Functionalized monomers, 2. Kinetics of radical polymerization of oxirane- and oxetane-derivatives of styrene and copolymerization of 4-vinylphenyloxirane with styrene

The kinetics of radical polymerization of 4-vinylphenyloxirane ( 1 ), 4-vinylbenzyloxirane ( 2 ), and 2-(4-vinylphenyl)oxetane ( 3 ) initiated by 2,2′-azoisobutyronitrile (AIBN) was studied. In the case of 1 the initial rate of polymerization was found to depend on the initiator and monomer concentrations as (r_p)_total ∝? [AIBN]^0,5 · [M₁]^1,36. The higher order of the polymerization rate with respect to 1 was interpreted as due to a concurrent thermally initiated polymerization; the rate of the latter was found to depend on the monomer concentration squared. The value of the ratio propagation rate constant over square root of termination rate constant k_p/k_t^1/2 = 2,8 · 10^?2 dm^3/2 · mol^?1/2 · s^?1/2 was determined from the measured dependences (r_p)_total = f[AIBN] and (r_p)_total = f([M₁]), corrected for the rate of thermally initiated polymerization of 1 . On the other hand, the kinetics of radical polymerization of 2 and 3 did not deviate from the standard scheme valid for radical polymerization; in both cases the observed reaction order with respect to initiator and monomer was 0,5 and 1, respectively. Radical copolymerization of 4-vinylphenyloxirane (M₁) with styrene (M₂) was characterized by monomer reactivity ratios r₁ = 1,06 and r₂ = 0,78, respectively, corresponding to the Q, e-scheme values Q = 0,9 and e = ?0,36 for monomer 1 .     

Kinetic and electron paramagnetic resonance studies on radical polymerization. Radical-initiated polymerization of methyl N-phenylitaconamate in N,N-dimethylformamide

The polymerization of methyl N-phenylitaconamate(methyl 2-methylenesuccinanilate ( 1 )) with dimethyl 2,2′-azodiisobutyrate ( 2 ) was studied in N,N-dimethylformamide (DMF) kinetically and by means of electron paramagnetic resonance (EPR) spectroscopy. The polymerization rate (R_p) at 55°C is given by the equation: R_p = k[ 2 ]^0,58 · [ 1 ]^1,6. The overall activation energy of the polymerization was calculated to be 54,2 kJ/mol. The number-average molecular weight of poly( 1 ) was in the range between 5000 and 17000. From an EPR study, the polymerization system was found to involve the EPR-detectable propagating polymer radical of 1 at practical polymerization conditions. Using the concentration of polymer radicals, the rate constants of propagation (k_p) and termination (k_t) were determined for 55°C. The rate constant of propagation k_p (between 8,4 and 12 L · mol^?1 · s^?1) tends to somehow increase with increasing monomer concentration. On the other hand, k_t (between 1,9. 10^?5 L · mol^?1 · s^?1) increases with decreasing monomer concentration, which results from a considerable dependence of k_t on the polymer-chain length. Such monomer-concentration-dependent k_p and k_t values are responsible for the high dependence of R_p on the monomer concentration. Thermogravimetric results showed that thermal degradation of poly( 1 ) occurs rapidly at temperatures higher than 200°C and the residue at 500°C amounts to 26% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 55°C in DMF the following copolymerization parameters were found: r₁ = 0,52, r₂ = 0,31, and Q, e values Q₁ = 1,09 and e₁ = +0,55.     

Kinetic and ESR studies on radical polymerization. Radical polymerization behavior of N-octadecylmaleimide in benzene

The polymerization of N-octadecylmaleimide ( 1 ) initiated with azodiisobutyronitrile ( 2 ) was investigated kinetically in benzene. The overall activation energy of the polymerization was calculated to be 94,2 kJ·mol^?1. The polymerization rate (R_p) at 50°C is expressed by the equation, R_p = k[ 2 ]^0,6[ 1 ]^1,7. The homogeneous polymerization system involves ESR-detectable propagating polymer radicals. Using R_p and the polymer radical concentration determined by ESR, the rate constants of propagation (k_p) and termination (k_t) were evaluated at 50°C. k_p (33 L · mol^?1 · s^?1 on the average) is substantially independent of the monomer concentration. On the other hand, k_t (0,3 · 10⁴ – 1,0 · 10⁴ L · mol^?1 · s^?1) is fairly dependent on the monomer concentration, which is ascribable to a high dependence of k_t on the chain length of rigid poly( 1 ). This is the predominant factor for the high order with respect to the monomer concentration in the rate equation. In the copolymerization of 1 (M₁) and St (M₂) with 2 in benzene at 50°C, the following copolymerization parameters were obtained: r₁ = 0,11, r₂ = 0,09, Q₁ = 2,1, and e₁ = +1,4.     

Kinetic and ESR studies of the radical-initiated polymerization of N-(2,6-dimethylphenyl)itaconimide

The polymerization of N-(2,6-dimethylphenyl)itaconimide (1) with azoisobutyronitrile (2) was studied in tetrahydrofuran (THF) kinetically and spectroscopically with the electron spin resonance (ESR) method. The polymerization rate (R_p) at 50°C is given by the equation: R_p = K [2] ^0,5 · [1] ^2,1. The overall activation energy of the polymerization was calculated to be 91 kJ/mol. The number-average molecular weight of poly (1) was in the range of 3500–6500. From an ESR study, the polymerization system was found to involve ESR-observable propagating polymer radicals of 1 under the actual polymerization conditions. Using the polymer radical concentration, the rate constants of propagation (k_p) and termination (k_t) were determined at 50°C. k_p (24–27 L · mol^?1 · s^?1) is almost independent of monomer concentration. On the other hand, k_t (3,8 · 10⁴–2,0 · 10⁵ L · mol^?1 · s^?1) increases with decreasing monomer concentration, which seems mainly responsible for the high dependence of R_p on monomer concentration. Thermogravimetric results showed that thermal degradation of poly (1) occurs rapidly at temperatures higher than 360°C and the residue at 500°C was 12% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 50°C in THF the following copolymerization parameters were found; r₁ = 0,29, r₂ = 0,08, Q₁ = 2,6, and e₁ = +1,1.     

Radical Polymerization and Copolymerization of 12‐[(N‐Methacryloyl)carbamoyloxy]octadecanoic Acid

Radical polymerization of 12‐[(N‐methacryloyl)carbamoyloxy] octadecanoic acid ( 1 ) was kinetically and ESR spectroscopically investigated in acetone, using dimethyl 2,2′‐azobisisobutyrate ( 2 ) as initiator. The polymerization rate (R_p) is given by R_p = k [2]^0.7[1]^1.4 at 50°C. Propagating poly( 1 ) radical was observed as a 13‐lines spectrum by ESR under the actual polymerization conditions. The ESR‐determined k_p values (1.8–7.9 L/mol·s) are much lower than those of usual methacrylate monomers. The rate constant (k_t) of termination was determined to be k_t = 1.0–2.7·10⁴ L/mol·s from decay curve of the propagating radical. The Arrhenius plots of k_p and k_t gave the activation energies of propagation (63 kJ/mol) and termination (24 kJ/mol). A significant solvent effect was observed on the radical polymerization of 1 . The copolymerizations of 1 with styrene(St) and acrylonitrile were examined at 50°C. Copolymerization parameters obtained for the 1  (M₁)/St (M₂) system are as follows; r₁ = 0.73, r₂ = 0.57, Q₁ = 0.83, and e₁ = 0.13.     

A kinetic study of butyl acrylate free radical polymerization in benzene solution

The free radical polymerization of butyl acrylate has been studied in benzene solutions ranging from 1 to 5 mol·L^–1 at 50°C using 2,2′‐azobisisobutyronitrile as initiator. Under the conditions of our experiments, both the effective rate coefficient for initiation, 2 f k_d , and the coupled parameter, k_p/k_t^1/2, (where k_p and k_t are the constants for propagation and termination reactions, respectively) are dependent on the monomer concentration. The 2 f k_d value shows little increase with monomer concentration. The variation of the k_p/k_t^1/2 parameter has been correlated with the chain length dependence of the termination rate coefficient. This effect is also responsible for the high dependence of the overall polymerization rate, R_p, onthe monomer concentration (1.49).     

Kinetic studies of the anionic polymerization of methyl methacrylate in tetrahydrofuran with Na+ as counter ion,using monofunctional initiators

A newly designed automatically controlled stirred reactor suitable for kinetic measurements of reactions with half lives ≥2s has been applied to follow the anionic polymerization of methyl methacrylate in THF with Na⁺ as a counter ion in the presence of an excess of NaB(C₆H₅)₄. As initiators were used: benzylsodium reacted with α-methylstyrene (I), fluorenylsodium (II), and 9-methylfluorenylsodium (III). With I the initiation is fast as compared with the polymerization reaction which is first order in monomer concentration. Within the range of ?50°C to ?100°C an almost unperturbed “living” polymerization is observed. The Arrhenius plot of the rate constants results in a straight line with activation energy E_a = 4,4kcal·mol^?1 (= 18kJ·mol^?1) and frequency exponent A = 7,0.II and III are slow initiators, II giving rise to side reactions because of the “acidic” proton in 9-position after initiation, III exhibiting a rate constant of initiation k_i = 1l·mol^?1·s^?1 at ?72°C. The termination reaction is becoming increasingly important with increasing temperature and seems to be a unimolecular reaction with E_a,t = 11,5kcal·mol^?1 (= 48 kJ·mol^?1) and A_t = 10. Since the basic feature of the reactor is the possibility of drawing samples, polymers from each state of the reaction were available to be investigated also with respect to their tacticity. The monomer addition was shown to follow Bernoullian statistics. A structure of the “living” end being in harmony with the results observed is discussed.     

Kinetic study of the polymerization of N,N′-methylenebis(acrylamide) in aqueous dispersion systems

The aqueous-phase polymerization of N,N′-methylenebis(acrylamide) initiated by potassium peroxodisulfate in the absence and in the presence of the anionic emulsifier sodium dodecylsulfate was kinetically investigated at 50°C by conventional gravimetric and dilatometric methods. The rate of polymerization is found to be proportional to the 0,75 and 0,24 oder with respect to potassium peroxodisulfate and N,N′-methylenebis(acrylamide) concentrations, respectively. On the other hand, it is independent of the concentration of sodium dodecylsulfate. This agrees with the polymerization of a monomer soluble in water. Therefore, the equations for a homogeneous polymerization were applied to evaluate the experimental results. The calculated ratio k_p/k_t^0,5 of the rate constants of propagation k_p and termination k_t for the N,N′-methylenebis(acrylamide) polymerization at zero conversion in the absence of emulsifier are scattered in the interval between 3,1 and 3,4 dm^1,5 · mol^?0,5 · s^?0,5 and in the presence of emulsifier in the interval between 2,4 and 3,5 dm^1,5 · mol^?0,5 · s^?0,5. They are close to those obtained for the homogeneous polymerization of acrylamide in the aqueous phase. The lower values of k_p/k_t^0,5 ≈ 0,3–0,6 dm^1,5 · mol^?0,5 · s^?0,5 determined for the polymerization of N,N′-methylenebis(acrylamide) for conversions between 30 and 60% follow from the hindered termination reaction within the polymer particles. The polymer dispersions formed are unstable. The growth of the polymer particles proceeds predominantly by coalescence. This suggests a kinetics which does not follow the Smith-Ewart theory but is characterized by a continuous particle nucleation and agglomeration. The interval 1 occurs at the beginning of the dispersion polymerization when polymer particles are being formed. Interval 2 follows, once the number of polymer particles has been fixed.     

Zur kinetik der radikalischen fällungspolymerisation von methyacrylonitril in substanz unter anfangsbedingungen

The AIBN initiated bulk polymerization of methacrylonitrile was carried out at 50 to 75°C. With regard to the initiator concentration the reaction order is 1/2. The overall activation energy was found to be 22,9 kcal. mole^?1. The parameter k²_w/k_a (k_w = propagation constant, k_a = termination constant) was calculated from the number average of the degree of polymerization (P?_n) and the overall reaction rate at the time t = 0 (v_br,o), taking into account only the low molecular weight fraction of polymers with a distribution of bimodal character. At a temperature of 60°C and with respect to initial state k²_w/k_a = 1,75 .10^?5 dm³ mol^?1.s^?1 and the efficiency of initiation f = 0,58 was obtained. The kinetic constants do not differ from those of solution polymerization.     

ESR determination of rate constants for the radical polymerization of methyl phenethyl itaconate

Phenethyl 2-(methoxycarbonylmethyl)acrylate (methyl penethyl itaconate) ( 1 ) was prepared and its polymerization with dimethyl 2,2′-azoisobutyrate ( 2 ) was investigated kinetically in benzene. The polymerization rate (R_p) was found to be expressed by R_p = k·[ 2 ]^0,5·[ 1 ]^1,8 (at 50°C). Further, a higher dependence of R_p (2nd order) on the concentration of 1 was observed at 61°C. The overall activation energy of the polymerization was calculated to be a low value of 50,3 kJ/mol. The initiator efficiency (f) of 2 was determined to be 0,48 to 0,22 at 50°Cand 0,50 to 0,28 at 61°C. f decreases with increasing monomer concentration due to the high viscosity of 1 . The poly( 1 ) radical is stable enough to be observable by ESR at high temperatures (50–60°C). Rate constants of propagation (k_p) and termination (k_t) were estimated using the poly( 1 ) radical concentration determined by ESR. k_p [6,0 to 121/(mol·s) at 50°Cand 7,1 to 15 1/(mol·s) at 61°C] shows a trend to increase with the concentration of 1 . On the other hand, k_t [2,9·10⁴ to 17·10⁴1/(mol·s) at 50°Cand 6,9·10⁴ to 45·10⁴1/(mol. s) at 61°C] decreases with increasing MPI concentration. This behavior is responsible for the high order with respect to monomer concentration. Copolymerization of 1 (M₁) with styrene (M₂) at 50°Cgave the following results: r₁ = 0,36, r₂ = 0,40, Q₁ = 0,82 and e₁ = + 0,59. Using the above results, the rate constants of the cross-propagations were estimated to be k₁₂ = 22 1/(mol·s) and k₂₁ = 308 1/(mol·s) at 50°C.     

Electron spin resonance (ESR) estimation of propagation rate constants of radical polymerization of butyl methacrylate

Propagation rate constants (k_p) for the free radical polymerization of butyl methacrylate (BMA) in toluene solution were measured by ESR spectroscopy in the temperature range from –30 to 90°C. The polymerization was photochemically induced at a constant initiator concentration of di‐tert‐butyl peroxide (tBPO). The Arrhenius parameters A and E_a were estimated to be (3.40 ± 0.4)>×10⁶ M^–1·s^–1 and (22.0 ± 1.0) kJ/mol, respectively. The k_p values estimated from ESR analysis were compared with the corresponding data obtained from the pulsed laser polymerization (PLP).     

Kinetics and molecular weight evolution during controlled radical polymerization

Automatic, continuous online monitoring of polymerization reactions (ACOMP) was applied to the controlled radical polymerization (CRP) of butyl acrylate (BA) using N‐tert‐butyl‐1‐diethylphosphono‐2,2‐dimethylpropyl nitroxide (SG1), to determine monomer conversion, evolution of molecular weight, reduced viscosity, and rate constants. The conversion is roughly first order, but depends only on the initial ratio of free SG1 to initiator; i.e., it is zeroeth order in initiator concentration. While it was found that the weight‐average molecular weight M _w, and viscosity‐average mass increase in approximately linear fashion with conversion, their values are finite at zero conversion. Although ACOMP involves no chromatographic separation columns, a useful measure of polydispersity evolution was found from combining M _w and viscosity‐based masses. CRP is contrasted with monitoring results for conventional free‐radical polymerization. Distinct light‐scattering signatures are expected, and found experimentally, for the two cases. The CRP kinetic findings allowed the determination of the equilibrium constant between active and dormant species at 120 °C (K_eq = 1.53 × 10⁻¹⁰ M ), as well as the corresponding kinetic constant of deactivation (k_deact = 2.8 × 10⁺⁷ L · mol⁻¹ · s⁻¹) and activation (k_act = 4.2 × 10⁻³ s⁻¹). Cross‐checks on the monitoring results were made with conventional Gel Permeation Chromatography (GPC), and kinetic behavior was also analyzed in the light of numerical integration software.

Raw data for PBA CRP reaction #6 in Table 1 vs. detector time, which lags the reaction time by 800 s. Shown are viscosity, RI, UV and 90 degree light‐scattering signals, together with the time at which stabilization, monomer flow, and polymerization began. The temperature curve is offset to the left 800 s with respect to the detector signals.     

Kinetik der startreaktion der radikalischen homo- und copolymerisation von styrol,N-vinyl-2-pyrrolidon und methylmethacrylat

In one, two and three component polymerizations of the monomer system styrene (St = M₁), N-vinyl-2-pyrrolidone (NVP = M₂) and methyl methacrylate (MMA = M₃) the rate constants of the initiation step 2k_df were measured in bulk at 60°C by means of the inhibition method with the stable Banfield radical as inhibitor and AIBN as initiator. The obtained 2k_df-values depend non-linearly on the monomer feed. It results from the evaluation of initiator decomposition constant k_d that the chain initiation step is mainly determined by the rate of initiator decomposition. In all polymerizations the efficiency factor f was almost equal to 0,5.     

Online Monitoring of Chain Transfer in Free‐Radical Polymerization

A recently introduced, automated method for online monitoring of polymerization reactions was used to study free‐radical transfer reactions. The persulfate initiated polymerization of acrylamide (AAm) in water was chosen as the test system. Chain transfer properties of ethanol (EtOH) and propanol (PrOH) were investigated. Different methods of computing the transfer constant are compared, including those based on the slope and intercept behavior of the monitored cumulative weight‐average molecular mass as a function of conversion, M̄_w (f), the reduced viscosity, and corresponding size exclusion chromatography analysis of the reaction end products. To a close approximation, the chain transfer agents were found to obey the form expected when ideal free‐radical polymerization takes place and radical transfer from propagating radicals to the chain transfer agent (CTA) is slower than from the CTA to monomer, that is, the polymer molar mass decreases with increasing chain transfer agent, but there is no appreciable effect on the kinetics of monomer conversion. The AAm kinetics were characterized in terms of the ratio k_p² /k_t, where k_p and k_t are the propagation and termination rate constants, respectively.     

Mechanischer Scherabbau von Polymeren in Lösung mittels turbulenter Strömung, 1. Kinetik,Molekulargewichtsabhängigkeit und Frage des Bruchmechanismus

The mechanical shear degradation of poly(decyl methacrylate) with weight average molecular weights 1,0·10⁶ ≤ M?_w ≤ 1,7·10⁶, and molecular polydispersity ratio M?_w/M?_n = 5 in dilute solutions is studied in turbulent flow as a function of molecular weight using a special apparatus consisting of two vessels connected by a capillary. Shear stress and shear rate remained constant during degradation. The rate of degradation was followed up by molecular weight distribution curves using gel permeation chromatography and described by ?dc_i/dt = k_i·cⁿ, i being a high molecular weight species of the distribution. The reaction was found to be of the first order (n = 1) independent of solvent and of capillary length. Rate constants k_i in the molecular weight range from 3,2·10⁶ to 13,5·10⁶ proved to be proportional to the hydrodynamic volume of the polymer molecules expressed in terms of the product of intrinsic viscosity and molecular weight [η]_i·M_i. This corresponds to a linear relationship between k_i and M_i^1,75. Additional experiments show that this type of dependence on molecular weight holds only for turbulent flow; in laminar flow the result of the literature could be confirmed that there is a linear relation between k_i and M_i¹. Both results are independent of capillary length. As to the mechanism of breakage in turbulent flow it seems that in one step each macromolecule is broken simultaneously into several smaller parts.     

Difference in propagation rate constants between photo-and thermal-initiated polymerization of styrene – effects of light irradiation

Propagation rate constants (k_p) for photo- and thermal-initiated radical polymerizations of styrene were determined by electron spin resonance (ESR) spectroscopy. The k_p value for the former at 70°C ((420 ± 30) M^?1 · S^?1) was more than twice as large as that for the latter ((190 ± 10) M^?1 · s^?1) although the former value was almost the same as that obtained with the pulsed-laser polymerization method. To evaluate the origin of the difference in the k_p values between photo- and thermal-initiated polymerization the absorption spectrum of a model of the propagating radical was measured and both the influence of the wavelength of the irradiation light on k_p values and the activation energy of the photo-initiated radical polymerization of styrene were determined. k_p decreases for wave-lengths shorter than 370 nm and is in agreement with the k_p value for thermal-initiated polymerization. No wavelength dependence of k_p was obtained for dienes whose propagating radical does not absorb above 300 nm. The difference in k_p values for photo- and thermal-initiated polymerizations is considered to be due to the photoactivation (photoinduced excitation) of the propagating radical of styrene.     

Determination of absolute rate constants for radical polymerization and copolymerization of ethyl α-cyanoacrylate in the presence of effective inhibitors against anionic polymerization

The absolute rate constants of propagation k_p and of termination k_t of ethyl α-cyanoacrylate (ECNA) were determined in bulk at 30°C by means of the rotating sector method under conditions to suppress anionic polymerization; k_p = 1 622 1 · mol^?1 · s^?1 and k_t = 4,11 · 10⁸ 1 · mol^?1 · s^?1 for the polymerization in the presence of acetic acid, and k_p = 1610 1 · mol^?1 · s^?1 and k_t = 4,04 · 10⁸ l · mol^?1 · s^?1 for the polymerization in the presence of 1,3-propanesultone. The magnitude of k/k_t determined was 6,39 · 10^?3 l · mol^?1 · s^?1. The absolute rate constants for cross-propagation in ECNA copolymerizations were also evaluated. Quantitative comparison of the rate constants with those of common monomers and polymer radicals shows that the strong electron-withdrawing power of the ethoxycarbonyl and cyano groups enable the poly(ECNA) radical to add to monomers as fast as the other polymer radicals. The relatively high reactivity of ECNA, regardless of the type of attacking polymer radical, is interpreted by a transition state greatly stabilized by both the ethoxycarbonyl and the cyano groups.     

Kinetic Study of Free‐Radical Solution Copoly‐ merization of N‐Acryloylmorpholine with an Activated Ester‐Type Monomer,N‐Acryloxysuccinimide

The kinetics of free radical solution (co)polymerization of N‐acryloylmorpholine (NAM) and N‐acryloxysuccinimide (NAS) have been investigated at 60°C in 1,4‐dioxane using 2,2′‐azoisobutyronitrile as the initiator. First, the k_p/k_t^1/2 value for the homopolymerization of NAM monomer was estimated to be 4.97 mol^–1/2·L^1/2· s^–1/2, which is indicative of a high ability of NAM to homopolymerize. Then, the reactivity ratios were determined to be r_NAS =  0.63 ± 0.03 and r_NAM =  0.75 ± 0.01. This binary comonomer system behaves like a perfectly random one, allowing the synthesis of macromolecules homogeneous in composition. Average copolymer compositions determined by ¹H NMR and ¹³C NMR spectroscopy were in good agreement with the theoretical ones. Finally, monomer sequence distribution was studied by ¹³C NMR analysis.     

Termination Kinetics of Dibutyl Itaconate Free‐Radical Polymerization Studied via the SP–PLP–ESR Technique

Summary: The termination kinetics of dibutyl itaconate (DBI) bulk polymerization was studied via SP–PLP–ESR single pulse–pulsed laser polymerization with time‐resolved detection of free‐radical concentration by electron‐spin resonance, at temperatures between 0 and 60 °C. As is characteristic of PLP experiments, termination rate coefficients, k_t(i,i), are measured for radicals of (almost) identical chain length (CL) i. CL‐averaged 〈k_t〉, for chain lengths up to 200 monomer units, and also kequation/tex2gif-stack-1.gif referring to termination of very small‐size radicals are directly deduced from measured DBI radical concentration vs time traces. At 45 °C, 〈k_t〉 is (3.4 ± 0.6) · 10⁵ L · mol^?1 · s^?1 and kequation/tex2gif-stack-2.gif is (7.2 ± 1.0) · 10⁵ L · mol^?1 · s^?1. Both rate coefficients are independent of monomer conversion up to the highest experimental conversion of 18%. The associated activation energies are E_A(〈k_t〉) = 23.0 ± 3.2 kJ · mol^?1 and E_A(kequation/tex2gif-stack-3.gif) = 27.6 ± 2.8 kJ · mol^?1, respectively. “Model‐dependent” and “model‐free” analyses of radical concentration vs time profiles indicate a pronounced CL dependence of k_t(i,i) for DBI radicals of moderate size, 5 < i < 50. The lowering of k_t(i,i) with CL corresponds to an exponent α close to 0.5 in a power‐law expression k_t(i,i) = kequation/tex2gif-stack-4.gif · i^?a. At higher chain lengths, the variation of k_t(i,i) with CL becomes weaker and may be represented by an α value of 0.16 or even below. These results are consistent with models according to which α varies to a larger extent at low CL and to a smaller extent at high CL with the crossover region between the two ranges being located somewhere around i = 100.

Conversion‐dependence of 〈k_t〉 and kequation/tex2gif-stack-5.gif from laser‐induced photopolymerization of DBI.     

Kinetic and ESR studies on radical polymerization. ESR and kinetic evidences for the penultimate effect in the radical-initiated copolymerization of N-cyclohexylmaleimide and bis(2-ethylhexyl) itaconate in benzene

The copolymerization of N-cyclohexylmaleimide ( 1 ) (M₁) and bis(2-ethylhexyl) itaconate ( 2 ) (M₂) with dimethyl 2,2′-azoisobutyrate ( 3 ) as an initiator was carried out at 50°C in benzene. Monomer reactivity ratios were estimated as r₁ = 0,34 and r₂ = 0,38. The copolymerization rate (R_p) and the molecular weight of the resulting copolymer increased with increasing concentration of 1 when the total concentration of comonomers was fixed at 1,00 mol. L^?1. R_p was proportional to [ 3 ]^0,5, indicating a usual bimolecular termination in the copolymerization. An electron spin resonance (ESR) spectrum of the propagating polymer radicals was observable in the actual copolymerization system at 50°C. The spectrum of the copolymerization system is inexplicable in terms of any superposition of spectra observed in the corresponding homopolymerization systems, revealing that some penultimate monomeric unit causes a change in the ESR spectrum, that is, the structure of propagating polymer radical. The apparent rate constant of propagation (k_p) and termination (k_t) were estimated by ESR. The k_p values (1,5–50 L · mol^?1 · s^?1) are fairly higher than those estimated on the basis of the terminal model, affording another piece of evidence for the penultimate effect. The k_t value (1,8–5,4·10³ L · mol^?1 · s^?1) shows a behaviour similar to that of the intrinsic viscosity of the resulting copolymer on varying the monomer feed composition, which seems to reflect diffusion-control of termination reactions.