A Novel Approach for Investigation of Chain Transfer Events by Pulsed Laser Polymerization

Pulsed laser polymerization (PLP) with subsequent analysis of molecular mass distribution (MMD) is used to determine the rate coefficient of chain transfer to an agent A, k_trA, by varying pulse repetition rate such that the contributions of PLP‐induced and chain‐transfer‐induced peaks to the MMD change to a significant extent. It is shown by simulation that the relative heights of these peaks may be used to estimate k_trA. The method is applied to evaluation of the rate coefficient of chain transfer to dodecyl mercaptan with butyl methacrylate polymerizations at ?11, 0, 20 and 40 °C. The Arrhenius parameters for this coefficient are determined to be: A(k_trA) = (2.2 ± 0.6) × 10⁶ L · mol^?1 · s^?1 and E_a(k_trA) = (22.1 ± 0.7) kJ · mol^?1.

     

Pulsed Laser Polymerization of Alkyl Acrylates: Potential Effects of the Oxygen Presence and High Laser Power

Summary: Unexpected difficulties are encountered in the determination of propagation rate coefficients (k_p) in free radical polymerization of alkyl acrylates by pulsed laser polymerization (PLP), mainly due to intramolecular transfer to polymer. 1 This article is focused on the role played by the high laser power in these difficulties and the possible reactions of mid‐chain radical with residual oxygen. Removing the oxygen by simple bubbling of nitrogen is sufficient to avoid alteration of the polymerization kinetics of acrylates by residual oxygen under PLP conditions. Moreover, no degradation of polymer (or solvent) has been detected after irradiation with the high laser power typically used in PLP experiments. However, it has been shown that this high laser power completely prevents from having a temporally and spatially homogeneous radical concentration in the PLP cell. A model is proposed here to simulate the pulsed laser polymerization taking initiator consumption and laser energy absorption into account. According to our simulation results, this non‐negligible initiator consumption and laser power absorption can indeed have a positive influence, i.e., it favors the obtainment of a bimodal molar mass distribution fulfilling the IUPAC consistency criteria. This observation may contradict the idea that PLP‐SEC is not suitable to determine accurate k_p values for acrylates above 20–30 °C.

Instantaneous MMDs formed after N pulses. Simulation taking initiator consumption and laser absorption into account.     

Investigations into the Mass Spectrometric Method for the Determination of the Mode of Termination in Radical Polymerization

The fraction of termination by disproportionation, λ, in radical polymerization may be determined by a mass spectrometric (MS) analysis of the resulting polymer. It is subjected to an array of stringent consistency checks, working with polymerizations of methyl methacrylate at 85 °C in three different solvents and with four different initiators. λ is shown to be unaffected by the choice of initiator, initiator concentration, or isoviscous solvent. These findings serve to allay any fears about the method being undermined by effects such as primary radical termination or chain transfer to solvent, thereby establishing its robustness. At the same time, direct evidence for the occurrence of chain transfer to the solvent methyl isobutyrate is uncovered, and the importance of knowing other rate‐parameter values accurately, if λ is to be determined accurately, is illustrated. By carrying out MS analyses, it is concluded that electrospray ionization with time‐of‐flight detection gives the best results for the present purpose.

     

Low Conversion 4‐Acetoxystyrene Free‐Radical Polymerization Kinetics Determined by Pulsed‐Laser and Thermal Polymerization

Summary: The free‐radical polymerization kinetics of 4‐acetoxystyrene (4‐AcOS) is studied over a wide temperature range. Pulsed‐laser polymerization, in combination with dual detector size‐exclusion chromatography, is used to measure k_p, the propagation rate coefficient, between 20 and 110 °C. Values are roughly 50% higher than those of styrene, while the activation energy of 28.7 kJ · mol⁻¹ is lower than that of styrene by 3–4 kJ · mol⁻¹. With known k_p, conversion and molecular weight data from 4‐AcOS thermal polymerizations conducted at 100, 140, and 170 °C are used to estimate termination and thermal initiation kinetics. The behavior is similar to that previously observed for styrene, with an activation energy of 90.4 kJ · mol⁻¹ estimated for the third‐order thermal initiation mechanism.

Joint confidence (95%) ellipsoids for the frequency factor A and the activation energy E_a from non‐linear fitting of k_p data for 4‐AcOS (black) and styrene (grey).     

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.     

Measurement of Transfer Coefficients to Monomer for n‐Butyl Methacrylate by Molecular Weight Distributions from Emulsion Polymerization

The average kinetic coefficient for chain transfer to monomer 〈k_tr,M〉 in the free‐radical polymerization of n‐butyl methacrylate (BMA) has been determined by the analysis of molecular weight distributions obtained by seeded emulsion polymerization under conditions such that chain transfer to monomer is the dominant chain‐stopping event. Measurements between 40 and 70 °C gave data fitting an Arrhenius‐type relationship with exponential factor E_A = 30 900 ± 4 500 J · mol^?1 and pre‐exponential factor log A = 3.45 ± 0.15. The value for E_A is comparable with published data for chain transfer to monomer from methyl methacrylate (MMA) and n‐butyl acrylate (BA). The A value, however, is 1–3 orders of magnitude smaller, suggesting that there is more hindrance for chain transfer to monomer for BMA than for either MMA or BA.

     

Atom Transfer Radical Polymerization of Glycidyl Methacrylate: A Functional Monomer

Summary: A detailed investigation of the polymerization of glycidyl methacrylate (GMA), an epoxy‐functional monomer, by atom transfer radical polymerization (ATRP) was performed. Homopolymers were prepared at relatively low temperatures using ethyl 2‐bromoisobutyrate (EBrIB) as the initiator and copper halide (CuX) with N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) as the catalyst system. The high polymerization rate in the bulk did not permit polymerization control. However, homopolymerization in solution enabled us to explore the effects of different experimental parameters, such as temperature, solvent (toluene vs. diphenyl ether) and initiator concentration, on the controllability of the ATRP process. SEC analysis of the homopolymers synthesized confirmed the importance of solvent character on molecular weight control, the lowest polydispersity indices ( ) and the highest efficiencies being found when the polymerizations were performed in diphenyl ether in combination with a mixed halide technique. A novel poly(glycidyl methacrylate)‐block‐poly(butyl acrylate) (PGMA‐b‐PBA) diblock copolymer was prepared through ATRP using PGMA‐Cl as a macro‐initiator. This chain growth experiment demonstrated a good living character under the conditions employed, while simultaneously indicating a facile synthetic route for this type of functional block copolymer. In addition, the isotacticity parameter for the PGMAs obtained was estimated using ¹H NMR analysis which gave a value of σ_GMA = 0.26 in agreement with that estimated in conventional radical polymerization.

SEC chromatograms of PGMA‐Cl macroinitiator and PGMA‐b‐PBA diblock copolymer.     

Termination Kinetics of tert‐Butyl Methacrylate and of n‐Butyl Methacrylate Free‐Radical Bulk Homopolymerizations

Summary: Termination kinetics in tert‐butyl methacrylate (tert‐BMA) and n‐butyl methacrylate (n‐BMA) bulk homopolymerizations has been studied via the single pulse‐pulsed laser polymerization‐near infrared (SP‐PLP‐NIR) method between 40 and 80 °C at pressures from 500 to 2 250 bar. Toward increasing monomer conversion, the chain‐length averaged termination rate coefficient, 〈k_t〉, for both monomers exhibits the methacrylate‐specific sequence of an initial plateau region, assigned to control by segmental diffusion, followed by a steep decrease of 〈k_t〉 at intermediate conversion, which is assigned to translational diffusion control, and a weaker decrease of 〈k_t〉, associated with reaction‐diffusion control, at still higher degrees of monomer conversion. Despite this similarity, the two isomeric monomers clearly differ in absolute size of 〈k_t〉 and in the monomer concentration ranges where the transitions between the different types of diffusion control occur. The differences are assigned to effects of chain mobility which is hindered to a larger extent in tert‐BMA than in n‐BMA. As a consequence, the 〈k_t〉 behavior of tert‐BMA at 80 °C is close to the one of n‐BMA at 40 °C. Investigations into the chain‐length dependence of k_t, in particular into k_t(i,i), the rate coefficient for termination of two radicals of identical size, support the evidence on the different types of diffusion control that operate as a function of monomer conversion. In the initial conversion range, the power‐law exponent which characterizes the chain‐length dependence of larger (entangled) radicals, is found for both monomers to be close to the theoretical value of α = 0.16.

Dependence of log(〈k_t〉/k_p) on monomer conversion, X, for n‐BMA and tert‐BMA bulk homopolymerizations at 2 000 bar and 70 °C. Circles and triangles represent independent data sets obtained from separate experiments.     

Termination Kinetics of Methyl Acrylate and Dodecyl Acrylate Free‐Radical Homopolymerizations up to High Pressure

The single pulse (SP) – pulsed laser polymerization (PLP) technique has been applied to measure k_t /k_p, the ratio of termination to propagation rate coefficients, for free‐radical bulk homopolymerizations of methyl acrylate (MA) and dodecyl acrylate (DA) between 10 and 50°C at pressures from 10 to 2 500 bar. k_t /k_p is obtained from experimental monomer concentration vs. time traces that are determined via time‐resolved (μs) near infrared monitoring of monomer conversion induced by single excimer laser pulses of about 20 ns width. With k_p being known from PLP–SEC experiments, chain‐length averaged k_t is immediately obtained from k_t /k_p. For MA, k_t remains constant up to about 15% monomer conversion and clearly decreases upon further polymerization. For DA at pressures of 100 bar and above, a plateau value of constant k_t is observed up to about 60% monomer conversion whereas at lower pressure, e. g. at 10 bar, k_t slightly increases in the very initial conversion region, but also exhibits a plateau k_t value at moderate and high conversions. The occurrence of such plateau k_t values and their pressure and temperature dependence are consistent with the view that plateau regions of k_t are best understood in terms of diffusion control via segmental mobility.     

Atom Transfer Radical Dispersion Polymerization of Styrene in the Presence of PEO‐based Macromonomer

Commercially available poly(ethylene oxide) macromonomers are successfully used as reactive stabilizers for atom transfer radical dispersion/precipitation polymerization of styrene. Polystyrene particles with PEO chains covalently anchored on the surfaces are obtained. Control over both the particle size and polymer chain growth is achieved using a two‐stage technique consisting of an initial free radical polymerization (FRP) for the nucleation followed by a reverse atom transfer radical polymerization (ATRP) for the controlled polymerization process. The limited monomer conversion at the FRP step ensures that the overall polymerization process is mainly controlled by the reverse ATRP step, resulting in the formation of particles containing well‐defined polymer chains.

     

Termination Kinetics of 1‐Vinylpyrrolidin‐2‐one Radical Polymerization in Aqueous Solution

Termination kinetics of 1‐vinylpyrrolidin‐2‐one radical polymerization in aqueous solution has been studied at 40 °C between 20 and 100 wt.‐% VP. The <k_t>/k_p values from laser single‐pulse experiments with microsecond time‐resolved NIR detection of monomer conversion, in conjunction with k_p from literature, yield chain‐length‐averaged termination rate coefficients, <k_t>. Because of better signal‐to‐noise quality, experiments were carried out at 2 000 bar, but also at 1 500, 1 000, and 500 bar, thus allowing for estimates of <k_t> at ambient pressure. The dependence of <k_t> on monomer conversion indicates initial control by segmental diffusion followed by translational diffusion and finally reaction diffusion control. To assist the kinetic studies, viscosities of VP–water mixtures at ambient pressure have been determined.

     

Dependence of termination kinetics in methyl acrylate–dodecyl acrylate free‐radical copolymerization on comonomer composition,pressure, temperature and conversion

The termination kinetics of the free‐radical bulk copolymerization of dodecyl acrylate (DA) and methyl acrylate (MA) has been investigated at various monomer mole fractions between 15 and 50 °C and up to 2000 bar. The ratio of termination to propagation rate coefficients, (k_t/k_p)_copo, is measured via the single pulse‐pulsed laser polymerization (SP‐PLP) technique. Chain‐length averaged k_t,copo are deduced from (k_t/k_p)_copo in conjunction with k_p,copo data that are estimated via a simplified version of the implicit penultimate unit effect (IPUE) model. At low and moderate degrees of monomer conversion extended ranges of almost constant k_t are observed where termination is controlled by segmental diffusion with important contributions of steric effects. These plateau k_t values are significantly – by almost two orders of magnitude – different for MA and DA. The increase with MA content of k_t,copo is adequately described by a penultimate unit model which uses the geometric mean approximation for estimating rate coefficients of cross‐termination between radicals of different free‐radical terminus. The model applies within the entire pressure and temperature range of the present study. At high degrees of monomer conversion, at and above 50%, homo‐k_t and k_t,copo are almost insensitive toward the composition of the monomer mixture. The termination rate under these conditions is essentially controlled by reaction diffusion.

Relative monomer conversion, c_M(t)/c_M,0, vs. time, t, plot of a methyl acrylate (MA) – dodecyl acrylate (DA) copolymerization (f_MA,0 = 0.5) induced by a single laser pulse at 30 °C and 10 bar. The difference between measured and fitted (to Equation (4)) data is illustrated by the plot of residuals (res) in the lower part of the figure.     

Pulsed Laser Polymerization of Styrene in Microemulsion: Determination of Band Broadening in Size Exclusion Chromatography with Multimodal Distributions

Summary: Styrene was polymerized in microemulsion by pulsed laser radiation. The resulting multimodal distributions are composed of equidistant Poisson distributions as bimolecular termination occurs mainly during the short laser pulses. For nearly all distributions the first peak was almost base line separated and higher order peaks were clearly distinguishable. The width of the first and second peak was used to determine the extent of band broadening (bb), σ, as a function of the retention volume. Results were collected for three combinations of columns differing in their separation range. These values and those obtained from anionically polymerized polystyrene standards show the same dependence on the retention volume and can be described by the van Deemter equation even beyond the exclusion limit. The results are interpreted in the context of obstructed diffusion and the van Deemter equation is extended to take into account the contribution of the individual columns with different pore sizes. Both theory and experiment advocate a non‐uniform bb thus emphasizing the necessity for the determination of bb for a given set of columns.

Comparison of experimental results (squares – standards, circles PLP‐ME samples) and theory (full line: according to Equation ( 6c ), broken lines: individual contributions of the columns).     

Modeling of Atom Transfer Radical Polymerization with Bifunctional Initiators: Diffusion Effects and Case Studies

Summary: A mathematical model for atom transfer radical polymerization (ATRP) with bifunctional initiators was developed. The model was validated with three case studies in bulk and solution polymerization. We used only polymer yield data to estimate some of the model parameters, while others were obtained from the literature. The model fits the polymer yield data and also predicts weight‐average molecular weights and polydispersities very well. The free volume theory was also incorporated to the model to study the effect of diffusion‐controlled reactions. The adjustable parameters in the free volume theory for the termination, propagation, activation, and deactivation reactions were varied to show the effect on monomer conversion, polymer chain length, and polydispersity. The model shows that diffusion‐limited termination reactions produce polymer with smaller polydispersities, while diffusion‐limited propagation reactions have the opposite effect. Both models, considering and neglecting diffusion effects on the kinetic rate constants, were compared with experimental data. Even though the model predictions for monomer conversion, number‐average molecular weight, and polydispersity are good in both cases, the simulations indicate that diffusion‐controlled reactions can be ignored for the cases studied in the three case studies described in this paper.

Comparison between model predictions and experimental data for BA polymerization of number‐average molecular weight in bulk at 90 °C.     

Synthesis of AB2 3‐ and AB4 5‐Miktoarm Star Copolymers Initiated from Dendritic Tri‐ and Penta‐Functional Initiators by Combination of Ring‐Opening Polymerization of ε‐Caprolactone and Nitroxide‐Mediated Radical Polymerization of Styrene

Summary: Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators. Initially, two kinds of dendritic initiators having one benzylic OH and two or four TEMPO‐based alkoxyamine moieties were prepared. Using them, ROP of ε‐caprolactone was carried out at room temperature to give poly(ε‐caprolactone)s carrying two or four alkoxyamine moieties. NMRP of styrene from the poly(ε‐caprolactone)s was carried out at 120 °C to give AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers, which were analyzed by ¹H NMR and SEC. The increased linearly with conversion and the were in the range 1.10–1.37, showing that well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were formed.

Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators.     

Concurrent Initiation by Air in the Atom Transfer Radical Polymerization of Methyl Methacrylate

The effect of air in atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) was studied. Air initiated polymerization was clearly noticed by the appearance of a low molecular weight peak in the synthesis of high molecular weight poly(isobutylene)‐graft‐poly(methyl methacrylate) (M _n = 5.0 × 10⁵ g/mol). The concentration of chains initiated by oxygen (air) was ≈8 × 10^?4 mol/L, determined using the Gladstone‐Dale relationship. The tentatively proposed mechanism for air initiated polymerization was supported by kinetic studies. Similar to typical ATRP systems, the rate of air initiated polymerization increased with temperature, [MMA], amount of air, and activity of the catalyst complex. Polymers with lower polydispersities (M _w/M _n = 1.13) were obtained in the presence of Cu(II) as compared to Cu(I) catalyst complex system.

Kinetic plots for the air initiated bulk polymerization of MMA at (?) 20 °C, (?) 50 °C, and (?) 90 °C.     

Photo‐Initiated Living Free Radical Polymerization in the Presence of Dibenzyl Trithiocarbonate

The polymerizations of styrene (St), methyl acrylate (MA), and butyl acrylate (BuA), carried out under UV irradiation at room temperature in the presence of dibenzyl trithiocarbonate (DBTTC) were found to display living free‐radical polymerization characteristics as evidenced by: narrow molecular weight distribution, linear increase of molecular weight with increasing conversion, well‐controlled molecular weight, and first‐order polymerization kinetics. The triblock copolymer, PMA‐PSt‐PMA, with narrow polydispersity and well‐defined structure was successfully prepared using PMA‐S‐C(=S)‐S‐PMA as macro‐photoinitiator under UV irradiation at room temperature. Based on GPC, NMR and FT‐IR analyses, the structures of the polymers were obtained and the mechanism of the polymerization was proposed.     

MALDI‐TOF Analysis of Halogen Telechelic Poly(methyl methacrylate)s and Poly(methyl acrylate)s Prepared by Atom Transfer Radical Polymerization (ATRP) or Single Electron Transfer‐Living Radical Polymerization (SET‐LRP)

Poly(methyl methacrylate)s (PMMA)s and poly(methyl acrylate)s (PMA)s are prepared by atom transfer radical polymerization (ATRP) or single electron transfer‐living radical polymerization (SET‐LRP) using methyl dichloroacetate (MDCA) and ethyl dibromoacetate (EDBA) as bifunctional initiators. The chain‐end functionality is determined by MALDI‐TOF mass spectrometry. The target PMMA (M_n = 2000 g mol^?1) and PMA (M_n = 2000 g mol^?1) samples obtained by ATRP of MMA and MA with MDCA as initiator have 12 and 81 mol% bis‐chloro end groups, respectively; those prepared by SET‐LRP have 57 and 100 mol% bis‐chloro end groups, respectively. The PMMAs obtained by ATRP or SET‐LRP with EDBA have no bromine end groups.

     

How to Fight Kinetics in Complex Radical Polymerization Processes: Theoretical Case Study of Poly(divinyl ether‐alt‐maleic anhydride)

Products of free‐radical polymerization (FRP) are usually not regulated on the molecular scale, consisting of blocks obtained through the fastest kinetic scheme pathways. The side or kinetically restricted products can be a source of impurities in a complex FRP case, or possess new properties if isolated solely. FRP synthesis of poly(divinyl ether‐alt‐maleic anhydride), known as “DIVEMA”, serves as a polymerization example with such kinetic and thermodynamic complexities. Uncertainty in factors regulating polymer structure is a challenge in advancement “DIVEMA” derivatives toward medical practice. In‐depth investigation via quantum‐chemical and molecular mechanics methods unveils mechanistic aspects of polymer stereoisomerism and confirms possible isolation of thermodynamically or kinetically controlled products on a large data set. Strategies toward regulation of 5‐exo/6‐endo cycloisomerism are theorized and then studied via microkinetic modeling. Thermodynamically controlled products can be isolated utilizing lower monomer concentrations, in range of 10^?3 to 10^?1 m , and/or application of a complexing agent that is better to realize via solvents, capable of formation π‐ and σ‐radical complexes. Change of electrophilic monomer is proposed as an approach for designing more molecularscale adjustable copolymerization processes. Methodology, obtained results, and conclusions for “DIVEMA” can be valuable to control other FRP processes on the molecular scale, unlocking polymers with improved or new functionalities.     

Direct Monitoring of the Cross‐Over from Diffusion‐Controlled to Decomposition‐Controlled Initiation in Free Radical Polymerization

High‐resolution free radical polymerization kinetics was obtained using automatic continuous online monitoring of polymerization reactions (ACOMP). A sharp cross‐over from diffusion‐controlled initiation at low [monomer] to initiator decomposition control at higher [monomer] was found, and agrees with the quasi‐steady state approximation (QSSA). The cross‐over was also measurable within individual experiments. The kinetic implications for polymer weight average molecular weight and intrinsic viscosity (η)_w were analyzed, and the QSSA‐predicted trends for the ratios of final to initial , and (η)_w, confirmed. Analytical expressions for conversion are contrasted, and it was found that first‐order fits, while not fully justified theoretically, nonetheless are robust, which simplifies calculations needed for controlling molecular weight distributions in “semi‐batch” reactions, where reagents are fed to the reactor through programmable flow profiles. At the low monomer concentrations used, there was no evidence that propagation or termination rate coefficients changed during the reactions.