Penultimate Unit Effect Research Articles

Microstructure analysis of copolymers of substituted itaconimide and methyl methacrylate: experimental and computational investigation

We present the microstructure analysis of copolymers of N-phenylitaconimide (PI) and methyl methacrylate (MMA) using experiment and computational study. The microstructure of copolymers, synthesized via atom transfer radical polymerization (ATRP) method, is characterized using 13C-NMR spectroscopy and elemental analysis. The copolymerization reactions are studied using the density functional theory (B3LYP/6–31 + G(d) level) and the reactivity ratios for the monomers are estimated from the rate constants of propagation (kp). Excellent agreement between the calculated and experimentally determined triad fractions is obtained when the polymeric species are modeled as trimers with the penultimate unit effect.

Role of N-substituents of maleimides on penultimate unit effect for sequence control during radical copolymerization

Radical copolymerization of N-substituted maleimides (RMIs) and olefins provides AAB sequence-controlled copolymers by penultimate unit (PU) control. In this study, we investigated the steric, resonance, and polar effects of N-substituents on sequence control during copolymerization of RMIs as the M2 monomer with diisobutene (DIB) and d-limonene (Lim) as the M1 monomer in chloroform at 60 °C. The monomer reactivity ratios (i.e., r2 (= k22/k21), r12 (= k122/k121), and r22 (= k222/k221)) were determined based on the terminal and PU models using a nonlinear least-squares method. For the copolymerization of RMIs with DIB, the introduction of a bulky N-alkyl group suppressed the PU effect and led to the formation of alternating copolymers. The copolymerization of N-phenylmaleimides with o- and p-substituents was also investigated to reveal the steric, resonance, and polar effects of the substituents. In conclusion, less bulky and more electron-donating substituents effectively induced the PU effect during the radical copolymerization of RMIs and olefins. Radical copolymerization of N-substituted maleimides and olefins provides AAB sequence-controlled copolymers by penultimate unit control. In this study, we investigated the steric, resonance, and polar effects of N-substituents on sequence control during copolymerization of various N-substituted maleimides with diisobutene and limonene in chloroform at 60 °C, and concluded that less bulky and more electron-donating substituents effectively induced the penultimate unit effect. We also demonstrated that 2:1 sequence-controlled maleimide copolymers was efficiently produced during radical copolymerization of limonene and N-phenylmaleimides with an electron-donating substituent.

Modeling of the Penultimate Unit Effect in Chain-Growth Copolymerizations

The present work addresses the modeling and simulation of the addition of copolymerizations of styrene and methyl methacrylate in batch mode, and the formation of tailored vinyl acetate/acrylic acid copolymers is evaluated through stochastic optimization procedures based on the Monte Carlo method. A kinetic model of the free-radical reaction was proposed in order to predict the behavior of the reaction system taking into consideration the presence of the penultimate unit effect. The profiles of conversion and copolymer composition were also evaluated considering the effect of the medium viscosity (kinetic phenomena related to gel and glass effects) on the reaction performance. It was shown that the proposed model for chain-growth copolymerization is able to describe strong nonlinear behaviors such as autoacceleration of the polymerization and drift of copolymer composition. It was also shown that copolymers with homogeneous composition can be successfully synthesized through manipulation of the monomer feed flow rate based on a stochastic optimization procedure.

Characterizations of radicals formed in radical polymerizations and transfer reactions by electron spin resonance spectroscopy

Abstract Electron spin resonance (ESR, aka electron paramagnetic resonance, EPR) investigations have been conducted on radicals formed during radical polymerizations and provide a detailed characterization of the active radical species. Active propagating radicals can be observed during actual radical polymerizations by ESR/EPR. The chain lengths of the observed radicals were estimated by a combination of atom transfer radical polymerization (ATRP) and ESR/EPR. The structures of the chain end radicals were determined by analysis of the ESR/EPR spectra. An increase in the dihedral angles between terminal p-orbital of radical and Cβ–H bonds was observed with increasing chain lengths of methacrylate polymers. Radical transfer reactions were observed during radical polymerization of acrylates. A combination of ATRP and ESR/EPR clarified a 1,5-hydrogen shift mechanism of the radical transfer reactions using model adamantyl acrylate radicals. Penultimate unit effects were also observed. Time-resolved ESR/EPR (TR ESR) spectroscopy clarified the initiation processes of an alternating copolymerization of styrene with maleic anhydride and the copolymerization of styrene with 1,3-butadiene. Several unsolved problems in conventional radical polymerization processes have been clarified using combinations of ATRP with ESR/EPR and TR ESR. Characterization of the radicals in radical polymerizations using various ESR techniques would definitely provide interesting and useful information on conventional radical polymerizations.

Ring‐Opening Metathesis Copolymerization of Cyclopentene Above and Below Its Equilibrium Monomer Concentration

AbstractCyclopentene (C) and norbornene (N) may both be enchained by ring‐opening metathesis polymerization (ROMP). N homopolymerizes to essential completion, but C has a significant equilibrium monomer concentration, [C]eq = 1.17 m at 30 °C. While C cannot homopolymerize below [C]eq, it can still be enchained in copolymerizations with N. A terminal kinetic model, where one monomer shows depropagation competitive with propagation, well represents experimental data for the “living” ROMP copolymerization of C and N with a Mo‐based “Schrock‐type” initiator, covering a broad range of monomer feed ratios, both above and below [C]eq. Incorporating a penultimate unit effect on C depropagation increases the model's accuracy when both the monomers are dilute. Below [C]eq, polymerization halts when all N is consumed; above [C]eq, a C homopolymer block forms once all N is consumed. As polycyclopentene may be hydrogenated to linear polyethylene, such copolymerizations offer a facile route to polyethylene‐containing block copolymers from a single charge of mixed‐monomer feed.

New Vistas on the Anionic Polymerization of Styrene in Non-Polar Solvents by Means of Density Functional Theory.

The elementary processes of anionic styrene polymerization in the gas phase and in cyclohexane were studied using M062X (a recently developed density functional theory (DFT) method) combined with the 6-31+G(d) basis sets, in order to clarify the complicated phenomena caused by the association of the active chain-ends and elucidate the details of the polymerization mechanism. Three types of HSt2Li (a model structure of polystyryllithium chain-ends) were obtained; the well-known first structure in which Li is coordinated to the side chain, the second structure in which Li is coordinated to the phenyl ring, (both without the penultimate unit coordination), and the third structure in which Li is coordinated to both the chain-end unit and the penultimate styrene unit. Although the third HSt2Li is the most stable as expected, the free energy for the transition state of its reaction with styrene is higher than those for the other two transition states due to its steric hindrance. The free energy for the transition state of the reaction of the second HSt2Li with styrene is the lowest, suggesting that the route through it is the predominant reaction path. The penultimate unit effect, slower addition of styrene to HSt2Li than to HStLi, is attributed to coordination of the penultimate styrene units of the polystyryllithium dimer (one of the starting materials) to its Li atoms. The calculated enthalpy for the reaction barrier of the second HSt2Li with styrene in cyclohexane was found to agree with the observed apparent activation energy in benzene.

A Combined Computational and Experimental Study of Copolymerization Propagation Kinetics for 1‐Ethylcyclopentyl methacrylate and Methyl methacrylate

The crosspropagation of 1‐ethylcyclopentyl methacrylate (ECPMA) and methyl methacrylate (MMA) has been studied using a combination of quantum chemistry calculations and experiment. Our computational work utilizes a trimer‐to‐tetramer reaction model, coupled with an ONIOM (B3LYP/6‐31G(2df,p): B3LYP/6‐31G(d)) method for geometry optimization and an M06‐2X/6‐311+G(2df,p) method plus SMD solvation model for single point energy calculations. The results show several trends: the identity of the ultimate unit of a trimer radical affects not only the preferred conformation of the region where the reaction takes place, but also the reactivity of the radical; the addition of an ECPMA monomer to the radicals is generally favored compared to an MMA monomer; the pen‐penultimate unit of a trimer radical shows a nonnegligible entropic effect; the penultimate unit effect is implicit for the ECPMA–MMA copolymer system. Finally, terminal model reactivity ratios fitted based on the explicit rate coefficients calculated from the quantum chemical results are compared with those from experimental measurements. The computations not only agree qualitatively with experimentally derived results in terms of the selectivity of ECPMA–MMA crosspropagation, but also give reasonable quantitative predictions of reactivity ratios. image

Penultimate Unit and Solvent Effects on 2:1 Sequence Control During Radical Copolymerization of N‐Phenylmaleimide With β‐Pinene

AbstractCopolymers with a 2:1 regulated sequence of N‐phenylmaleimide with β‐pinene units (AAB‐type repeating units) are produced during radical copolymerization. The monomer reactivity ratios are determined using the penultimate unit model, and it is demonstrated that they vary depending on the solvent used (tetrahydrofuran, 1,2‐dichloroethane, 2,2,2‐trifluoroethanol, and perfluoro‐tert‐butanol) and the copolymerization temperature (0–100 °C). The interaction of the PhMI repeating units with solvents lead to the production of the 2:1 controlled sequence and a significant change in the relative reactivity of the PhMI radicals by polar and steric effects.

A Combined Computational and Experimental Study on the Free‐Radical Copolymerization of Styrene and Hydroxyethyl Acrylate

AbstractBulk free‐radical copolymerization of styrene and 2‐hydroxyethyl acrylate (HEA) is investigated experimentally at 50 °C using pulsed‐laser polymerization and computationally using ab initio simulations. Arrhenius parameters for HEA chain‐end homopropagation are A = 1.72 × 107 L mol−1 s−1 and Ea = 16.8 kJ mol−1, based on experiments between 20 and 60 °C. Copolymer composition data are well fitted by the terminal model with reactivity ratios rST = 0.44 ± 0.03 and rHEA = 0.18 ± 0.04, but the variation in the propagation rate coefficient with monomer composition is underpredicted. Results are compared with computational predictions assuming the terminal as well as the penultimate unit effect (PUE) model. Intramolecular H‐bonding is shown to have a significant influence on PUE calculations. Discrepancies between computational predictions and experiment are attributed to the influence of intermolecular H‐bonding.

Free Radical Copolymerization Kinetics of γ-Methyl-α-methylene-γ-butyrolactone (MeMBL)

The propagation kinetics and copolymerization behavior of the biorenewable monomer γ-methyl-α-methylene-γ-butyrolactone (MeMBL) are studied using the pulsed laser polymerization (PLP)/size exclusion chromatography (SEC) technique. The propagation rate coefficient for MeMBL is 15% higher than that of its structural analogue, methyl methacrylate (MMA), with a similar activation energy of 21.8 kJ·mol(-1). When compared to MMA, MeMBL is preferentially incorporated into copolymers when reacted with styrene (ST), MMA, and n-butyl acrylate (BA); the monomer reactivity ratios fit from bulk MeMBL/ST, MeMBL/MMA, and MeMBL/BA copolymerizations are r(MeMBL) = 0.80 ± 0.04 and r(ST) = 0.34 ± 0.04, r(MeMBL) = 3.0 ± 0.3 and r(MMA) = 0.33 ± 0.01, and r(MeMBL) = 7.0 ± 2.0 and r(BA) = 0.16 ± 0.03, respectively. In all cases, no significant variation with temperature was found between 50 and 90 °C. The implicit penultimate unit effect (IPUE) model was found to adequately fit the composition-averaged copolymerization propagation rate coefficient, k(p,cop), for the three systems.

Toward block copolymers from nonliving isospecific single-site catalytic systems

Ethene/1-olefin blocky copolymers were obtained through nonliving insertion copolymerizations promoted by an isospecific single site catalyst. Propene or 4-methyl-1-pentene were copolymerized with ethene with metallocenes endowed with different stereospecificity in propene polymerization: (i) aspecific “constrained geometry” half-sandwich complex, {η1:η5-([tert-butyl-amido)dimethylsilyl](2,3,4,5-tetramethyl-1-cyclopentadienyl)}titanium dichloride [Me2Si(Me4Cp)(N-tBu)TiCl2] (CG), (ii) moderately isospecific rac-ethylenebis(indenyl)zirconium dichloride [rac-(EBI)ZrCl2] (EBI), (iii) slightly more isospecific hydrogenated homologue, rac-ethylenebis(tetrahydroindenyl)zirconium dichloride [rac-(EBTHI)ZrCl2] (EBTHI), (iv) highly iso-specific rac-[methylenebis(3-tert-butyl-1-indenyl)]zirconium dichloride [rac-H2C-(3-tBuInd)2ZrCl2] (TBI), (v) most isospecific rac-[isopropylidene-bis(3-tert-butyl-cyclopentadienyl)]zirconium dichloride [rac-Me2C-(3-tBuCp)2ZrCl2] (TBC). Copolymerizations were described by a 2nd order Markovian copolymerization model and data are proposed to correlate the formation of 1-olefin sequences with catalytic site isospecificity, made by the cooperation of organometallic complex and growing chain. Blocky copolymers were prepared over wide ranges of compositions: with any of the isospecific metallocenes when 4-methyl-1-pentene was the 1-olefin and only with the highly isospecific ones (TBI, TBC) when propene was the comonomer. A penultimate unit effect was observed with TBI as the metallocene, whereas a 1st order Markov model described the ethene/propene copolymerization from TBC. A moderately isospecific metallocene, such as EBI, is shown to be able to prepare blocky ethene copolymers with 4-methyl-1-pentene. These results pave the way for the synthesis of new ethene based materials. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2063–2075, 2010

Investigation of Free-Radical Copolymerization Propagation Kinetics of Vinyl Acetate and Methyl Methacrylate

The free-radical copolymerization propagation kinetics of vinyl acetate (VAc) and methyl methacrylate (MMA) at 50 degrees C were investigated through an experimental study combined with a computational analysis based on quantum chemistry. Copolymer composition data, obtained using pulsed laser polymerization followed by size exclusion chromatography (PLP-SEC) and proton nuclear magnetic resonance (NMR), were well represented by the terminal model using monomer reactivity ratios obtained with the computational approach (r(VAc) = 0.001 and r(MMA) = 27.9). Concerning the composition-averaged copolymerization propagation rate coefficient k(p,cop), the differences between the terminal model and the implicit penultimate unit effect (IPUE) model (s(MMA) = 0.544 and s(VAc) = 0.173) are small for VAc/MMA, with the terminal model sufficient to describe the experimental k(p,cop) data measured by PLP-SEC. Monomer and radical charge distributions determined computationally are used to explain the reactivity exhibited by the VAc/MMA system.

An Investigation of Free Radical Copolymerization Kinetics of the Bio‐renewable Monomer γ‐Methyl‐α‐methylene‐γ‐butyrolactone with Methyl methacrylate and Styrene

AbstractThe free‐radical copolymerization propagation kinetics of the bio‐renewable monomer γ‐methyl‐α‐methylene‐γ‐butyrolactone (MeMBL) with styrene (ST) and with methyl methacrylate (MMA) have been investigated by the pulsed laser polymerization (PLP)—size exclusion chromatography (SEC) technique, in addition to DSC and proton NMR measurements. Monomer reactivity ratios for bulk MeMBL/ST and MeMBL/MMA copolymerizations are rMeMBL = 0.80 ± 0.04 and rST = 0.34 ± 0.04 and rMeMBL = 3.0 ± 0.3 and rMMA = 0.33 ± 0.01, respectively, with no significant variation with temperature found between 22 and 90 °C. The implicit penultimate unit effect (IPUE) model best represents the composition‐averaged copolymerization propagation rate coefficient, kp,cop for the MeMBL/ST system. Arrhenius parameters were estimated for MeMBL homopolymerization and found to be quite similar to those of MMA. DSC results show that copolymers of well controlled composition can be produced to suit specific Tg requirements over a much higher temperature range than possible with current acrylic glasses.magnified image

An Investigation of Free-Radical Copolymerization Propagation Kinetics of Styrene and 2-Hydroxyethyl Methacrylate

Free-radical copolymerization propagation kinetics of styrene (ST) and 2- hydroxyethyl methacrylate (HEMA) have been investigated using pulsed laser polymerization (PLP) combined with size exclusion chromatography (SEC) and proton NMR. Monomer reactivity ratios for bulk ST/HEMA copolymerization are rHEMA = 0.49 and rST = 0.27, with no significant variation with temperature found between 50 and 120 °C. The composition-averaged copolymerization propagation rate coefficient, kp,cop, is well represented by the implicit penultimate unit effect (IPUE) model. The copolymerization kinetics of HEMA with ST is quite similar to that of glycidyl methacrylate (GMA) with ST. A computational study based on quantum chemistry supports the finding that GMA and HEMA are more reactive toward ST radicals compared to alkyl methacrylates.

Comprehensive Modeling Study of Nitroxide-Mediated Controlled/Living Radical Copolymerization of Methyl Methacrylate with a Small Amount of Styrene

This article presents a comprehensive kinetic study of the SG1 nitroxide-mediated copolymerization of methyl methacrylate with a small percentage of styrene using the PREDICI software. The aim of this study was to confirm the results from a previous publication showing that a living polymerization can be achieved for this system. The PREDICI simulations based on the penultimate unit effect model were also able to give a better insight into the complex mechanism of nitroxide-mediated controlled radical copolymerization. The model showed the copolymerization kinetics and the evolution of the number average molar mass, the fraction of living and dead chains, and the concentration of the four types of alkoxyamines and propagating radicals with monomer conversion. It was applied for different initial percentages of styrene and different initiator concentrations.

Tacticity control by conformational isomerization in free radical polymerization of acrylate

Contribution of conformational isomerization to polymer tacticity has been studied in free radical polymerization of (−)-menthyl 2-acetamidoacrylate with azo initiators. It has been demonstrated that the chain end of the growing polymer radical, which is generated from the s-trans and s-cis conformers of monomer, produces stereoselectively new R and S configurational quaternary carbons, respectively, for the attack of monomer. In addition, polymerization reactivity of both conformers is indistinguishable under the present conditions, and the polymerization is considered to proceed through a chain-end controlled mechanism, which excludes a penultimate unit effect on tacticity in the polymerization. The results obtained would give a clue to understand an origin of tacticity in conventional free radical polymerization of acrylates.

Penultimate-Unit Effect in Ethene/4-Methyl-1-pentene Copolymerization for a “Sequential” Distribution of Comonomers

This paper presents a novel microstructure for an ethene/1-olefin copolymer. Ethene/4-methyl-1-pentene copolymers were found to be essentially made of sequences of ethene alternating with sequences of 4-methyl-1-pentene, with a minor amount of isolated ethene units distributed in the 1-olefin sequence. This microstructure is produced by highly isospecific catalytic sites and can be explained on the basis of a penultimate unit effect in copolymerization. Two isospecific organometallic complexes were used to prepare said copolymer: a sterically hindered, highly regio- and stereospecific metallocene, rac-[methylenebis(3-tert-butyl-1-indenyl)]zirconium dichloride [rac-H2C-(3-tBuInd)2ZrCl2] (TBI), and the prototypical moderately isospecific hydrogenated metallocene without any substituent on the indenyl ligand, rac-ethylenebis(tetrahydroindenyl)zirconium dichloride [rac-(EBTHI)ZrCl2] (EBTHI). As a comparison, copolymers were also prepared with the so-called “constrained geometry” half-sandwich complex, {η1:η5- [(tert-butyl-amido)dimethylsilyl)(2,3,4,5-tetramethyl-1-cyclopentadienyl)}titanium dichloride [Me2Si(Me4Cp)(N-tBu)TiCl2] (CG). Comonomer sequences at the triad level were determined through 13C NMR analysis, and reactivity ratios of copolymerizations were elaborated through a statistical method, investigating the effect of ultimate and penultimate inserted comonomer units. Values of r1r2 products larger than 1 were calculated for copolymerizations from both the isospecific metallocenes, with relatively high values of both r1 and r2. The investigation of the penultimate-unit effect brought to calculate high values of both r11 and r22 and unprecedented contemporary low values of r12 and, surprisingly, r21 to indicate the occurring of comonomer sequences and the unusual absence of alternate comonomers. In particular, the low r21 value indicates that the 4-methyl-1-pentene unit in the penultimate position causes a remarkable decrease of the ethene reactivity, with the insertion of a minor amount of isolated units in 4-methyl-1-pentene sequences. A product of reactivity ratios r1r2 close to 1 was obtained with CG as the catalyst precursor, not only confirming the tendency of this organometallic complex to promote almost random ethene/1-olefin copolymerizations, already observed with propene as the 1-olefin, but also suggesting that the isospecificity of the catalytic site plays a key role in affording the novel copolymer microstructure. With 4-methyl-1-pentene as the comonomer, for the first time long 1-olefin sequences are observed in an ethene copolymer obtained with a catalytic system poorly isospecific in propene homopolymerization, such as EBTHI. This seems to suggest that the branched comonomer 4-methyl-1-pentene contributes to the formation of a highly isospecific site, whose stereospecificity is once more confirmed to arise from the cooperation of the catalyst and of the growing chain. These findings seem also to indicate that when a catalytic site becomes highly isospecific, it is likely to form sequences of both comonomers in an ethene/1-olefin copolymer.

Ab Initio Study of the Penultimate Effect for the ATRP Activation Step Using Propylene, Methyl Acrylate, and Methyl Methacrylate Monomers

High-level ab initio molecular orbital calculations are used to study the magnitude and origin of the penultimate unit effect in atom transfer radical polymerization (ATRP) of dimers involving the comonomers methyl acrylate (MA), methyl methacrylate (MMA), and propylene (P). The penultimate unit effects depend on the nature of the terminal unit and the halogen and can be significant, with the MMA unit in particular altering the equilibrium constant for the bond dissociation equilibrium by as much as 2 orders of magnitude. Specifically, the ratios of the equilibrium constants (K) for the bond dissociation reactions of H−M2−M1−Cl at 298 K, relative to the equilibrium constant (K0) of the corresponding unimer H−M1−Cl, for penultimate units M2 = P, MA, and MMA are respectively 0.81, 1.27, and 92.46 for M1 = P; 8.73, 2.64, and 32.69 for M1 = MA; and 5.73, 0.78, and 1.57 for M1 = MMA. For the bromides, H−M2−M1−Br, the corresponding ratios K/K0 for M2 = P, MA, and MMA are respectively 0.55, 0.70, and 54.79 (M1 = P); 5.44, 0.63, and 2.38 (M1 = MA); and 0.24, 12.18, and 43.77 (M1 = MMA). It is shown that the penultimate unit effects arise in both the entropy and enthalpy of the equilibrium and are the result of a complex interplay of stereoelectronic effects which, for the ester linkages, are heavily influenced by intramolecular hydrogen bonding. The penultimate unit effects have important implications for initiator design; for example, they can account for the experimental observation that the isobutyrate halide is an inefficient initiator for MMA polymerization. The results also imply that penultimate unit effects need to be taken into account in the synthesis of block, gradient, and random copolymers.

Propagation Kinetics of Binary Acrylate–Methacrylate Free‐Radical Bulk Copolymerizations

AbstractRate coefficients of propagation, kp,copo, for MA–MMA, MA–DMA, DA–MMA, and DA–DMA bulk copolymerizations have been determined via PLP–SEC. The kp,copo values are analyzed in terms of the terminal and penultimate unit effect models. Alkyl acrylate and alkyl methacrylate reactivity ratios, as obtained from copolymer composition, are not significantly dependent on the size of the alkyl ester moiety, whereas kp,copo clearly depends on the size of the alkyl group. For MA–DMA, an almost perfect simultaneous fit of copolymer composition and kp,copo is obtained via the terminal model. For DA–MMA, even the penultimate unit effect model affords no adequate simultaneous fit. The observed effects are assigned to hindrance of internal rotational motion in the transition state due to intermolecular and intramolecular mobility terms.magnified image

Further Effects of Chain-Length-Dependent Reactivities on Radical Polymerization Kinetics

In the present paper, we finalize some threads in our investigations into the effects of chain-length-dependent propagation (CLDP) on radical polymerization kinetics, confirming all our previous conclusions. Additionally, and more significantly, we uncover some unexpected and striking effects of chain-length-dependent chain transfer (CLDTr). It is found that the observed overall rate coefficients for propagation and termination (and therefore the rate of polymerization) are not significantly affected by whether or not chain transfer is chain-length dependent. However, this situation is different when considering the molecular weight distributions of the resulting polymers. In the case of chain-length-independent chain transfer, CLDP results in a considerable narrowing of the distribution at the low molecular weight side of the distribution in a chain-transfer controlled system. However, the inclusion of both CLDP and CLDTr yields identical results to classical kinetics – in these latter two cases, the molecular weight distribution is governed by the same chain-length-independent chain transfer constant, whereas in the case of CLDP only, it is governed by a chain-length-dependent chain transfer constant that decreases with decreasing chain length, thus enhancing the probability of propagation for short radicals. Furthermore, it is shown that the inclusion of a very slow first addition step has tremendous effects on the observed kinetics, increasing the primary radical concentration and thereby the overall termination rate coefficient dramatically. However, including possible penultimate unit effects does not significantly affect the overall picture and can be ignored for the time being. Lastly, we explore the prospects of using molecular weight distributions to probe the phenomena of CLDP and CLDTr. Again, some interesting insights follow.