Single‐pulse pulsed laser polymerization–electron paramagnetic resonance investigations into the termination kinetics of n‐butyl acrylate macromonomers

The termination of model mid‐chain radicals (MCRs), which mimic radicals that occur in acrylate polymerization over a broad range of reaction conditions, has been studied by single‐pulse pulsed laser polymerization (SP‐PLP) in conjunction with electron paramagnetic resonance spectroscopy. The model radicals were generated by initiator‐fragment addition to acrylic macromonomers that were preformed prior to the kinetic experiments, thus enabling separation of termination from the propagation reaction, for these model radicals propagate sparingly, if at all, on the timescale of SP‐PLP experiments. Termination rate coefficients of the MCRs were determined in the temperature range of 0–60°C in acetonitrile and butyl propionate solution as well as in bulk macromonomer over the range of 0–100 °C. Termination rate coefficients slightly below those of the corresponding secondary radicals were deduced, demonstrating the relatively high termination activity of this species, even when undergoing MCR–MCR termination. For chain length of 10, a reduction by a factor of 6 is observed. Unusually high activation energies were found for the termination rate coefficient in these systems, with 35 kJ mol^?1 being determined for bulk macromonomer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Kinetics of polymerization rate based on the dependence of termination rate on chain length

When the structure of a primary radical resembles that of the chain end of the polymer radical, the rate of the primary radical termination is approximately the same as the termination rate between the oligomer radical and the polymer radical. The rate constant of termination between polymer radicals of chain length n and s, which involve the primary radicals, is k_t,ns = const.(ns)^?a. In the polymerization of methacrylonitrile initiated by 2,2′-azobisisobutyronitrile in dimethylformamide at 60.0°C, the value of a is found to be 0.091. From data obtained previously in the bulk polymerization of styrene initiated by 1-azobis-2-phenylethane at 60.0°C, the value of a is found to be 0.167. Because such a values are so large that they are not estimated by the excluded volume, the termination rates are discussed by adding the dependence of the diffusion of the segments to that for chain length.     

Chain-length dependent termination in pulsed-laser polymerization, 2. On the prospects of determining the bimolecular termination constant kt from the second moment of the chain-length distribution

The correct (event weighted) average of k_t, 〈k_t〉, has been calculated for pseudostationary laser-induced polymerization for a kinetic scheme with chain-length dependent termination and compared to the average k̄_t obtained by formally solving for k_t the expression for the second moment of the chain-length distribution valid for chain-length independent termination (represented by the product of rate of polymerization ν_p and weight average degree of polymerization P̄_w). It is shown that there is a fair agreement between the two quantities. This may be used to recover experimentally the power-law governing the dependence of k_t on chain-length, especially its exponent.     

Chain-length dependent termination in pulsed-laser polymerization, 3. On the prospects of determining the bimolecular termination constant kt from rate data

The correct (event-weighted) average of k_t, 〈k_t〉, has been calculated from simulation data for pseudostationary laser-induced polymerization for a kinetic scheme with chain-length dependent termination and compared to the average k̄_t which is obtained by employing the formal procedures, originally designed for the evaluation of individual rate constants from rate data in the case of chain-length independent termination. Satisfactory (and in fact excellent) results are obtained only if the complete equation for the conversion per laser pulse is solved for k̄_t. This leads to an almost perfect recovery of the power-law governing the dependence of k_t on chain-length, especially the exponent.     

Polar,functionalized diene‐based material. III. Free‐radical polymerization of 2‐[(N,N‐dialkylamino)methyl]‐1,3‐butadienes

The bulk free‐radical polymerization of 2‐[(N,N‐dialkylamino)methyl]‐1,3‐butadiene with methyl, ethyl, and n‐propyl substituents was studied. The monomers were synthesized via substitution reactions of 2‐bromomethyl‐1,3‐butadiene with the corresponding dialkylamines. For each monomer the effects of the polymerization initiator, initiator concentration, and reaction temperature on the final polymer structure, molecular weight, and glass‐transition temperature (T_g) were examined. Using 2,2′‐azobisisobutyronitrile as the initiator at 75 °C, the resulting polymers displayed a majority of 1,4 microstructures. As the temperature was increased to 100 and 125 °C using t‐butylperacetate and t‐butylhydroperoxide, the percentage of the 3,4 microstructure increased. Differential scanning calorimetry indicated that all of the T_g values were lower than room temperature. The T_g values were higher when the majority of the polymer structure was 1,4 and decreased as the percentage of the 3,4 microstructure increased. The Diels–Alder side products found in the polymer samples were characterized using NMR and gas chromatography‐mass spectrometry methods. The polymerization temperature and initiator concentration were identified as the key factors that influenced the Diels–Alder dimer yield. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4070–4080, 2000     

Kinetic analysis of styrene atom transfer radical polymerization: Extraction of radical‐radical termination rate coefficients

Kinetic studies of the atom transfer radical polymerization (ATRP) of styrene are reported, with the particular aim of determining radical‐radical termination rate coefficients (<k_t>). The reactions are analyzed using the persistent radical effect (PRE) model. Using this model, average radical‐radical termination rate coefficients are evaluated. Under appropriate ATRP catalyst concentrations, <k_t> values of approximately 2 × 10⁸ L mol^?1 s^?1 at 110 °C in 50 vol % anisole were determined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5548–5558, 2004     

Radical polymerization of methyl methacrylate in the presence of stereoregular poly(methyl methacrylate). V. Kinetics of initial template polymerization

The influence of stereoregular poly(methyl methacrylate) (PMMA) as a polymer matrix on the initial rate of radical polymerization of methyl methacrylate (MMA) has been measured between ?11 and +60°C using a dilatometric technique. Under proper conditions an increase in the relative initial rate of template polymerization with respect to a blank polymerization was observed. Viscometric studies showed that the observed effect could be related to the extent of complex formation between the polymer matrix and the growing chain radical. The initial rate was dependent on tacticity and molecular weight of the matrix polymer, solvent type and polymerization temperature. The accelerating effect was most pronounced (a fivefold increase in rate) at the lowest polymerization temperature with the highest molecular weight isotactic PMMA as a matrix in a solvent like dimethylformamide (DMF), which is known to be a good medium for complex formation between isotactic and syndiotactic PMMA. The acceleration of the polymerization below 25°C appeared to be accompanied by a large decrease in the overall energy and entropy of activation. It is suggested that the observed template effects are mainly due to the stereoselection in the propagation step (lower activation entropy Δ S_p^?) and the hindrance of segmental diffusion in the termination step (higher activation energy Δ E_t^?) of complexed growing chain radicals.     

Polymerization of o‐quinodimethanes. V. Ionic polymerization of o‐quinodimethanes generated by thermal isomerization of corresponding benzocyclobutenes

The ionic polymerization of substituted o‐quinodimethanes via thermal isomerization of benzocyclobutenes is described. In the cationic polymerizations of 1‐methoxy‐o‐quinodimethane in the presence of various cationic initiators at 110 °C for 12 h, chain transfer reactions also considerably underwent besides the polymerization. Meanwhile, cationic polymerizations of 1‐trimethylsilyloxy‐o‐quinodimethane under the same conditions gave good yields of the corresponding polymer. Anionic polymerizations of 1‐cyano‐o‐quinodimethane in the presence of anionic initiators such as n‐BuLi or t‐BuOK were performed at various temperatures for 12 h. Good yields of hexane‐insoluble polymer, which was produced by anionic polymerization of corresponding o‐quinodimethane as an intermediate, were obtained above 120 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 844–850, 2008     

Anomalous solubility of polyacrylamide prepared by dispersion (precipitation) polymerization in aqueous tert‐butyl alcohol

Polyacrylamide prepared by dispersion (precipitation) polymerization in an aqueous t‐butyl alcohol (TBA) medium is only partially soluble when the TBA concentrations in the polymerization media are in the range 82 vol % < TBA < 95 vol %. Independent experiments with a soluble (linear) sample of polyacrylamide show that the polymer swells sufficiently in the aforementioned media to lower the glass‐transition temperature of the polymer below the polymerization temperature (50 °C). The anomalous solubility has been attributed to the crosslinking of polymer chains that occurs during the solid‐phase polymerization of acrylamide in the swollen polymer particles. It is postulated that some of the radical centers shift from the chain end to the chain backbone during solid‐phase polymerization by chain transfer to neighboring polymer molecules, and when pairs of such radicals come into close vicinity, crosslinking occurs. However, dispersion (precipitation) polymerization in other media such as aqueous methanol and aqueous acetone yields polymers that are soluble. This result has been attributed to the fact that the polymer radical undergoes a chain‐transfer reaction with these solvents at a much faster rate than with TBA, which overcomes the effect of the polymer‐transfer reaction. Even the addition of as little as 5% methanol to a TBA–water mixture (TBA:water = 85:10) gives rise to a soluble polymer. The chain‐transfer constants for acetone, methanol, and TBA have been determined to be 9.0 × 10^?6, 6.9 × 10^?6, and 1.48 × 10^?6, respectively, at 50 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3434–3442, 2001     

Radical polymerization and copolymerization of methyl α-(2-carbomethoxyethyl)acrylate,a dimer of methyl acrylate,as a polymerizable α-substituted acrylate

Polymerization and copolymerization of methyl α-(2-carbomethoxyethyl)acrylate (MMEA), which is known as a dimer of methyl acrylate, were studied in relation to steric hindrance-assisted polymerization. The propagating polymer radical from MMEA was detected as a five-line spectrum and quantified by ESR spectroscopy during the bulk polymerization at 40–80°C. The absolute rate constants of propagation and termination (κ_p and κ_t) for MMEA at 60°C (κ_p = 19 L/mol s and κ_t = 5.1 × 10⁵ L/mol s) were evaluated using the concentration of the propagating radical at the steady state. The balance of the propagation and termination rates allows polymer formation from MMEA. The polymerization rate of MMEA at 60°C was less than that of MMA by a factor of about 4 at a constant monomer concentration. Although no influence of ceiling temperature was observed at a temperature ranging from 40 to 70°C, addition-fragmentation in competition with propagation reduced the molecular weight of the polymer. The content of the unsaturated end group was estimated to be 0.1% at 60°C to the total amount of the monomer units consisting of the main chain. MMEA exhibited reactivities almost similar to those of MMA toward polymer radicals. It is concluded that MMEA is one of the polymerizable acrylates bearing a substituted alkyl group as an α-substituent. Characterization of poly(MMEA) was also carried out. © 1996 John Wiley & Sons, Inc.     

Highly heterotactic polymerization of methacrylates — Higher order stereoregulation and stereochemical significance

A combination of tert-butyllithium (t-BuLi) and bis(2,6-di-t-butylphenoxy)methylaluminium (MeAI(ODBP)₂) was found to be an efficient initiator for heterotactic living polymerization of certain alkyl methacrylates in toluene at low temperatures. The polymerization of methyl methacrylate (MMA) with t-BuLi/MeAI(ODBP)₂ (AI/Li=5 mol/mol) in toluene at −78°C gave heterotactic-rich poly(methyl methacrylate) (PMMA) with narrow molecular weight distributions (MWDs) (heterotactic triad fraction mr = 68%, ratio of weight- to number-average molecular weights M̄w/M̄n = 1.06-1.17). Other alkyl methacrylates also gave heterotactic polymers under the same conditions; in particular, ethyl and butyl methacrylates gave polymers with heterotactic triad fractions of 87%. The highest triad heterotacticity of 91.6% was obtained for the polymerization of ethyl methacrylate at −95°C. Some characteristic features of this stereospecific polymerization were discussed based on the polymerization results combined with other structural information of the polymer such as chain-end stereostructure and stereosequence distribution in the main chain.     

Low‐temperature controlled free‐radical polymerization of vinyl monomers in the presence of a novel cyclic dixanthate under γ‐ray irradiation

The free‐radical polymerization of methyl acrylate (MA) has been studied in the presence of a novel cyclic dixanthate under γ‐ray irradiation (80 Gy min^?1) at room temperature (～28 °C), ?30 °C, and ?76 °C respectively. The resultant polymers have controlled molecular weights and relatively narrow molecular weight distributions, especially at low temperatures (i.e., ?30 and ?76 °C). The polymerization control may be associated with the temperature: the lower the temperature is, the more control there is. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry analysis of poly(methyl acrylate) (PMA) samples shows that there are at least three distributions: [3‐(MA)_n‐H]⁺ cyclic polymers, [3‐(MA)_n‐THF‐H]⁺, and [3‐(MA)_n‐(THF)₂‐H]⁺ linear PMAs. The relative content of the cyclic polymers markedly increases at a lower temperature, and this may be related to the reduced diffusion rate and the suppressed chain‐transfer reaction at the low temperature. It is evidenced that the good control of the polymerization at the low temperature may be associated with the suppressed chain‐transfer reaction, unlike reversible addition–fragmentation chain transfer polymerization. In addition, styrene bulk polymerizations have been performed, and gel permeation chromatography traces show that there is only one cyclic dixanthate moiety in the polymer chain. This article is the first to report the influence of a low temperature on controlled free‐radical polymerizations. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2847–2854, 2007     

Oligomer formation in the emulsifier‐free emulsion polymerization of styrene

The formation of oligomers in emulsifier‐free emulsion polymerization of styrene was characterized by means of gel permeation chromatography and surface tension measurements. GPC analysis showed incessant oligomer formation throughout the emulsion polymerization process. Oligomers spanned a molecular weight range of 200–1,500, have an M̄_w of 800–900, an M̄_n of 600–800 and a polydispersity index of 1.3. On average, the oligomers contain 4 to 6 styrene units. UV detection could not be utilized to acquire the weight ratio of oligomers to polymers without correction. Combination was the major mode of termination of free radicals in the aqueous phase, but disproportionation was not negligible: for every three‐combination reactions there was about 1 disproportionation. Surface tension measurements showed that oligomers minimized the surface tension of the latex at about 50 min reaction to only 30 mN/m. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1323–1336, 2000     

Anionic polymerization of N,N‐dialkyl‐2‐methylenebut‐3‐enamide: Effect of alkyl substituents on polymerization behavior

Anionic polymerizations of three 1,3‐butadiene derivatives containing different N,N‐dialkyl amide functions, N,N‐diisopropylamide (DiPA), piperidineamide (PiA), and cis‐2,6‐dimethylpiperidineamide (DMPA) were performed under various conditions, and their polymerization behavior was compared with that of N,N‐diethylamide analogue (DEA), which was previously reported. When polymerization of DiPA was performed at ?78 °C with potassium counter ion, only trace amounts of oligomers were formed, whereas polymers with a narrow molecular weight distribution were obtained in moderate yield when DiPA was polymerized at 0 °C in the presence of LiCl. Decrease in molecular weight and broadening of molecular weight distribution were observed when polymerization was performed at a higher temperature of 20 °C, presumably because of the effect of ceiling temperature. In the case of DMPA, no polymer was formed at 0 °C and polymers with relatively broad molecular weight distributions (M_w/M_n = 1.2) were obtained at 20 °C. The polymerization rate of PiA was much faster than that of the other monomers, and poly(PiA) was obtained in high yield even at ?78 °C in 24 h. The microstructure of the resulting polymers were exclusively 1,4‐ for poly(DMPA), whereas 20–30% of the 1,2‐structure was contained in poly(DiPA) and poly(PiA). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3714–3721, 2010     

Gamma radiation-initiated polymerization of styrene at high pressure. II. Chain termination

The pressure dependence of the termination rate constant k_t for the free radical polymerization of monomers such as styrene is a function of polymer chain length, chain stiffness, and monomer viscosity, all of which influence the rate of segmental diffusion of an active radical chain end out of the coiled polymer chain to a position in which it can react with a proximate radical. Although k_t is not sensitive to changes in chain length, the large increase in molecular weight is responsible for a significant reduction in k_t at high pressures. For most of the common vinyl polymers, which exhibit some degree of chain stiffness, k_t is inversely proportional to a fractional power of the monomer viscosity because it depends in part on the resistance of chain segments to movement and in part on the influence of viscosity in controlling diffusion of the chain ends. The fractional exponent appears to increase with pressure and this is interpreted as evidence that the polymer chains become more flexible in a more viscous solvent. Because the fractional exponent is higher for more flexible chains, the value of the activation volume for chain termination is an indication of the degree of flexibility of the polymer chains, provided that the monomer is a good solvent for the polymer and that chain transfer is negligible.     

Kinetic study on the radical polymerization of 2‐methacryloyloxyethyl phosphorylcholine

Polymerization of 2‐methacryloyloxyethyl phosphorylcholine (MPC) was kinetically investigated in ethanol using dimethyl 2,2′‐azobisisobutyrate (MAIB) as initiator. The overall activation energy of the homogeneous polymerization was calculated to be 71 kJ/mol. The polymerization rate (R_p) was expressed by R_p = k[MAIB]^0.54±0.05 [MPC]^1.8±0.1. The higher dependence of R_p on the monomer concentration comes from acceleration of propagation due to monomer aggregation and also from retardation of termination due to viscosity effect of the MPC monomer. Rate constants of propagation (k_p) and termination (k_t) of MPC were estimated by means of ESR to be k_p = 180 L/mol · s and k_t = 2.8 × 10⁴ L/mol · s at 60 °C, respectively. Because of much slower termination, R_p of MPC in ethanol was found at 60 °C to be 8 times that of methyl methacrylate (MMA) in benzene, though the different solvents were used for MPC and MMA. Polymerization of MPC with MAIB in ethanol was accelerated by the presence of water and retarded by the presence of benzene or acetonitrile. Poly(MPC) showed a peculiar solubility behavior; although poly(MPC) was highly soluble in ethanol and in water, it was insoluble in aqueous ethanol of water content of 7.4–39.8 vol %. The radical copolymerization of MPC (M₁) and styrene (St) (M₂) in ethanol at 50 °C gave the following copolymerization parameters similar to those of the copolymerization of MMA and St; r₁ = 0.39, r₂ = 0.46, Q₁ = 0.76, and e₁ = +0.51. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 509–515, 2000     

Triptycene‐containing polyetherolefins via acyclic diene metathesis polymerization

Several new triptycene‐containing polyetherolefins were synthesized via acyclic diene metathesis (ADMET) polymerization. The well‐established mechanism, high selectivity and specificity, mild reaction conditions, and well‐defined end‐groups make the ADMET polymerization a good choice for studying systematic variations in polymer structure. Two types of triptycene‐based monomer with varying connectivities were used in the synthesis of homopolymers, block copolymers, and random copolymers. In this way, the influence of the triptycene architecture and concentration in the polymer backbone on the thermal behavior of the polymers was studied. Inclusion of increasing amounts of triptycene were found to increase the glass transition temperature, from ?44 °C in polyoctenamer to 59 °C in one of the hydrogenated triptycene homopolymers ( H‐PT2 ). Varying the amounts and orientations of triptycene was found to increase the stiffness ( H‐PT1 ), toughness ( PT1₁‐b‐PO₁ ) and ductility ( PT1₁‐ran‐PO₃ ) of the polymer at room temperature. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Polar,functionalized diene‐based materials. V. Free‐radical polymerization of 2‐[(N‐benzyl‐n‐methylamino)methyl]‐1,3‐butadiene and copolymerization with styrene

2‐[(N‐Benzyl‐N‐methylamino)methyl]‐1,3‐butadiene (BMAMBD), the first asymmetric tertiary amino‐containing diene‐based monomer, was synthesized by sulfone chemistry and a nickel‐catalyzed Grignard coupling reaction in high purity and good yield. The bulk and solution free‐radical polymerizations of this monomer were studied. Traditional bulk free‐radical polymerization kinetics were observed, giving polymers with 〈M_n〉 values of 21 × 10³ to 48 × 10³ g/mol (where M_n is the number‐average molecular weight) and polydispersity indices near 1.5. In solution polymerization, polymers with higher molecular weights were obtained in cyclohexane than in tetrahydrofuran (THF) because of the higher chain transfer to the solvent. The chain‐transfer constants calculated for cyclohexane and THF were 1.97 × 10^?3 and 5.77 × 10^?3, respectively. To further tailor polymer properties, we also completed copolymerization studies with styrene. Kinetic studies showed that BMAMBD incorporated into the polymer chain at a faster rate than styrene. With the Mayo–Lewis equation, the monomer reactivity ratios of BMAMBD and styrene at 75 °C were determined to be 2.6 ± 0.3 and 0.28 ± 0.02, respectively. Altering the composition of BMAMBD in the copolymer from 17 to 93% caused the glass‐transition temperature of the resulting copolymer to decrease from 64 to ?7 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3227–3238, 2001     

Modeling termination kinetics in free‐radical polymerization using a reduced number of parameters

An approach for modeling chain‐length dependent termination rate coefficients is presented. The method is based on the assumption that free‐radical chain length may be considered as a continuous variable. As compared to discrete numerical methods, in continuous modeling the number of independent dimensionless parameters can be significantly reduced. As a consequence, for a wide variety of monomers the conversion dependence of k_t can be predicted without extensive numerical calculations. The method may also be used to determine polymerization conditions under which simpler models of k_t (which neglect effects arising from the dependence of k_t on chain length) may be applied. Calculations for methyl methacrylate, styrene, and butyl acrylate bulk polymerizations up to high degrees of monomer conversion show that the impact of chain length on termination varies with conversion and strongly depends on the type of monomer.     

Alkoxyamine‐mediated “living” radical polymerization: MS investigation of the early stages of styrene polymerization initiated by cumyl‐TEISO

A new series of 1,1,3,3‐tetraethylisoindoline‐2‐oxyl (TEISO)‐based alkoxyamines was prepared. The half‐lives for thermal dissociation indicated that the most sterically congested cumyl‐TEISO alkoxymine had the greatest potential as an initiator for the polymerization of monomers at lower temperatures. The polymerization of styrene at 110 °C gave a linear evolution of M_n with conversion in the early stages. Further evidence for the “living” nature was given by the polydispersities of the polymers that remained low (M_w/M_n = 1.13–1.27) throughout the polymerization (up to 80% conversion). No polymer was formed for the styrene system in a reasonable time below 100 °C. High‐performance liquid chromatographic/mass spectrometric investigations of the distribution of trapped oligomers containing one to nine monomer units formed at 60 °C revealed that the trapping of oligomeric cumyl–styryl radicals by TEISO is irreversible at this temperature. Methyl methacrylate polymerized with cumyl‐TEISO at 60–70 °C, although the initial high rates of polymerization soon decreased to zero at low conversions (10–15%), and the high polydispersities (M_w/M_n = 1.42–1.73) indicated significant side reactions. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1232–1241, 2001