Thermal polymerization of styrene: New data on the conversion dependence of the initiation rate

With the aid of new acceptors of free radicals usable at high temperatures (T > 100°C), the rate of initiation w _i has been measured experimentally for the thermal polymerization of styrene at 122.5°C in a wide range of conversions C = 0–80%. It has been shown that the value of w _i tends to increase in the course of polymerization transformation in agreement with the w _i = f(C) relationship calculated from the kinetic data on the thermal polymerization of styrene in the absence of counters of free radicals. Hypothetical reasons for this non-trivial tendency have been formulated. The experimental dependence w _i = f(C) has been measured for the first time and has been invoked to refine currently available mathematical models for the thermal polymerization of styrene that assume that w _i remains invariable in the course of polymerization transformation.     

Free-radical vinyl polymerization in bulk: Conversion dependence of initiation efficiency

The systematic array of data concerning the conversion C dependence of initiation rate constants k _i has been collected for the first time via direct experimental measurements (via the inhibitor method) of the rate of initiation in the course of free-radical polymerization carried out at various temperatures (50, 60, and 70°C) for three most representative monomers (styrene, methyl methacrylate, and vinyl acetate) in the presence of AIBN as a typical initiator. The k _i = f(C) dependence demonstrates an exponential behavior with a sharp change in the exponent at a certain critical point C _cr whose position on the conversions scale correlates with the kinetic features of polymerization for each of the monomers. The results of this study are necessary for correction of approaches useful for development of a new generation of mathematical models of free-radical polymerization processes, since, in terms of previously advanced models, the values of k _i are assumed as conversion-independent. On the basis of the experimental data, the mechanisms of some physical processes that occur in monomerpolymer systems and determine diffusion-controlled chemical transformations are discussed. Specifically, factors controlling the diffusion exit of initiator radicals from the Franck-Rabinowitch cage and factors governing the quadratic termination of growing polymers chains are compared.     

Cinetique de polymerisation du styrene en presence de peroxyde de benzoyle p,p′ bis-bromomethyle synthese de polymeres peroxydes

The free radical polymerization of styrene has been studied by using p,p′-bisbromomethyl benzoyl peroxide as initiator containing a chain transfer group. The rate constant of decomposition (k_d) of this peroxide has been determined at various temperatures, as well as the efficiency factor f and the transfer constant to initiator C₁. At 60°, f = 0·70 ± 0·05 and C₁ = 0·5. Polystyrene containing peroxide groups has been prepared by using this initiator. The highest yield in polymeric peroxide has been obtained for polymerization in emulsion at 40°.     

Bulk polymerization of rac-lactide initiated by guanidinate alkoxide complexes of rare earth metals. The molecular structure of the cluster [{(Me3Si)2NC(NPri)2}Nd]4(μ3-OPri)8Li7(μ2-Cl)3(μ3-Cl)2(μ4-Cl)2

The complexes {(Me₃Si)₂NC(NPrⁱ)₂}₂LnOBu^t (Ln = Y (1), Lu (2)) initiate the bulk polymerization of racemic lactide (LA) at 130 °C. At the monomer: initiator molar ratio ([LA]: 1) equal to 1000: 1, the quantitative conversion of the monomer is achieved within 6 h. The resulting polymers are characterized by a rather narrow monomodal molecular weight distribution (M _w/M _n = 1.46–1.82) and molecular weights up to 33400 g mol^?1. The molecular weights of the resulting polylactides measured by gel permeation chromatography are 3–11 times lower than the values calculated from the monomer: initiator ratio on the assumption of one growing polymer chain per catalytic center. The reaction of the in-situ prepared complex [(Me₃Si)₂NC(NPrⁱ)₂]NdCl₂ with 2 equiv. of PrⁱOLi produced the 11-nuclear cluster [{(Me₃Si)₂NC(NPrⁱ)₂}Nd]₄(μ³-OPrⁱ)₈Li₇(μ²-Cl)₃(μ³-Cl)₂(μ⁴-Cl)₂ (3), which was isolated in 62% yield. The structure of compound 3 was established by X-ray diffraction. Complex 3 initiates both the bulk and solution polymerization of rac-lactide. In the bulk polymerization at the molar ratio [LA]: [Nd] = 500: 1, the 89% conversion of the monomer was achieved within one hour. The polylactide thus synthesized has the molecular weight M _n = 19720 g mol^?1 and a rather narrow polydispersity M _w/M _n = 1.54.     

Measurement of state-to-state rotational transfer rates in collision of I2*(B 0u+, υ = 15, j) with 3He, 4He,Ne, Ar,H2, D2 and I2

The selective laser excitation and induced fluorescence observation technique has been used to study rotationally inelastic collisions of I₂^*(B 0_u⁺, υ = 15,j) with I₂, ³He, ⁴He, Ne, Ar, H₂ and D₂. For each collision partner, several initial rotational levels ranging from j_i = 12 up to j_i = 146 have been excited. For purely rotational transfer within the υ = 15 level, our data are perfectly consistent with energy sudden (eventually corrected) scaling laws. Thus, any thermally averaged rate constant, k(j_i → j_f), can be expressed as a function of the basis rate constants k(l → 0). Furthermore, these k(l → 0) are found to follow simple empirical fitting laws. Consequently any k(j_i → j_f) can be predicted given a set of two or three fitting parameters. Collisions with relatively heavy particles (I₂, Ar and Ne) are well described by using the inverse power fitting law k(l → 0) = b[l(l+1)]^?γ, where b = 1.7, 1.2 and 1.2×10^?10 cm³ s^?1 and γ = 1.08, 1.02 and 1.17 for I₂^*-Ne, I₂^*-Ar and I₂^*-I₂ collisions respectively. For collisions with light particles (³He, ⁴He, H₂ and D₂), k(l → 0) shows a sharp decrease with l which can be accounted for by a hybrid power-exponential fitting law k(l → 0) = b[l(l+1)]^?γ exp[-l(l+1)/l^* (l^*+1)], where b = 0.84, 0.71, 2.77 and 2.78×10^?10 cm³ s^?1l⁺ = 20.6, 23.1, 18.8 and 31.4, and γ = 0.66, 0.66, 0.78 and 0.91 for I₂^*-³He, I₂^*-He, I₂^*-H₂ and I₂^*-D₂ collisions, respectively. We confirm that the rotational transfer dynamics in heavy molecules is mainly governed by angular momentum exchange.     

A chemical approach to the problem of slow initiation in living cationic polymerizations

The effect of slow initiation on initiation efficiency and MWD has been investigated, with regard to the living carbocationic polymerization of isobutylene. The initiating system trans−2,5-diacetoxy-2,5-dimethyl−3–hexene(DiOAcDiMeH₆)BCl₃ has been investigated at −35°C in CH₃Cl. Based on considerations valid also for anionic polymerizations, the following methods have been applied to increase the initiation efficiency: 1. Increasing [M_o]/[I_o] (batch, AMI) or [ΔM].j/[I_o] (IMA). The applicability of this method is limited by the solubility of the polymer. 2. Increasing k_i/k_p, by (i) increasing k_i, e.g., by introducing an electron withdrawing substituent into the initiator, (ii) decreasing k_p, e.g., by the addition of a strong electron donor (DMSO) to the system, (iii) using Cl-CH₂-CH₂-Cl.     

Polymerization of vinylene carbonate in dimethyl sulfoxide

Pure vinylene carbonate polymerizes readily in dimethyl sulfoxide solutions upon initiation by azobisisobutyronitrile (AIBN). The monomer conversion is characterized by a limiting value which appears to be a function of the temperature and the initial concentrations of both the initiator and the monomer. Increasing both initiator concentration and temperature results in higher final conversions, whereas a maximum conversion is indicated for initial monomer concentrations in the range of 80% to 90%. Principal kinetic quantities were found to be adequately represented by the equations k_d = 24.3 × 10⁵ exp {?11300/RT} and k_p(f/k_t)^1/2 = 46.3 × 10⁵ exp {?8900/RT} for the temperature range of 50–80°C. The average degree of polymerization was found to be affected by chain transfer to the solvent. A value of 5.8 × 10^?4 was determined for the corresponding chain transfer constant.     

Telomerization kinetics by redox catalysis

A new formula for redox catalyzed telomerization kinetics was set up: 1/DP _n = C_Me(Me)/(M) + C_XY(XY)/(M), where C_Me is the transfer constant for the metallic ion Me, C_XY is the transfer constant for the telogen XY, M the monomer, and DP _n is the degree of polymerization. This study has shown that classic transfer and termination reactions can be neglected because of their slow rates and that the metal transfer constant (C_Me) is the predominant factor influencing telomer molecular weight. This formula was verified by telomerizations of three monomers [chlorotrifluoroethylene (CTFE), acrylic acid, and ethyl acrylate] with CCl₄, catalyzed with ferric chloride-benzoin. In this last case the initiation rate constant (fK_i), the initiation and termination rates, and the radical concentration were measured.     

Caesium as counter-ion in the anionic polymerization of ethylene oxide—II: Kinetics in hmpt

Results are reported for the kinetics of the anionic polymerization of ethylene oxide in hexamethylphosphoramide using the caesium alkoxide of the monoethylether of diethylene glycol as initiator at 40·0°.The reactions were found to be first order in monomer and 0·5 in initiator; rates are depressed by adding caesium tetraphenyl boride. These results indicate that propagation takes place by free ions and ion pairs; the respective propagation rate constants are k_(?) = 22 and k_(±) = 0·2 M 'sec.     

Kinetics of the liquid-phase oxidation of 2-hydroxycyclohexanone

The kinetics of oxygen uptake in the azobisisobutyronitrile-initiated oxidation of 2-hydroxycy-clohexanone (RCH(OH)C(O)R) at 323 K has been investigated (chlorobenzene solvent, [RCH(OH)C(O)R] = 0.15-1.30 mol/L, initiation rate of w _i = 0.016–0.473 mol L^?1 s^?1). The effective oxidizability parameter k _{p, eff}(2k _t,eff)^?0.5 has been determined by solving the inverse kinetic problem using the entire data array obtained while varying [RCH(OH)C(O)R] and w _i. The rate constants of chain propagation and termination and the equilibrium constant of the addition of the hydroxyperoxyl radical to the ketoalcohol have been derived from the dependence of the oxidizability parameter on the substrate concentration. The rate constant of bimolecular chain initiation has been calculated to be k ₀ = 9.7 × 10^?7 L mol^?1 s^?1. The ratio of the rate constants of quadratic-law peroxyl radical recombination reactions without and with chain termination (k _eff′/k _t,eff′) increases with increasing [RCH(OH)C(O)R], passes through a maximum at a substrate concentration of 0.8 mol/L, and then falls off. It is demonstrated that this effect is due to the variation of the proportions of the hydroperoxyl and organic (1-hydroxy-2-oxocyclohexylperoxyl or 1,2-dihydroxy-1-cyclo-hexylperoxyl) radicals in the reaction medium.     

Seeded dispersion polymerization of MMA using submicron PMMA particles as seed: a mechanistic study

Micron-size poly(methyl methacrylate) (PMMA) particles having a narrow particle size distribution were prepared by seeded dispersion polymerization of methyl methacrylate (MMA) using submicron PMMA particles as seed. The processes of particle aggregation and nucleation were controlled by the initial seed size, initial seed number, and initiator concentration, determining the formation of the mature particles and the number (N _(final)) and size of the final particles. It was found that N _(final) was equal to the number of particles produced in the absence of seed (N _{(ab initio)}) when the initial number of seed particles (N _(initial)) was less than N _{(ab initio)}. When N _(initial) was greater than N _{(ab initio)}, N _(final) was equal to k?×?N _(initial), where the value of k was a function of seed size and initiator concentration. k increased with seed size and was less than 1 at high initiator concentrations (0.52 and 1.00 %), while at low initiator concentrations (0.23 and 0.30 %), a maximum value of k was found for a 198 nm seed size. k could be greater than unity in some cases.     

Molecular weight distribution of a living polymer generated from a bifunctional initiator with non-equal initiation rate constants

This work deals with the kinetics of the living polymerization initiated by a bifunctional initiator with different rate constants of the first and the second step of initiation. The expressions of the molecular weight distribution (MWD) function, the number- and weight-average degrees of polymerization and the polydispersity index were derived rigorously. Numerical results show that the nonequal initiation rate constants often lead to a bimodal MWD except the ratio of k_i to k_i′ being very large (>500, for instance) or relatively small (<1, for example). The MWD of the resultant polymer is rather narrow even in the presence of double peaks. With decreasing initiator concentration, the MWD at full conversion becomes narrower, and the shoulder peak of the MWD curve declines.     

Kinetics of emulsion polymerization of methyl methacrylate

The kinetics of the emulsion polymerization of methyl methacrylate at 50°C have been studied in seeded systems using both chemical initiation and γ-radiolysis initiation. Both steady-state rates and (for γ-radiolysis) the relaxation from the steady state were observed. The average number of free radicals per particle was quite high (e.g., ～0.7 for 10^?3 mol dm^?3 S₂O²₈ initiator). The data are quantitatively interpreted using a generalized Smith–Ewart–Harkins model, allowing for free radical entry, exit, biomolecular termination within the latex particles, and aqueous phase hetero-termination and re-entry. From this treatment, there results (i) the dependence of the termination rate coefficient (k_t) on the weight fraction of polymer (w_p), (ii) lower bounds for the dependence of the entry rate coefficient on initiator concentration, and (iii) the conclusion that most exited free radicals undergo subsequent re-entry into particles rather than hetero-termination. The results for k_t(w_p) are consistent with diffusion control at temperatures below the glass transition point. Comparisons are presented of the behavior of methyl methacrylate, butyl methacrylate, and styrene in emulsion polymerization systems.     

The photophysics of trans-stilbene

The fluorescence lifetime of trans-stilbene in dilute methylcyclohexane/iso-hexane solution has been measured and the mean S₁ radiative (k_F), radiationless (k_I) and cis-isomerization (k_C) rate parameters have been determined from ?90 to 60°C. S_i consists of a fluorescent trans (¹B_u^*) state (k_F⁰ = 6.0 × 10⁸ s^?1) which undergoes reversible thermal-activated rotational internal conversion (ΔH = 1.75 kcal mole^?1, ΔS = 10.6 cal deg^?1 mole^?1) to a non-fluorescent perp (¹A_g^*) state. p(¹A_g^*) lies 610 cm^?1 above t (¹B_u^*) with an intermediate S₁ potential maximum. p(¹A_g^*) undergoes internal conversion(k_I.^∞ = 5.8 × 10⁸ s^?1) to p (¹A_g) leading to cis-isomerization. This is the main isomerization channel over the whole temperature range.     

Titanium complexes of dialkanolamine ligands as initiators for living ring‐opening polymerization of ε‐caprolactone

The titanium complexes with one ( 1a , 1b , 1c ) and two ( 2a , 2b ) dialkanolamine ligands were used as initiators in the ring‐opening polymerization (ROP) of ε‐caprolactone. Titanocanes 1a and 1b initiated living ROP of ε‐caprolactone affording polymers whose number‐average molecular weights (M_n) increased in direct proportion to monomer conversion (M_n ≤ 30,000 g mol^?1) in agreement with calculated values, and were inversely proportional to initiator concentration, while the molecular weight distribution stayed narrow throughout the polymerization (M_w/M_n ≤ 1.2 up to 80% monomer conversion). ¹H‐NMR and MALDI‐TOF‐MS studies of the obtained poly(ε‐caprolactone)s revealed the presence of an isopropoxy group originated from the initiator at the polymer termini, indicating that the polymerization takes place exclusively at the Ti–OⁱPr bond of the catalyst. The higher molecular weight polymers (M_n ≤ 70,000 g mol^?1) with reasonable MWD (M_w/M_n ≤ 1.6) were synthesized by living ROP of ε‐caprolactone using spirobititanocanes ( 2a , 2b ) and titanocane 1c as initiators. The latter catalysts, according MALDI‐TOF‐MS data, afford poly(ε‐caprolactone)s with almost equal content of α,ω‐dihydroxyl‐ and α‐hydroxyl‐ω(carboxylic acid)‐terminated chains arising due to monomer insertion into “Ti–O” bond of dialkanolamine ligand and from initiation via traces of water, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1230–1240, 2010     

Radical sensitization of acetylene pyrolysis

The kinetics of acetylene polymerization initiated by neopentane (Np) or acetone (Ac) decompositions has been investigated in a static reactor dynamically coupled to a modulated beam mass spectrometer between 850–950 K. Overall rates follow the expression: R = ?d[C₂H₂]/dt = k_s[X]^1/2[C₂H₂] + k_u[C₂H₂]² (I), where X represents Np or Ac and k_s, k_u the rate constants of the sensitized and unsensitized reactions, respectively. The rate law of the sensitized reaction clearly suggests a chain polymerization mechanism with k_s = k_p(k_i/k_t)^1/2 (i, t, and p stand for initiation, termination, and propagation, respectively). Remarkably, the derived values of k_p are nearly independent of the sensitizer, although Ac acts as a source of methyl radicals whereas Np also produces hydrogen atoms, and fall in the expected range for the addition of vinylic radicals to acetylene. It is shown that a chain transfer process involving the fast [1,5] intramolecular hydrogen atom shift in 4-methyl-buta-1,3-dien-1-yl radicals (CH₃? CH ? CH? CH ??H) followed by further addition to C₂H₂ and aromatization, transforms methyl radicals into hydrogen atoms and is able to account for the presence of toluene among the products of the sensitized reactions. Based on current thermochemical data for the but-1-en-3-yn-2-yl radical (CH₂??? C?CH) and present rates of propagation it is argued that if the unsensitized polymerization of acetylene also proceeded by a vinyl radical chain, then even the most favorable self-initiation reaction: 2C₂H₂ = C₄H₃ + H (a), would be far too slow. Finally, present results also show that acetone at impurity levels (? 0.1%) can not provide fast enough spurious initiation rates in chain mechanisms for the “unsensitized” acetylene pyrolysis at pressures above 10 torr.     

A neutral Ni(II) acetylide-mediated radical polymerization of methyl methacrylate using the atom transfer radical polymerization method

A neutral nickel σ-acetylide complex [Ni(CCPh)₂(PBu₃)₂] (NBP) is used for possible atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in conjunction with an organic halide as an initiator [R-X: CCl₄, CH₃Cl, BrCCl₃, C₂H₅Br, and C₅H₉Br] in toluene at 80 °C. Among these initiating systems, BrCCl₃/NBP gave the best controlled radical polymerization of MMA and produced polymer with relatively narrow molecular weight distribution (M_w/M_n≈1.3). The ATRP of MMA is preliminarily identified by the following facts: (1) the present MMA polymerization initiated by BrCCl₃/NBP is completely hindered by the addition of TEMPO; (2) the conversion shows a typical linear variation with time in semilogarithmic coordinates; (3) the measured number-average molecular weights of polymer show a linear increase with conversion and agree closely with the theoretical values; (4) the resulting polymer chain contains a dormant carbon-halogen terminal.     

Conversion-time curves in the diffusion-controlled free radical polymerization of styrene

In order to calculate conversion-time curve in radical polymerization, the termination rate constant for interacting polymer radicals with chain lengths n and s is written as: k_t,ns = k(ζ_n + ζ_s) where k ∝ exp(−0.40/v_f) (v_f: free volume), and ζ = 1 at n ⩽ n_c and ζ = (n_c/n)^1,5 at n ⩾ n_c + 1 (n_c: a critical chain length of polymer moving by reptation). The curves are applicable to the experimental data obtained from 20°C to 154°C in the thermal polymerization of styrene. Further, it is shown that the curves can be applied to the experimental data for polymerization in the presence of initiator.     

The initiation process in the polymerization of tetrahydrofuran with carbonium ion salts

The initiation process for the polymerization of tetrahydrofuran with (C₆H₅)₃C⁺SbCl₆^? has been studied. Two mechanisms have been considered: a cation-addition process, and a process in which tetrahydrofuran donates a hydride ion to the cation of the initiator to form triphenylmethane. The biscarbonium salt [(C₆H₅)₂C⁺C₆H₄CH₂]₂(SbCl₆^?)₂ has been synthesized and used to initiate the polymerization of tetrahydrofuran. The results are consistent with the hydride-ion mechanism but may be inconclusive because of chain transfer. NMR experiments with 0.05–0.2M solutions of initiator in tetrahydrofuran show that triphenylmethane is rapidly produced in an amount equal to the molar amount of initiator originally present. Some NMR evidence for the presence of an acetal end group in the polymer has been obtained. It is concluded that the initiation process in this system definitely involves the formation of triphenylmethane, although a detailed, unique mechanism cannot be selected at this time.     

Steady-state and pulsed studies of the radiation-induced polymerization of methyl methacrylate

In order to quantify our fluorogenic molecular probe studies on the radiation-induced polymerization of methyl methacrylate (MMA), measurements have been made of the monomer conversion, C _M, and polymer molecular weight distribution as a function of dose for bulk MMA polymerized by steady-state (⁶⁰Co γ-rays) and nanosecond-pulsed (3 MeV electrons) radiation. In all cases, C _M was found to increase close to linearly with dose up to ca. 30%. Above this conversion, autoacceleration of polymerization (the gel or Trommsdorff effect) occurs in the γ-irradiated samples. From the low-conversion steady-state data, using dose-rates of 0.21 and 2.7 Gy/s, the parameter 〈k _p〉 ² G(R^.)/2〈k _t〉 = 0.011 and 0.015 lmol^-1 s^-1 (100 eV)^-1 respectively have been determined (with 〈k _p〉 and 〈k _t〉 the average rate coefficients for bimolecular propagation and termination respectively, and G(R^.) the yield of the chain-initiating free radicals per 100 eV (G = 1 (100 eV)^-1 0.1036 μ mol J^-1)). From the 5 Hz repetitive pulse data the value of 〈k _p〉 G(R^.) = 1700 lmol^-1 s^-1 (100 eV)^-1 has been determined. Taking 〈k _p〉 = 342 lmol^-1 s^-1 from the literature results in G(R^.) = 5.0 (100 eV)^-1 and 〈k _t〉 = 2.7 × 10⁷ lmol^-1 s^-1.