EPR Study of Backbiting in the Aqueous-Solution Polymerization of Acrylamide.

Via electron paramagnetic resonance (EPR) spectroscopy, the type of radicals occurring during acrylamide (AAm) homopolymerization in aqueous solution is investigated between -5 and +100 °C. The radicals are produced photochemically under stationary conditions. Midchain AAm radicals (MCRs) are clearly identified by EPR which demonstrates that secondary propagating AAm radicals (SPRs) undergo backbiting reactions. Above 50 °C, the fraction of MCRs even exceeds the one of SPRs. The extent of backbiting is however well below the one in butyl acrylate polymerization at identical temperature.

Solvent Effects on Acrylate kp in Organic Media: A Response.

Radical propagation rate is adequately understood in the light of fundamental kinetic theory. Differences in bulk and solution k(p) are primarily of entropic origin, with the effects depending on the differences in polarity and size of monomer and solvent molecules, respectively. Experimental data for the propagation rate coefficient of secondary acrylate radicals demonstrate that bulk and solution-in-toluene k(p) exhibit distinctly different behavior. Bulk k(p) is clearly enhanced in passing from methyl acrylate (MA) to dodecyl acrylate (DA), whereas solution-in-toluene k(p) is approximately constant with a slight tendency to decrease from MA to DA. This kp behavior has also been found in a recent study, in which it has, however, been concluded that k(p) in solution of toluene (and of butyl acetate) displays a similar behavior to bulk. This surprising and ina-dequate conclusion requires the present comment and rectification to be made.

High-pressure atom transfer radical polymerization of n-butyl acrylate.

High-pressure atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) is performed in acetonitrile (MeCN) with Cu(I) Br/TPMA [TPMA: tris(2-pyridylmethyl)-amine] as the catalyst up to 5 kbar. Increasing either pressure or temperature significantly enhances the rate of polymerization, while retaining control over the polymerization. The polymerizations under high pressure could be efficiently performed with very low levels of Cu catalyst in the absence of any reducing agents. For example, 100 ppm Cu is sufficient to catalyze the polymerization of BA with targeted degree of polymerization (DPT ) = 1000. The conversion reached 79% in 3.0 h at 80 °C providing PBA with Mn = 112 000, Mw /Mn = 1.12. Since the initial Cu(I) -to-initiator molar ratio is 0.05:1, the molar percentage of terminated chains should remain <5%. For DPT = 10 000 using only 50 ppm Cu catalyst, a polymer with molecular weight Mn = 612 000 (DP = 4800) was obtained at 67% conversion.

"Assessing the RAFT equilibrium constant via model systems: an EPR study"--response to a comment.

We have presented an EPR-based approach for deducing the RAFT equilibrium constant, K(eq), of a dithiobenzoate-mediated system [Meiser, W. and Buback M. Macromol. Rapid Commun. 2011, 32, 1490]. Our value is by four orders of magnitude below K(eq) from ab initio calculations for the identical monomer-free system. Junkers et al. [Macromol. Rapid Commun. 2011, 32, 1891] claim that our EPR approach would be model dependent and our data could be equally well fitted by assuming slow addition of radicals to the RAFT agent and slow fragmentation of the so-obtained intermediate radical as well as high cross-termination rate. By identification of all side products, our EPR-based method is shown to be model independent and to provide reliable K(eq) values, which demonstrate the validity of the intermediate radical termination model.

Determination of ATRP Equilibrium Constants under Polymerization Conditions.

Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were measured during polymerization of methyl acrylate (MA) with CuIBr/CuIIBr2 in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris(2-pyridylmethyl)amine (TPMA) or tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln(KATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for KATRP to bulk conditions, which were 2 × 10-9 and 3 × 10-8 for TPMA and Me6TREN ligands at 25 °C, respectively. The temperature effect on KATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The KATRP values increased with temperature providing ΔH = 36 kJ mol-1 in MeCN.

Assessing the RAFT equilibrium constant via model systems: an EPR study.

Reversible addition-fragmentation chain transfer (RAFT) equilibrium constants, K(eq), for the model system cyano-iso-propyl dithiobenzoate (CPDB) - cyano-iso-propyl radical (CIP) have been deduced via electron paramagnetic resonance (EPR) spectroscopy. The CIP species is produced by thermal decomposition of azobis-iso-butyronitrile (AIBN). In solution of toluene at 70 °C, K(eq) has been determined to be (9 ± 1) L · mol(-1). Measurement of K(eq) = k(ad)/k(ß) between 60 and 100 °C yields ΔE(a) = (-28 ± 4) kJ · mol(-1) as the difference in the activation energies of k(ad) and k(ß). The data measured on the model system are indicative of fast fragmentation of the intermediate radical produced by addition of CIP to CPDB.

SP-PLP-EPR Investigations into the Chain-Length-Dependent Termination of Methyl Methacrylate Bulk Polymerization.

Termination kinetics of methyl methacrylate (MMA) bulk polymerization has been studied via the single pulsed laser polymerization-electron paramagnetic resonance method. MMA-d(8) has been investigated to enhance the signal-to-noise quality of microsecond time-resolved measurement of radical concentration. Chain-length-dependent termination rate coefficients of radicals of identical size, k ti,i, are reported for 5-70 °C and up to i = 100. k ti,i decreases according to the power-law expression $k_{\rm t}^{i,i} = k_{\rm t}^{{\rm 1,1}} \cdot i^{ - \alpha }$. At 5 °C, k(t) for two MMA radicals of chain-length unity is k t1,1 = (5.8 ± 1.3) · 10(8) L · mol(-1) · s(-1) . The associated activation energy and power-law exponent are: E(A) (k t1,1) ≈ 9 ± 2 kJ · mol(-1) and α ≈ 0.63 ± 0.15, respectively.

EPR Analysis of n-Butyl Acrylate Radical Polymerization.

Via electron paramagnetic resonance (EPR) spectroscopy, concentrations of secondary propagating radicals (SPRs) and tertiary mid-chain radicals (MCRs) in n-butyl acrylate solution polymerization were measured. The EPR spectrum is dominated by the 4-line spectrum of SPRs at -50 °C and by the 7-line spectrum of MCRs at +70 °C. At intermediate temperatures, a third spectral component is seen, which is assigned to an MCR species with restricted rotational mobility. The MCR components are produced by 1,5-hydrogen shift (backbiting) of SPRs. The measured ratio of MCRs to SPRs allows for estimating the rate coefficient k pt for monomer addition to a mid-chain radical. For 70 °C, k pt is obtained to be 65.5 L · mol(-1) · s(-1) .

SP-PLP-EPR study of chain-length-dependent termination in free-radical polymerization of n-dodecyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate: evidence of "composite" behavior.

The chain-length dependence of the termination rate coefficient in n-dodecyl methacrylate (DMA), cyclohexyl methacrylate (CHMA), and benzyl methacrylate (BzMA) bulk free-radical homopolymerizations at ambient pressure and at temperatures from -20 to 0 degrees C is deduced using the recently developed technique of SP-PLP-EPR: pulsed-laser polymerization (PLP) in which time-resolved EPR measurement of radical concentration, cR, is made following each single pulse (SP) of an excimer laser. The decay of cR results from termination of radicals of almost identical size. Their chain length, i, increases linearly with time, t, after applying a SP. The rate coefficient, kt(i,i), for termination of two radicals of size i is determined by fitting the experimental cR vs t data. This process demonstrates that (at least) two power-law exponents are necessary to describe kt(i,i) over the extended chain-length range of i = 1 to 1000. This is consistent with the so-called "composite model" , which uses power-law exponents alpha(S) and alpha(L) to describe termination of radicals either shorter or longer, respectively, than a crossover chain length, ic. The fourth parameter obtained from fitting the SP-PLP-EPR data with this model is kt(1,1), the termination rate coefficient for two radicals of degree of polymerization 1. Previous DMA experiments are reanalyzed while new experimental results are reported and analyzed for CHMA and BzMA. The parameter values for CHMA and BzMA termination at 0 degrees C are almost identical-kt(1,1) approximately 3 x 10(7) L mol(-1) s(-1), alpha(S) approximately 0.50, ic approximately 90, and alpha(L) approximately 0.21-and they are close to those for DMA at 0 degrees C: kt(1,1) approximately 1 x 10(7) L mol(-1) s(-1), alpha(S) approximately 0.64, ic approximately 50, and alpha(L) approximately 0.18. The results fully support the composite model in that the chain-length dependence is more pronounced for shorter than for longer radicals, i.e., alpha(S) > alpha(L). Moreover, the power-law exponent that characterizes termination of long-chain radicals is close to the theoretical value of alpha(L) = 0.16. In fact all parameter values-including the small differences between DMA and CHMA/BzMA-are more-or-less in accord with expectations based on polymer dynamics. Furthermore, our results suggest that termination of methacrylate radicals with large cyclic or long n-alkyl substituents may be affected by steric shielding of the radical functionality.

Characterization of n-butyl acrylate centered triads in poly(n-butyl acrylate-co-carbon monoxide-co-ethylene) by isotopic labeling and two dimensional NMR.

UNLABELLED: Poly(n-butylacrylate-co-carbon monoxide-co-ethylene) (polyEBC) samples prepared from 13C-labeled monomer, n-butyl acrylate, were characterized using two dimensional (2D) pulsed field gradient (PFG) 750 MHz NMR spectroscopy. To elucidate the complex structure of the terpolymer, 2D-1H/13C-heteronuclear single quantum coherence (HSQC) and heteronuclear multiple bond correlation (HMBC) experiments were conducted by selectively exciting the enhanced resonances in the spectra of two polymer samples, one polymer resulting from synthesis with 1-13C-n-butylacrylate monomer and a second polymer obtained from a synthesis with 2-13C-n-butylacrylate monomer. High-resolution 2D-NMR combined with 13C-labeling of the polymer greatly simplifies the 2D-NMR spectra, selectively enhances the weak peaks from low occurrence B-centered triad structures, and aids in their resonance assignments. In all experiments, the sample temperature was 120 degrees C, to ensure a homogeneous solution and sufficient molecular mobility. ELECTRONIC SUPPLEMENTARY MATERIAL: Supplementary material (1D 13C NMR spectra of the 13C-labeled and unlabeled polymers) is available in the online version of this article at http://dx.doi.org/100.1007/s00216-003-2402-3.

A seemingly well understood light-induced peroxide decarboxylation reaction reinvestigated with femtosecond time resolution.

The photoinduced (266 nm) ultrafast decarboxylation of the peroxyester tert-butyl 9-methylfluorene-9-percarboxylate (TBFC) in solution has been studied with femtosecond time resolution. While the photodissociation of TBFC occurs too fast to be resolved, the intermediate 9-methylfluorenylcarbonyloxy radical (MeFl-CO(2)) decarboxylates on a picosecond time scale. The latter process is monitored by pump-probe absorption spectroscopy at wavelengths between 400 and 883 nm. The measured transient absorbance signals reveal a dominant fast decay with a lifetime of a few picoseconds and, to a minor extent, a slow component with a lifetime of about 55 ps. Statistical modeling of MeFl-CO(2) decarboxylation employing molecular parameters calculated by density functional theory suggests that the fast component is associated with the decarboxylation of vibrationally hot radicals, whereas the 55 ps decay reflects the dissociation of thermally equilibrated MeFl-CO(2) at ambient temperature. The vast majority of MeFl-CO(2) radicals thus decarboxylate on a time scale about an order of magnitude faster than expected from the time constant of 55 ps reported by Falvey and Schuster for this reference reaction. This literature value turns out to refer to decarboxylation rate of MeFl-CO(2) at ambient temperature.