Browsing by Author "Buback, M."

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-CO2) 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-CO2 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-CO2 at ambient temperature. The vast majority of MeFl-CO2 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-CO2 at ambient temperature.

Detailed kinetic studies into free-radical polymerization via pulsed laser experiments ideally require photoinitiators which almost instantaneously dissociate into primary free-radical fragments that rapidly add to monomer molecules and thus induce macromolecular growth. 2-Methyl-4'-(methylthio)-2-morpholinopropiophenone (MMMP) is shown to be such a suitable photoinitiator. Measurement of monomer conversion induced by a single laser pulse, within the so-called single-pulse pulsed laser polymerization (SP-PLP) experiment, provides direct information about the chain-length dependence of the termination rate coefficient if MMMP is used as the photoinitiator.

Pulsed laser polymerization (PLP) in conjunction with analysis of the resulting polymer by size-exclusion-chromatography (SEC) was used to measure propagation rate coefficients, kp, of acrylic acid (AA) in aqueous solution at concentrations of 20 and 40 wt % and temperatures between 2.6 and 28.5 degreesC. Under these conditions, more than 99% of acrylic acid exists in the nonionized form. The molecular weight distribution MWD) of poly(AA) is directly measured by aqueous SEC. The kp values at 20 wt 7( are about 60% higher than at 40 wt %. The activation energy of hp is close to 12.0 kJ . mol(-1) at both concentrations. At 25 degreesC and 20 wt %, k(p) of AA in aqueous solution exceeds 100 000 L . mol(-1).s(-1) which is the highest kp value determined by the PLP-SEC technique so far.

The chain-length dependence of the termination rate coefficient, k(t), in butyl acrylate free-radical polymerization has been determined by two independent methods, RAFT-SP-PLP and RAFT-CLD-T, both employing control of radical chain length by reversible addition fragmentation chain transfer (RAFT) polymerization. Within RAFT-SP-PLP, the polymerization induced by a laser single pulse is monitored via near-IR spectroscopy with a time resolution of microseconds. In RAFT-CLD-T, isothermal reaction rate measurements are carried out via DSC under stationary polymerization conditions. The resulting k(t) data refer to the situation of living/controlled radical polymerization, where both radical chain length and monomer conversion increase during the course of the reaction. The RAFT-SP-PLP measurements were carried out at 60 degrees C and two pressures, 5 and 1000 bar. The RAFT-CLD-T experiments were run at ambient pressure and at two temperatures, 60 and 80 degrees C, respectively. In absolute value, the termination rate coefficients for identical pressure and temperature deduced from the two methods differ by less than a factor of 2. For the dependence of k(t) on chain length, i, almost identical information is provided by the two techniques. The chain-length dependence of kt may be described by the power-law expression k(t)(i) = k(t)(1,1)i(-alpha) with, however, a being different for short-chain and long-chain radicals. RAFT-SP-PLP yields alpha(1) = 1.25 for the short-chain regime from 1 < i < 30, and alpha(2) = 0.22 for chain lengths above i = 50. RAFT-CLD-T results in alpha(1) = 1.04 and alpha(2) = 0.20 in identical chain length regimes. kt(1,1) values are found to be close to 1 x 10(9) L mol(-1) s(-1).

Termination rate coefficients of free-radical polymerization are accessible from SP-PLP studies where monomer conversion induced by a laser single pulse is measured with a time resolution of microseconds. Previous experiments with 2,2-dimethoxy-2-phenylacetophenone (DMPA) acting as the initiator revealed that upon of the DMPA concentration, the resulting monomer conversion vs. time traces intersect. A detailed kinetic analysis of this unexpected type of behavior is presented. It turns out that such crossings occur in situations where k(t) is chain-length dependent and, at the same time, the primary initiator-derived free-radical species differ in reactivity toward the monomer. As is known from the literature, this difference in radical reactivity is particularly pronounced with DMPA, which photo-decomposes to a propagating and to a non-propagating free radical. Modeling of the crossing behavior opens a novel route for determining chain-length dependent k(t). Results for methyl acrylate (MA) and styrene homopolymerizations at low degrees of monomer conversion, to a maximum of 10%, are presented. The decrease of k(t) with chain length is modeled via an exponential function. The dependence is significantly larger for MA than for styrene. The exponents derived from simulation studies via PREDICI(R) are in excellent agreement with corresponding data reported by Olaj et al. for styrene and by de Kock for methyl acrylate.

Termination rate coefficients, k(t), of alkyl acrylate and alkyl methaerylate homopolymerizations at 40 degreesC and pressures of 1000 and 2000 bar have been measured up to high degrees of monomer conversion using the time-resolved single-pulse-pulsed-laser polymerization (SP-PLP) technique. The chain-length dependence (CLD) of k(t) has been deduced from SP-PLP data by adopting the power-law model, k(t) = k(t)(o)i(-alpha), where i is the chain length. For methacrylates at low degrees of monomer conversion, t alpha is close to the theoretically predicted value of 0.16. At conversions above 20% the exponent a increases significantly with increasing conversion. This effect becomes particularly pronounced in the gel effect region, where alpha, e.g. for MMA, reaches values close to unity. In the case of acrylates with small alkyl ester side chain, such as methyl acrylate, alpha is also close to 0.16 at low conversions and increases toward higher conversions. In the case of acrylates with larger alkyl ester side chain, such as dodecyl acrylate and 2-ethylhexyl acrylate, however, alpha is close to 0.4 even at low degrees of monomer conversion. The latter effect is strongly indicative of intramolecular chain transfer, which generates significant amounts of midchain radicals in the system. The fact that such transfer processes take place is supported by SP-PLP data on alkyl acrylates polymerized in mixtures with supercritical carbon dioxide.

The molar mass distribution (MMD) of synthetic polymers is frequently analyzed by size exclusion chromatography (SEC) coupled to multi angle light scattering (MALS) detection. For ultrahigh molar mass (UHM) or branched polymers this method is not sufficient, because shear degradation and abnormal elution effects falsify the calculated molar mass distribution and information on branching. High temperatures above 130 degrees C have to be applied for dissolution and separation of semi-crystalline materials like polyolefins which requires special hardware setups. Asymmetrical flow field-flow fractionation (AF4) offers the possibility to overcome some of the main problems of SEC due to the absence of an obstructing porous stationary phase. The SEC-separation mainly depends on the pore size distribution of the used column set. The analyte molecules can enter the pores of the stationary phase in dependence on their hydrodynamic volume. The archived separation is a result of the retention time of the analyte species inside SEC-column which depends on the accessibility of the pores, the residence time inside the pores and the diffusion ability of the analyte molecules. The elution order in SEC is typically from low to high hydrodynamic volume. On the contrary AF4 separates according to the diffusion coefficient of the analyte molecules as long as the chosen conditions support the normal FFF-separation mechanism. The separation takes place in an empty channel and is caused by a cross-flow field perpendicular to the solvent flow. The analyte molecules will arrange in different channel heights depending on the diffusion coefficients. The parabolic-shaped flow profile inside the channel leads to different elution velocities. The species with low hydrodynamic volume will elute first while the species with high hydrodynamic volume elute later. The AF4 can be performed at ambient or high temperature (AT-/HT-AF4). We have analyzed one low molar mass polyethylene sample and a number of narrow distributed polystyrene standards as reference materials with known structure by AT/HT-SEC and AT/HT-AF4. Low density polyethylenes as well as polypropylene and polybutadiene, containing high degrees of branching and high molar masses, have been analyzed with both methods. As in SEC the relationship between the radius of gyration (R(g)) or the molar mass and the elution volume is curved up towards high elution volumes, a correct calculation of the MMD and the molar mass average or branching ratio is not possible using the data from the SEC measurements. In contrast to SEC. AF4 allows the precise determination of the MMD, the molar mass averages as well as the degree of branching because the molar mass vs. elution volume curve and the conformation plot is not falsified in this technique. In addition, higher molar masses can be detected using HT-AF4 due to the absence of significant shear degradation in the channel. As a result the average molar masses obtained from AF4 are higher compared to SEC. The analysis time in AF4 is comparable to that of SEC but the adjustable cross-flow program allows the user to influence the separation efficiency which is not possible in SEC without a costly change of the whole column combination. (C) 2011 Elsevier B.V. All rights reserved.

Poly[ethylene-co-(butyl acrylate)-co-(carbon monoxide)] (polyEBC) samples, prepared from C-13-labeled carbon monoxide, were characterized using two dimensional (2D) pulsed field gradient (PFG) 750 MHz NMR spectroscopy. To elucidate the complex mixture of structures present in this terpolymer, 2D H-1/C-13 heteronuclear multiple bond correlation (HMBC) experiments were conducted by selectively exciting the enhanced peaks resulting from C-13-labeling. High resolution 2D NMR combined with C-13-labeling of the polymer greatly simplifies the 2D NMR spectra, selectively enhances the weak peaks from low occurrence C-centered triad structures and aids in their resonance assignments.

Cloud-point pressures of ethylene-(meth)acrylic acid ester copolymers in supercritical ethene have been measured to maximum pressures and temperatures of 3000 bar and 533 K, respectively. The copolymers under investigation were prepared in a continuously operated stirred tank reactor (CSTR), which ensures production of chemically homogeneous polymer. After isolation and purification of the copolymer, the phase behavior was mapped out in a discontinuously operated high-pressure cell. The influence of the type and of the content of (meth)acrylate units within the copolymer on cloud-point behavior in mixtures with ethene was systematically studied for ethylene-ethyl acrylate (poly(E-co-EA)), ethylene-propyl acrylate (poly(E-co-PA)), ethylene-methyl methacrylate (poly(E-co-MMA)), and ethylene-butyl methacrylate (poly(E-co-BMA)) copolymers mostly covering the entire comonomer composition range including the limiting homopolymer systems ethene-polyethylene and ethene-poly(meth)acrylate. The data is compared with previously measured cloud-point pressures for ethylene-methyl acrylate and ethylene-butyl acrylate copolymers in fluid ethene. Starting from the limiting ethene-polyethylene system, cloud-point pressures decrease upon increasing the content of polar comonomer segments. For poly(E-co-MA), poly(E-co-EA), poly(E-co-MMA), and poly(E-co-BMA) this tendency is reversed at higher (meth)acrylic acid ester contents where the cloud-point pressures increase again. No such minimum in cloud-point pressure as a function of comonomer content occurs with poly(E-co-PA) and poly(E-co-BA). The variation of cloud-point pressure with copolymer composition is assigned to effects resulting from (i) short-chain branches on the polymer backbone, (ii) intersegmental interactions of carbonyl groups being shielded to different extents by the various types of alkyl ester groups, and (iii) "entropy penalty" contributions associated with the introduction of the a-methyl groups in case of the methacrylates. The experimental cloud-point-pressure curves are satisfactorily modeled by the Perturbed-chain (PC)-statistical associating fluid theory (SAFT) equation of state. The entire set of pure-component parameters and the three binary interaction parameters are independent of temperature, pressure and polymer molecular weight. The parameters from PC-SAFT modeling allow for estimates of the cloud-point behavior of ethene-poly(E-co-(meth)acrylate) systems in wide ranges of pressure and temperature. (C) 2003 Elsevier B.V. All rights reserved.

Ethene-methacrylic acid (MAA) and etheneacrylic acid (AA) copolymers of narrow polydispersity and high chemical homogeneity have been synthesized at acid, unit copolymer contents up to 9 mol-% within a continuously operated stirred tank reactor at overall monomer conversions of about 2%. Cloud-point pressures (CPPs) of mixtures of 3 wt.-% copolymer in ethene (E) have been measured in an optical high-pressure cell at pressures and temperatures up to 3 000 bar and 260degreesC, respectively. The CPP weakly, increases with acid copolymer content up to about 3.5 mol-%. Toward higher Acid contents, the CPP is strongly enhanced, in particular at the tower edge of the experimental temperature range at around 200degreesC. This increase in CPP is more pronounced for the AA than for the MAA systems. The data suggest that hydrogen-bonding interactions are operative in the pressurized E/poly(E-co-(M)AA) mixtures at temperatures of 260degreesC and perhaps even above. E-AA and E-MAA copolymers with acid contents of about 5.6 mol-% have also been completely methyl-esterified to yield the associated methyl esters. The CPPs of the resulting E-methyl acrylate and E-methyl methacrylate copolymers in mixtures with E are significantly below the CPPs of the corresponding E/poly(E-co-(M)AA) systems.

Free-radical terpolymerizations of styrene, methyl methacrylate, and glycidyl methacrylate were carried out in a tubular reactor in the presence of 20 wt.% CO2 at temperatures between 120 and 180degreesC and pressures of 300 and 350 bar. The number average molecular weights, M-N, were mostly between 2000 and 3000 g(.)mol(-1) and polydispersity indices around 2. In part of the experiments molecular weights were controlled by n-dodecyl mercaptan serving as the chain-transfer agent. PREDICI modeling indicates that the targeted molecular weights of M(N)similar to2500 g(.)mol(-1) and polydispersities around 2 may also be reached by using an initiator cocktail, a mixture of two initiators with significantly different decomposition rate coefficients. The predictions are confirmed experimentally.

Pulsed-laser polymerization (PLP) in conjunction with the analysis of the molecular weight distribution (MWD) via size-exclusion chromatography (SEC) remains recommended by the IUPAC Working Party on Modeling of polymerisation kinetics and processes as the method of choice for the determination of propagation rate coefficients, k(p), in free-radical polymerization. k(p) data from PLP-SEC studies in several laboratories for ethyl methacrylate (EMA), butyl methacrylate (BMA) and dodecyl methacrylate (DMA) bulk free-radical polymerizations at low conversion and ambient pressure are collected. The data fulfill consistency criteria and the agreement among the data is remarkable. These values are therefore recommended as constituting benchmark data sets for each monomer. The results are best fit by the following Arrhenius relations: EMA: k(p) = 10(6.61) L.mol(-1).s(-1) exp(-23.4 kJ.mol(-1)/R.T) BMA: k(p) = 10(6.58) L.mol(-1).s(-1) exp(-22.9 kJ.mol(-1)/R.T) DMA: k(p) = 10(6.40) L.mol(-1).s(-1) exp(-21.0 kJ.mol(-1)/R.T) For the methacrylates under investigation k(p) increases with the size of the ester group. For example, in going from MMA to DMA, k(p) at 50 degrees C is enhanced by a factor of 1.5.

Propagation rate coefficients, k(p), which have been previously reported by several groups for free-radical bulk polymerizations of cyclohexyl methacrylate (CHMA), glycidyl methacrylate (GMA), benzyl methacrylate (BzMA), and isobomyl methacrylate (iBoMA) are critically evaluated. All data were determined by the combination of pulsed-laser polymerization (PLP) and subsequent polymer analysis by size-exclusion chromatography (SEC). This-so-called PLP-SEC technique has been recommended as the method of choice for the determination of k(p) by the IUPAC Working Party on Modeling of Polymerisation Kinetics and Processes. The present data fulfill consistency criteria and the agreement among the data from different laboratories is remarkable. The values for CHMA, GMA, and BzMA are therefore recommended as constituting benchmark data sets for each monomer. The data for iBoMA are also considered reliable, but since SEC calibration was established only by a single group, the data are not considered as a benchmark data set. All k(p) data for each monomer are best fitted by the following Arrhenius relations: CHMA: k(P) = 10(6.80) L . mol(-1) . s(-1) exp( -23.0 kJ.mol(-1) / (R.T), GMA: k(p) = 10(6.79) L . mol(-1) . s(-1) exp (-22.9 kJ.mol(-1)) / (R.T), BzMA: k(p) = 10(6.83) L . mol(-1) .s(-1) exp(-22.9 kJ.mol(-1)) / (R.T), iBoMA: k(p) =10(6.79)L . mol(-1) . s(-1) exp(-23.1 kJ.mol(-1)) / (R.T). Rather remarkably, for the methacrylates under investigation, the k(p) values are all very similar. Thus, all data can be fitted well by a single Arrhenius relation resulting in a pre-exponential factor of 4.24 x 10(6) L . mol(-1) . s(-1) and an activation energy of 21.9 kJ . mol(-1). All activation parameters refer to bulk polymerizations at ambient pressure and temperatures below 100degreesC. Joint confidence intervals are also provided, enabling values and uncertainties for k(p) to be estimated at any temperature.

Propagation rate coefficients, k(P), for free-radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed-laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP-SEC technique has been recommended as the method of choice for the determination of k(P) by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain-growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid-chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP-SEC determination of k(P) of chain-end radicals to low temperatures (<20 degreesC) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of -65 to 20 degreesC mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 2.21 x 10(7) L (.) mol(-1) (.) s(-1) and an activation energy of 17.9 kJ (.) mol(-1). It must be emphasized that these PLP-determined k(P) values are for monomer addition to a chain-end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers. [GRAPHICS] Arrhenius plot of propagation rate coefficients, k(P), for BA as measured by PLP-SEC.

This is the first publication of an IUPAC-sponsored Task Group on "Critically evaluated termination rate coefficients for free-radical polymerization." The paper summarizes the current situation with regard to the reliability of values of termination rate coefficients k(t). It begins by illustrating the stark reality that there is large and unacceptable scatter in literature values of k(t), and it is pointed out that some reasons for this are relatively easily, remedied. However, the major reason for this situation is the inherent complexity of the phenomenon of termination in free-radical polymerization. It is our impression that this complexity is only incompletely grasped by many workers in the field, and a consequence of this tendency to oversimplify is that misunderstanding of and disagreement about termination are rampant. Therefore this paper presents a full discussion of the intricacies of k(t): sections deal with diffusion control, conversion dependence, chain-length, dependence, steady state and non-steady state measurements, activation energies and activation volumes, combination and disproportionation, and theories. All the presented concepts are developed from first principles, and only rigorous, fully-documented experimental results and theoretical investigations are cited as evidence. For this reason it can be said that this paper summarizes all that we, as a cross-section of workers in the, field, agree on about termination in free-radical polymerization. Our discussion naturally leads to a series of recommendations regarding measurement of k(t) and reaching a more satisfactory understanding of this very important rate coefficient. Variation of termination. rate coefficient k(t) with inverse absolute temperature T-1 for bulk, polymerization of methyl methacrylate at ambient pressure.([6]) The plot contains all methacrylate at ambient pressure. tabulated values([6]) (including those categorized as "recalculated") except ones from polymerizations noted as involving. solvent or above-ambient pressures.

The knowledge of accurate rate coefficients for individual steps of free-radical polymerization (FRP) is of scientific interest and of application-oriented importance. For a wide variety of homopolymerizations and for many copolymerizations, reliable propagation rate coefficients, k(P), are accessible via the IUPAC-reconiniended method of PLP-SEC (pulsed laser polymerization-size-exclusion chromatography). For termination rate coefficients, k(t). the situation is less favorable. Even for very common monomers, no k(t) benchmark data sets are available. Moreover, instead of having one recommended technique for measuring k(t). there are a plethora of such methods. Seventeen of the most prominent approaches for measuring k(t) are here reviewed, including innovative ones that have emerged over the last decade. The methods have been subdivided into two categories: (i) 'Kinetic methods', in which analysis of the time dependence of concentrations is essential, and (ii) 'MWD methods', in which the analysis of the molecular weight distribution plays the dominant role. The methods are evaluated with respect to their potential for providing routine access to measuring k(t) as a function of monomer conversion and of free-radical chain length. Moreover, it has been considered whether expensive instrumentation or highly demanding analysis is required for a particular method and whether a method is applicable to many types of monomers. A table summarizes all these evaluations in a readily accessible form. The use of kinetic methods appears to be generally preferable over MWD-based methods. The largest potential is currently seen for methods in which polymerization is induced by a single laser pulse and where the subsequent time evolution of either monomer concentration or free-radical concentration is measured. (c) 2005 Elsevier Ltd. All rights reserved.

Self-initiated reversible addition fragmentation chain transfer (RAFT) polymerizations of styrene at temperatures of 120, 150, and 180 degrees C, using cumyl dithiobenzoate (CDB) at concentrations between 5.0 x 10(-3) and 2.0 x 10(-2) mol L-1 as the RAFT agent were performed at 1000 bar. The increase of average molecular weight with monomer conversion, the shape of the molecular weight distributions, and polydispersity indices below 1.5 at monomer conversions up to about 50% indicate control of styrene bulk polymerization even at the high experimental temperatures. Neither a substantial decomposition of the RAFT agent nor a change in the overall polymerization process, e.g., by ionic side reactions, is observed. Polymerization rates are lower than in conventional styrene polymerization. The rate retardation effect increases with CDB concentration but is clearly reduced toward higher temperature. The lower retardation effect at high temperatures is assigned to a lower equilibrium concentration of the intermediate RAFT radical. The experimental rate data can be consistently described in terms of the concept of irreversible termination of the intermediate RAFT radical. On the other hand, the data are qualitatively and semiquantitatively inconsistent with the idea of slow fragmentation of intermediate radicals. The analysis of the kinetic data results in a reaction enthalpy of about 50 M mol(-1) for the beta-scission reaction of the intermediate RAFT radical.

Rate coefficients of beta-scission reactions in tertiary alkoxy radicals, R(CH3)(2)CO (R = methyl, ethyl, tert-butyl and neo-pentyl) have been estimated via density functional theory (DFT) calculations in conjunction with statistical unimolecular rate theory. For tert-butoxy, results obtained by employing different basis sets are compared with experimental values, indicating that UB3LYP/6-31G(d,p) excellently predicts kinetic data. Rate coefficients for inter- and intramolecular hydrogen abstraction are also reported. Depending on R, the P-scission rate may vary by orders of magnitude. The predicted temperature dependence of the alcohol-to-ketone product ratios for alkoxy radical decomposition in a hydrocarbon environment is in remarkably close agreement with the corresponding ratios measured on the product mixtures from decomposition of tert-butyl and tert-amyl peroxyacetates in solution of n-heptane.

The termination 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 50degreesC and up to 2000 bar. The ratio of termination to propogation rate coefficients, (k(t)/k(p))(copo), is measured via the single pulse-pulsed laser polymerization (SP-PLP) technique. Chain-length averaged kt,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 cenversion extended ranges of almost constant kt are observed where termination is controlled by segmental diffusion with important contributions of steric effects. These plateau k(t) values are significantly - by almose two orders of magnitude - different for MA and DA. The increase with MA content of k(t,copo) is adequately described buy a penultimate unit model which uses the geometric mean approximation for estimating rate coefficients of cross-termination betwen 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.