Stable Free Radical Polymerization Kinetics of Alkyl Acrylate Monomers Using in situ FTIR Spectroscopy: Influence of Hydroxyl‐Containing Monomers and Additives

Summary: The homopolymerization and copolymerization of various alkyl acrylate monomers was studied under stable free radical polymerization (SFRP) conditions using in situ FTIR spectroscopy to monitor polymerization kinetics. The IR absorbance corresponding to the C? H deformation of the monomer (968 cm^?1) was measured to determine monomer conversion in real‐time fashion. The monomer disappearance profiles were subsequently converted to pseudo‐first order kinetic plots. Altering the alkyl ester chain length and configuration did not reveal a significant trend in the resulting polymerization kinetics. However, addition of 2‐hydroxyethyl acrylate (HEA) to a polymerization of n‐butyl acrylate (nBA) substantially accelerated the rate of total monomer conversion, increasing the observed rate constant almost two times. ¹H NMR spectroscopy also showed that the resulting HEA/nBA copolymers were enriched with the HEA monomer. Moreover, a similar but enhanced effect was also observed upon the addition of small amounts of dodecanol to an n‐butyl acrylate homopolymerization resulting in more than a doubling of the observed rate constant.

Resonance forms associated with the DEPN nitroxide and stabilization resulting from hydrogen bonding.     

VUV‐Induced Photopolymerization of Acrylates

Acrylates and methacrylates are photopolymerized without photoinitiator by exposure to 172 nm radiation. The kinetics of the polymerization is studied using real‐time Fourier‐transform infrared attenuated total reflectance (FTIR‐ATR) spectroscopy. It is shown that layers with a thickness of ≈500 nm can be polymerized very rapidly. The effect of structure, viscosity, functionality, and absorption of the acrylates as well as the influence of temperature and oxygen concentration on the reactivity are studied. A strong conversion gradient is observed in layers up to ≈2 μm thickness, which reflects the intensity gradient within the layer. However, the penetration of the polymerization into the layer exceeds the initial penetration depth of the VUV radiation, which indicates strong bleaching of the acrylates during irradiation.     

Reinvestigation of the Mechanism of the Free Radical Polymerization Photoinitiation Process by Camphorquinone–Coinitiator Systems: New Results

Summary: Comparative studies of photoinitiation processes using camphorquinone (CQ) and benzophenone (BP) as light absorbers were performed. The experimental results show that after the transformation of (phenylthio)acetic acid (PTAA) into its tetrabutylammonium salt (PTAA AS), a substantial decrease of the polymerization photoinitiation ability for the CQ–PTAA AS pair in comparison to the CQ–PTAA pair is observed. The mechanism of the photoinitiated polymerization for the tested photoredox pair was clarified based on laser flash photolysis experiments obtained using benzophenone as an electron acceptor and (phenylthio)acetic acid and its tetrabutylammonium salt as electron donors in solution in MeCN. It is documented and deduced that the photoreduction of benzophenone in the presence of (phenylthio)acetic acid and its tetrabutylammonium salt occurs by a photoinduced electron transfer process, while for CQ as initiator, the free radicals are formed by hydrogen atom abstraction by the triplet state of camphorquinone.

Schematic of the transients formed after an electron‐transfer process for benzophenone–PTAA and benzophenone–PTAA AS pairs.     

Synthesis and copolymerization of vinylidene fluoride (VDF) with Trifluorovinyl Monomers, 11

The radical telomerization of vinylidene fluoride with 1‐chloro‐1,2‐dibromotrifluoroethane is presented. This dibrominated transfer agent was produced readily from the addition of bromine to chlorotrifluoroethylene in high yields. Four different ways of initiation (thermal, photochemical, or in the presence of redox catalysts or radical initiators) were used in order to obtain optimized yields and degrees of telomerization. Interestingly, the thermal process carried out at 210 °C led to fair to good yields in contrast to photochemically induced reaction or that catalyzed by redox systems (this latter reaction produced the monoadduct only). When radical initiators were used, tert‐butyl peroxypivalate was the most efficient one, despite the lower reaction temperature. ¹H and ¹⁹F NMR spectra enabled the microstructure of VDF telomers to be described and to determine the cumulative average degrees of telomerization (DP _n). Similarly, gas chromatography, able to detect even the ninth‐order adduct, was also useful to assess the DP _n. The kinetics of telomerization of this reaction allowed the determination of the first three order transfer constants ( ) and led to = 1.3 at 75 °C.

     

Polymerization Kinetics of Ethylene Oxide Methacrylates in Ionic Media

The radical polymerization of ethylene oxide (PEGylated) methacrylates in ionic media has been studied. Lithium salts interact with the monomer causing a significant increase in the propagation rate constant, k_p, and also providing an ionic and highly viscous medium that sharply decreases the termination rate coefficient, k_t. Both features make the polymerization reactions with lithium salts faster compared to the bulk monomer. The systems are studied by means of FT‐IR spectroscopy to identify the interactions between the monomer and the lithium salt. In addition, an extensive kinetic study by PLP‐SEC has been performed to study the influence of the lithium salt on k_p and k_t in the polymerization of these monomers. These results are compared to those obtained when ionic liquids are used as polymerization medium.

     

Temperature Dependence of the Oxygen Solubility in Acrylates and its Effect on the Induction Period in UV Photopolymerization

Summary: The influence of temperature on the kinetics of the UV photopolymerization of two diacrylates carried out under air was studied by real‐time FTIR‐ATR spectroscopy. In the temperature range up to 100 °C, the rate of polymerization obeys the Arrhenius law. The induction period was found to decrease with increasing temperature. It was shown that this decay is exclusively due to the decreasing solubility of oxygen in the acrylate. Moreover, from the induction period and the oxygen solubility, quantum yields of the α‐cleavage of some photoinitiators were estimated.

Induction period calculated from the solubility of oxygen in TPGDA in comparison with experimental data from RT‐FTIR measurements.     

Effect of Pressure on Activation–Deactivation Equilibrium Constants for ATRP of Methyl Methacrylate

Atom transfer radical polymerization (ATRP) rate for methyl methacrylate (MMA) in acetonitrile solution has been measured up to 2500 bar. The increase in rate, by up to two orders of magnitude for the CuBr/Me₆TREN system, is not compromised by an increase in dispersity. Activation–deactivation equilibrium constants were measured for MMA, K_ATRP, and for two monomer‐free model systems, K_EBiB, and K_PMMABr, with Me₆TREN, TPMA, PMDETA, and HMTETA being the ligands. K_ATRP and K_EBiB agree within one order of magnitude for Me₆TREN and TPMA, but differ by about two orders of magnitude for PMDETA and HMTETA, whereas K_ATRP and K_PMMABr are close to each other for PMDETA and HMTETA. The observations indicate an important role of back‐strain effects in ATRP of MMA.     

Formation of Inorganic/Organic Nanocomposites by Nitroxide‐Mediated Polymerization in Bulk Using a Bimolecular System

Summary: A series of organic‐inorganic nanoparticles were synthesized by nitroxide‐mediated polymerization (NMP) of butyl acrylate initiated by a self‐assembled monolayer of an azo initiator. The azo initiator was immobilized on silica particles in the presence of a stable nitroxide radical, SG1 (an acyclic β‐phosphonylated nitroxide, N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethyl)propyl nitroxide). After preliminary qualitative characterization by X‐ray spectroscopy (XPS) and Fourier‐transform infrared (FTIR) measurements, the nanoparticles were studied by thermogravimetric analysis (TGA) to determine the polymer grafting density and to permit a comparison with corresponding values of the initiator monolayer. It was demonstrated that the grafting from polymerization exhibits a controlled character with a low polydispersity ( < 1.2) in a large range of molecular weights of the grafted chains (from 4 000 up to 145 000 g · mol^?1) under the conditions when the stable radical SG1, acting as chain growth moderator tethered to the inorganic core, was used.

     

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.     

Cyclic‐Amine‐Based Dithiocarbamate Chain Transfer Agents for the RAFT Polymerization of Less Activated Monomers

A series of pyrrolidine‐, piperidine‐, and imidazoline‐based dithiocarbamate CTAs have been synthesized and utilized to control the polymerization of vinyl monomers. The controlling ability of the CTAs depends on the presence of both activating and re‐initiating groups and the monomer used. Pyrrolidine‐ and piperidine‐based CTAs proved reasonably good for controlling vinyl acetate polymerizations. The living nature of the vinyl acetate polymerizations was studied by kinetics and chain extension reactions. Interestingly the vinyl acetate polymers synthesized using these pyrrolidine and piperidine based CTAs were almost colorless when precipitated and, therefore, were interesting considering the industrial applications of these polymers.

     

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).     

Solvent Effects,Diffusion Control and Mechanistic Aspects of Catalytic Chain Transfer Polymerization

In the catalytic chain transfer polymerization of methyl methacrylate, butyl methacrylate and 2‐ethylhexyl methacrylate in toluene, it was found that the chain transfer constants (C_T) were independent of solvent concentration and therefore of solution viscosity, and did not differ from the bulk polymerization values. For a diffusion controlled system, theoretical calculations predict an increase in C_T when the solution viscosity is lowered. This points at a non‐diffusion controlled catalytic chain transfer step. These results were obtained when the solvent was thoroughly purified using a Grubbs‐type set‐up, whereas a large reduction in C_T was found in unpurified solvent. Similar results were obtained for butyl acetate. Therefore it is concluded that the decrease of C_T in non‐purified solvents is not related to the solvent itself, but to solvent impurities. Further we found that methyl methacrylate ended radicals do not covalently bind to the cobalt complex, in contrast to styrene and acrylate ended radicals.

Chain transfer constant for the CCT polymerization of 2‐ethylhexyl methacrylate in toluene, and predictions for diffusion controlled systems.     

Free‐Radical Polymerization of N‐Vinylimidazole and Quaternized Vinylimidazole in Aqueous Solution

Aqueous‐phase free‐radical batch polymerizations of N‐vinylimidazole (NVI) and quaternized N‐vinylimidazole (QVI) are conducted with varying initial monomer and initiator concentrations at 70 and 85 °C. The polymerization rate of NVI is very slow at the natural pH of 9 due to degradative radical addition to monomer. The rates are increased by lowering the pH, wherein the degradative addition to NVI monomer is partially (at pH 4) and completely (at pH 1) hindered, with the polymerization rate matching that of QVI at pH 1. The initial rates of polymerization for both NVI and QVI are independent of temperature. A kinetic model developed in Predici that includes the pH‐dependent side reactions can reasonably represent both QVI and NVI polymerization.

     

Influence of Monomer Structure on the Propagation Kinetics of Acrylate and Methacrylate Homopolymerizations Studied via PLP‐SEC in Fluid CO2

Summary: Propagation kinetics of free‐radical homopolymerizations of methyl acrylate, dodecyl acrylate, butyl methacrylate, dodecyl methacrylate, glycidyl methacrylate, cyclohexyl methacrylate, and isobornyl methacrylate in solutions containing 40 wt.‐% CO₂ were studied applying the PLP‐SEC technique. The obtained apparent propagation rate coefficients, k_p,app, are by up to 40% below the associated bulk k_p values. This reduction is assigned to a lowering of local monomer concentration, c_M,loc, at the site of the free‐radical chain end rather than to a decrease of the actual propagation rate coefficient. With the alkyl (meth)acrylates, intersegmental interactions between polar groups of the same polymer molecule are responsible for deviations of c_M,loc from the analytical overall monomer concentration, c_M,a. Increasing size of the flexible alkyl ester group reduces the differences between c_M,loc and c_M,a due to shielding effects. Methacrylates with cyclic ester groups do not follow this trend. In case of isobornyl methacrylate, which polymerizes to a rigid material with large side groups, relative size of monomer and CO₂ matters and reduces c_M,loc significantly below c_M,a.

     

Catalytic Chain Transfer Copolymerization of Methyl Methacrylate and Butyl Acrylate

In this work it is shown that catalytic chain transfer is a very efficient way of controlling molecular weight in the copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA). The experimental data are compared to a previously developed model based on copolymerization kinetics and the mechanisms for catalytic chain transfer and for cobalt‐mediated living radical polymerization that can describe the observed transfer constants. Secondly, it is shown that the presence of a catalytic chain transfer agent does not affect the reactivity ratios within the concentration range studied. Finally, the effect of conversion and therewith composition drift on the catalytic chain transfer polymerization of MMA and BA is investigated and it is shown that under the conditions employed in the experiments a certain degree of macromer copolymerization is present at high partial conversions of MMA.

Evolution of MWD with conversion for the CCT copolymerization of MMA and BA in toluene at 60 °C, [AIBN] = 6 × 10^?3 mol · L^?1.     

Radical Propagation Kinetics of N‐Vinylpyrrolidone in Organic Solvents Studied by Pulsed‐Laser Polymerization–Size‐Exclusion Chromatography (PLP–SEC)

Pulsed‐laser polymerization with subsequent analysis of the polymer molar mass distribution by size‐exclusion chromatography, PLP–SEC, is used to measure the propagation rate coefficient, k_p, of N‐vinylpyrrolidone (NVP) in a series of organic solvents, varying the NVP concentration from 5 to 100 wt% and varying the temperature between –5 and +80 °C. In contrast to the 20‐fold increase observed in aqueous solution upon decreasing the NVP concentration from bulk to dilute conditions, the k_p values of NVP in butyl acetate, iso‐propyl acetate, N‐ethylpyrrolidone, and N‐ethylformamide stay within 20% of the bulk value and exhibit no significant dependence on monomer concentration. The k_p behavior of NVP in methanol and n‐butanol is intermediate between the one in water and in the other organic solvents, with k_p increasing by about a factor of 2 upon lowering the monomer concentration from bulk to 5 wt% NVP. The activation energies for propagation in organic solvents agree within experimental uncertainty with the value reported for bulk NVP. The data demonstrate that hydrogen bonding is responsible for the increase in k_p upon dilution, with this effect being much stronger in an aqueous environment than in a solution of alcohol.

     

A Highly Efficient Iron‐Mediated AGET ATRP of Methyl Methacrylate Using Fe(0) Powder as the Reducing Agent

A new method, AGET ATRP mediated by an iron(III) catalyst using Fe(0) powder as a reducing agent and MMA as a model monomer, is reported. The polymerizations can be carried out in the absence or presence of a limited amount of air and show the features of a “living”/controlled radical polymerization. MMA conversions of 90.3 and 80.0% can be obtained in 3.5 and 4.0 h in the absence/presence of a limited amount of air, respectively, for the iron‐mediated AGET ATRP with a molar ratio of [MMA]₀/[EBiB]₀/[FeCl₃ · 6H₂O]₀/[PPh₃]₀/[Fe(0)]₀ = 600:1:0.5:2:0.1 at 90 °C. PMMA with molecular weights of 55 060 and 47 790 g · mol^?1 and with molar‐mass dispersity of 1.24 and 1.28, respectively, can be obtained correspondingly.

     

Investigation of the Peak Width of Polystyrene Prepared by Rotating‐Sector Polymerization in Microemulsion

Styrene was polymerized by intermittent photoinitiation (at a constant sector speed and a light to dark ratio of 1:5) in microemulsion. The molecular weight distribution was measured by size‐exclusion chromatography and turned out to be extremely well structured. When the points of inflection were used for the calculation of the rate constant of propagation in analogy to pulsed‐laser polymerization/size‐exclusion chromatography (PLP‐SEC), values were obtained which were lower than those accepted for bulk polymerization but good agreement was achieved with a slightly modified equation. The absolute peak width (introduced as the difference between two successive points of inflection) turned out to be an invariant quantity no matter which type of distribution curve (number, molar mass, or hyper distribution) was analyzed, thus demonstrating the Poissonian character of the peaks. Furthermore, the relative peak widths (defined as the ratio of two successive points of inflection) for the first peak scattered between 1.35–1.45, those for the second peak between 1.26–1.30. After subtracting the respective value for the theoretical peak width, the same value for the first and second peaks was obtained for all experiments over a broad range of experimental parameters. As the appearing peaks are well resolved and narrow, the influence of axial dispersion on the results is, therefore, conclusively demonstrated. A simple procedure for the direct determination of axial dispersion is presented based on the relative peak widths and the additivity of the peak variances. The invariance of the peak widths is an easily verifiable theoretical and experimental prerequisite for this procedure.

Comparison of GPC traces of a microemulsion polymerization of styrene (t₀ = 1.4 s, [M] = 4.326 mol · l^?1; light‐to‐dark ratio 1:5; full line) and of a standard mixture (styrene, peak masses: 1 950, 89 400, 993 000; dashed line).     

The Influence of Hydrogen Bonding on the Propagation Rate Coefficient in Free‐Radical Polymerizations of Hydroxypropyl Methacrylate

The propagation rate coefficient k_p was determined for hydroxypropyl methacrylate by applying pulsed laser initiated polymerizations and subsequent analysis of the polymer by size‐exclusion chromatography. k_p data were derived for polymerizations in bulk and in several solvents: toluene, tetrahydrofuran (THF), benzyl alcohol, and supercritical CO₂. With the exception of THF, no solvent influence on k_p was observed. For polymerizations in THF k_p values 40% below the corresponding bulk data were obtained. In addition, the activation energy of k_p for polymerizations in THF is higher than for the other systems. The results are explained by a complexation of the OH group contained in the ester group with THF. As a consequence, H bonds between OH groups and carbonyl O atoms, which occur in the other systems, are not formed in the presence of THF. This explanation is supported by Raman spectra, which show that association of carbonyl groups does not occur for systems containing THF, whereas for all other systems the occurrence of two peaks at 1 703 cm^?1 and 1 720 cm^?1 is indicative of the vibrations of two different – associated vs. not associated – types of carbonyl groups. Based on the change in activation energy it is suggested that a true kinetic solvent effect occurs.

Temperature dependence of k_p for HPMA polymerizations in bulk and in solution of THF. The literature data for bulk polymerizations are taken from ref. 22 . Open symbols refer to ν_rep = 10 Hz and filled symbols to ν_rep = 25 Hz.     

Kinetics and Modeling of the Radical Polymerization of Trimethylaminoethyl methacrylate chloride in Aqueous Solution

For the radical polymerization of ionized trimethylaminoethyl methacrylate chloride (TMAEMA) in aqueous solution, two strategies to determine the propagation rate coefficient (k_p) are proposed for systems where the pulsed‐laser polymerization–size‐exclusion chromatography (PLP–SEC) method fails. This problem occurs with some fully ionized or sterically highly hindered monomers, where termination may become too slow. As TMAEMA is a borderline case with k_p being accessible by PLP–SEC and from single‐pulse–pulsed‐laser polymerization with electron paramagnetic resonance (SP–PLP–EPR) spectroscopy, studies into this monomer allow for judging the quality of the suggested alternative approaches of k_p measurement and serve for consistency checks of the previously published k_p and termination rate coefficient (k_t) data. Within both approaches, k_p/〈k_t〉 ^0.5 is measured via chemically initiated polymerization, with 〈k_t〉 referring to chain‐length‐averaged termination. The k_p/〈k_t〉 ^0.5 data are combined either with k_p/〈k_t〉 values from highly time‐resolved near‐infrared detection of monomer conversion induced by a single laser pulse (SP–PLP–NIR) or with Predici modeling on the basis of known chain‐length‐dependent termination kinetics. As coupled rate coefficients are measured, the obtained k_p data also provide 〈k_t〉 for a particular chain‐length distribution. The differences between propagation and termination rates of nonionized and fully ionized monomers are discussed.