRPP) process] that occurs at temperatures above the lower critical solution temperature. In this process, conversion-time plots for styrene polymerization in ether show autoacceleration at the beginning, followed by a relatively long period of reduced conversion rate starting at conversions as low as 30% and at operating temperatures way below the glass transition of the reacting system. Molecular weight and polydispersity index data also indicate early autoacceleration (in the form of overshoots in these values), whereas the latter period of slow conversion rate is accompanied by stable levels of molecular weight and polydispersity index. Polymer radical concentration measurements show an initial sharp rise, followed by an asymptotic value, even after almost all the initiator molecules have already decomposed into radicals. With end-group analyses of product polystyrene and polymer radical data, we calculate a proportion of live polymeric radicals to asymptote at levels of 80–84% of all polymeric species, even after almost all initiator molecules have already decomposed into radicals. All the data presented herein verify the postulate of a controlled polymerization mechanism for the FRRPP process. Our results have become the basis for an anti-gel effect phenomenon that is derived from prior theoretical and experimental observations, in which phenomenological diffusivities vanish at the spinodal curve of the phase envelope. The universality of this behavior in FRRPP systems is manifested from similar observations in styrene polymerization in acetone and methacrylic acid polymerization in water. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 761–774, 1999

Characterization of ultrahigh molecular weight polyethylene irradiated with ?-rays and electron beams to high doses

Recently, γ-irradiation of ultrahigh molecular weight polyethylene (UHMWPE) to high doses such as 1 MGy was shown by Oonishi et al. to be very effective in improving the wear resistance of UHMWPE. The present work was undertaken to characterize the UHMWPE irradiated with γ-rays and electron beams to doses ranging from 25 KGy to 5 MGy. The radical concentration was determined by electron spin resonance (ESR) spectroscopy, while infrared spectroscopy was used to study the carbonyl and double bond formation on the irradiated UHMWPE. The change of the melting temperature of UHMWPE upon irradiation was examined by differential scanning calorimetry, while mechanical tests were performed to measure the compressive creep deformation, surface hardness, and tensile strength of the irradiated UHMWPE. The UHMWPE irradiated with γ-rays in air and then stored in air showed that the decrease in polymer radical concentration with time became lower with the increase in radiation dose. The decay of polymer radicals formed upon irradiation with γ-rays in N2 gas and stored in N2 gas was slow except for UHMWPE irradiated to 25 KGy. Irradiation with electron beams in air and the subsequent storage in air yielded a high concentration of polymer radicals, but the decay was much faster than that of γ-irradiated UHMWPE. The g-factor evaluated from the ESR spectra revealed that the polymer radicals transformed into oxidized ones during storage in air. The UHMWPE radicals formed by γ-irradiation in N2 gas to 1–5 MGy remained almost unchanged during storage in N2 gas. The gel fraction and the carbonyl and double bonds concentration increased with the increase in radiation dose when UHMWPE was γ-irradiated in air, while the melting temperature of UHMWPE increased upon irradiation in air, so far as a UHMWPE block, not powder, was γ-irradiated. The creep deformation decreased and surface hardness increased by γ-irradiation in air, suggesting that crosslinking was introduced into UHMWPE molecules upon irradiation to high doses. In conclusion, irradiation of UHMWPE with γ-rays and electron beams to high doses could introduce crosslinking to UHMWPE, resulting in increase in surface hardness and decrease in creep deformation. The trapped radicals in the irradiated UHMWPE decreased more quickly for electron-beam irradiation than for γ-ray irradiation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 159–168, 1999

Mechanism and Kinetics of Iodide-Mediated Polymerization of Styrene

The kinetics of the bulk polymerization of styrene in the presence of a model ω-polystyryl iodide (Mn ≈ 2000, and Mw/Mn ≈ 1.26) as mediator and benzoyl peroxide as initiator was studied. The rate of polymerization, Rp, was found to be independent of the iodide concentration, showing that the stationary concentration of polymer radicals, [P*], is determined by the balance of initiation and termination rates, as in the conventional (iodide-free) system. The pseudo-first-order activation rate constant kact of the model iodide was determined as a function of BPO concentration and temperature (50−80 °C) by both the GPC curve-resolution and polydispersity-analysis methods. The results showed that kact is directly proportional to [P*], which means that degenerative transfer (active species-exchanging transfer) is the only important mechanism of activation in this system. The activation energy for the transfer rate constant kex was found to be 27.8 kJ mol-1, somewhat smaller than the known activation energy for the styrene propagation rate constant kp of 32.5 kJ mol-1. This indicates that lowering, rather than raising, the reaction temperature will be more effective in preparing polystyrenes with a narrower polydispersity by the iodide-mediated polymerization. This is because the most important parameter for determining the polydispersity of a degenerative-transfer-type system is the kex/kp ratio, as has been known for some time.

Radical polymerization of benzylN-(2,6-dimethylphenyl)itaconamate

The polymerization of benzyl N-( 2,6-dimethylphenyl)itaconamate (BDMPI) with benzoyl peroxide (BPO) in N,N-dimethylformamide (DMF) was studied kinetically by ESR. The polymerization rate (R P ) at 70°C was given by R P = k[BPO] 0.78 [BDMPI] 11 . The overall activation energy of polymerization was determined to be 83.7 kJ/mol. The number-average molecular weight of poly( BDMPI) was in the range of 1500-2000 by gel permeation chromatography. From the ESR study, the polymerization system was found to involve ESR-observable propagating radicals of BDMPI under practical polymerization conditions. Using the polymer radical concentration by ESR, the rate constants of propagation (k P ) and termination (k t ) were determined in the temperature range of 50-70°C. The k P value seemed dependent on the chain-length of propagating radical. The analysis of polymers by the MALDI-TOF mass spectrometry suggested that most of the resulting polymers contain the dimethylamino terminal group. The copolymerization of BDMPI (M 1 ) and styrene (M 2 ) at 50°C in DMF gave the following copolymerization parameters; r 1 = 0.49, r 2 = 0.26, Q 1 = 1.2, and e 1 = +0.63. The thermal behavior of poly(BDMPI) was examined by dynamic thermogravimetry and differential scanning calorimetry.