Rate Of Scission Research Articles

Experimental study of polystyrene thermolysis to moderate conversion

A general model for polymer degradation by concurrent random and chain-end processes was developed using continuous distribution kinetics. Population balance equations based on fundamental, mechanistic free radical reactions were solved analytically by the moment method. The model, applicable to any molecular weight distribution (MWD), reduces to the cases of independent random or chain-end scission. Polystyrene degradation experiments in mineral oil solution at 275–350°C supported the model and determined reaction rate parameters. The degradation proceeded to moderate extents requiring a MW-dependent random scission rate coefficient. Throughout the experiments, the polymer MWD, which was measured by GPC, maintained the exponential form, so that the logarithm of MWD varies linearly with MW, with slope directly related to the number-average MW. Polystyrene random scission activation energy was 7.0 kcal/mol, which agrees well with other thermolysis investigations but is lower than that found by pyrolysis due to fundamental differences between the processes. Magnesium oxide, added as a heterogeneous catalyst in solution, was found to have no effect on polystyrene degradation rate.

Free-radical-induced chain breakage and depolymerization of poly(methacrylic acid): equilibrium polymerization in aqueous solution at room temperature.

Hydroxyl radicals, generated by ionizing radiation in N2O saturated aqueous solutions, abstract H atoms from poly(methacrylic acid) at the methyl and methylene groups, and radicals 1 and 2 are formed, respectively. The reactions of the poly(methacrylic acid) radicals were investigated by pulse radiolysis (using optical and conductometric detection), EPR, product analysis, and kinetic simulations. The conductometric detection allowed us to measure the rate of chain scission and monomer release. Under conditions in which the polymer is largely deprotonated, the primary radical 1 abstracts a hydrogen (k= 3.5 x 10(2)s(-1)) from the methylene group, and this yields the more stable secondary radical 2. This radical undergoes chain scission by beta-fragmentation (k= 1.8 s(-1)), and the terminal (end-of-chain) radical 3 is formed. The polymer radicals terminate only slowly (2k= 80 dm3mol(-1)s(-1)). This allows effective depolymerization (depropagation) to take place (k=0.1 s(-1)). The yield of monomer release is higher than the original radical yield by up to two orders of magnitude. Once monomer is formed, it reacts with 3 (propagation, k= 15 dm3mol(-1)s(-1)), and a situation close to an equilibrium radical polymerization is approached. From these data, the equilibrium monomer concentration is calculated at 6.7 x 10(-3) mol dm(-3) at room temperature. The standard entropy of propagation is estimated at -185 to -150 J mol(-1)K(-1). Because the monomer reaches concentrations in the millimolar range, the *OH radicals increasingly react with monomers (results in oligomerization) rather than with the polymer. This effect is reflected by, for example, a lowering of chain-scission yields upon prolonged irradiation. In acid solutions, the decay of the polymer radicals becomes much faster (estimated at about 10(7)dm3mol(-1)s(-1) at pH3.5), and monomer release is no longer observed.

Bond Scission in a Perfect Polyethylene Chain and the Consequences for the Ultimate Strength

We present ab initio calculations concerning the bond scission rate in a polyethylene chain. The energy barrier for scission is calculated both by using an effective potential scheme based on ab initio data and from the ab initio transition state itself. In the latter case the prefactor for scission is calculated. In both methods the relaxation of the polymer chain is taken into account, leading to a strong strain dependence of the energy barrier. The two schemes agree quantitatively. The barrier is reduced from 3.9 eV at zero strain to 1.7 eV at a strain of 5%. The calculated scission rate is used to estimate the strength of a polyethylene fiber consisting of perfectly aligned, independent chains in a constant strain rate experiment resulting in 18 GPa at a strain of 8%. This value of the ultimate strength is a factor 2 larger than found in experiments. This contrasts previously reported quantum chemical calculations, which reported values a factor 5−10 larger than the experimental values.

Simulation of the random scission of C–C bonds in the initial stage of the thermal degradation of polyethylene

We performed molecular dynamics simulations to analyze the initial stage of the thermal degradation of polyethylene, which is dominated by the random scission reaction. The simulations were initiated from structures that were taken from previously equilibrated snapshots of the amorphous polymer and of a free-standing thin film. Isolated chains were also used for comparison. Our systems were coupled to a thermal heat bath, and the effect of different coupling constants was studied. Rate of random scission increases as the strength of the temperature coupling increases. Rates of reaction are almost similar in thin films and the bulk, whereas the rates are much faster in isolated chains. Expansion of the free-standing thin film accompanies degradation, producing fragments of various sizes. Chains of higher molecular weights than the initial chains can be produced due to recombination of fragments during the expansion of thin films. The polydispersity index of the resulting fragments is higher in thin films compared to the bulk. The bonds at the low density portion of the thin films have a higher probability of being broken.

Substitutions of Asn-726 in the Active Site of Yeast DNA Topoisomerase I Define Novel Mechanisms of Stabilizing the Covalent Enzyme-DNA Intermediate

Eukaryotic DNA topoisomerase I (Top1p) catalyzes changes in DNA topology and is the cellular target of camptothecin. Recent reports of enzyme structure highlight the importance of conserved amino acids N-terminal to the active site tyrosine and the involvement of Asn-726 in mediating Top1p sensitivity to camptothecin. To investigate the contribution of this residue to enzyme catalysis, we evaluated the effect of substituting His, Asp, or Ser for Asn-726 on yeast Top1p. Top1N726S and Top1N726D mutant proteins were resistant to camptothecin, although the Ser mutant was distinguished by a lack of detectable changes in activity. Thus, a basic residue immediately N-terminal to the active site tyrosine is required for camptothecin cytotoxicity. However, replacing Asn-726 with Asp or His interfered with distinct aspects of the catalytic cycle, resulting in cell lethality. In contrast to camptothecin, which inhibits enzyme-catalyzed religation of DNA, the His substituent enhanced the rate of DNA scission, whereas the Asp mutation diminished the enzyme binding of DNA. Yet, these effects on enzyme catalysis were not mutually exclusive as the His mutant was hypersensitive to camptothecin. These results suggest distinct mechanisms of poisoning DNA topoisomerase I may be explored in the development of antitumor agents capable of targeting different aspects of the Top1p catalytic cycle.

High-energy mechanical milling of poly(methyl methacrylate), polyisoprene and poly(ethylene-alt-propylene)

High-energy mechanical milling has been performed on poly(methyl methacrylate) (PMMA) at ambient and cryogenic temperatures, as well as on polyisoprene (PI) and poly(ethylene-alt-propylene) (PEP) at cryogenic conditions only. Milling conducted at ambient temperature has a substantially greater impact on the molecular characteristics of PMMA than milling at cryogenic temperatures. An increase in the milling time is accompanied by substantial reductions in PMMA molecular weight and, hence, glass transition temperature and impact strength under both sets of experimental conditions. An unexpected trend identified here is that the PMMA molecular weight distribution initially broadens and subsequently narrows with increasing milling time. Solid-state mechanical milling promotes comparable decreases in molecular weight and glass transition temperature in PEP (at a slower rate relative to PMMA), but induces chemical crosslinking in PI, as confirmed by FTIR spectroscopy. Charlesby–Pinner analysis yields not only the degree of PI crosslinking, but also the relative crosslinking and scission rates of PI, during cryogenic milling.

Analysis of chain-scission and crosslinking rates in the photo-oxidation of polystyrene

A study of the chain-scission and crosslinking rates in polystyrene photodegraded in the laboratory with fluorescent tubes (UVA-340) was made using GPC molecular weight distributions. The analysis was based on the assumption that scission and crosslinking occur randomly and employed a Monte Carlo procedure to compute the changes in molecular weight distribution for chosen values of scission and crosslinking rates and compared the computed profiles with measurements made on the photodegraded samples. Results were obtained for three different exposures and at various depths within 3.2-mm-thick bars. The scission/crosslinking ratio, λ, was between 3 and 8 for all samples measured in this study. The lowest values of λ were found near the exposed surface and the highest near the bar center. Both scission and crosslinking rates were much lower in the interior, presumably the result of oxygen starvation. Some bars were exposed while loaded to 10 MNm−2 in uniaxial tension. The stress appeared to increase the reaction rates somewhat near the surface and to depress the rates in the interior correspondingly. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3015–3023, 2000

The β C–C bond scission in alkoxy radicals: thermal unimolecular decomposition of t-butoxy radicals

The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323–383 K. The low and the high pressure limiting rate constants as well as the broadening factor Fc have been extracted from a complete falloff analysis of the experimental results: k0=[He]×1.5×10−8 exp(−38.5 kJ mol−1/RT) cm3 s−1, k∞=1.0×1014 exp(−60.5 kJ mol−1/RT) s−1, and Fc=0.87−T/870 K. We anticipate an uncertainty for these rate constants of ±30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the β C–C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for β C–C bond scission rate constants of all alkoxy radicals of A=1014±0.3 s−1. Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k∞=1.0×1014 exp(−78.2 kJ mol−1/RT) and 1.0×1014 exp(−63.1 kJ mol−1/RT) s−1, respectively. For the reverse reactions, the addition of CH3 radicals to CH2O, CH3CHO, and (CH3)2CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol−1, respectively.

Rheology and Microstructural Changes during Enzymatic Degradation of a Guar−Borax Hydrogel

Hydrogels composed of borax cross-linked guar galactomannans are enzymatically degraded using endo-β-mannanase, an enzyme which cleaves the polymer chain backbone. Dynamic rheological measurements show the elastic (G‘) and viscous (G‘ ‘) moduli to be sensitive to gel structure and to reduce significantly during the enzymatic hydrolysis process. The reduction in rheological properties shows three distinct regimes: an initial large decrease, a slower reduction rate at intermediate times, and an accelerated reduction at longer degradation times. In contrast, the polymer chain molecular weight, obtained from gel permeation chromatography, reduces rapidly at short times and at a slower rate subsequently. We therefore find the kinetics of moduli reduction to be dictated by the relationship between gel structure and rheological properties, rather than purely the rates of chain scission. At short times, the large decrease in moduli is analogous to changes in molecular weight and can directly be attributed to chain scission. At long times, corresponding to when the product of polymer concentration and intrinsic viscosity, c[η], reaches a critical value (≤2.5), the chains are too short to overlap and the long range network breaks down rapidly, leading to accelerated moduli reduction. Additionally, a synergistic increase in the degradation rate is observed on using a combination of backbone-cleaving β-mannanase enzyme and a side-chain-cleaving α-galactosidase enzyme, as compared to using only β-mannanase. This can be attributed to an enhancement of mannanase activity due to removal of the sterically hindering galactose side chains. Finally, a comparison of gel and solution degradation reveals very similar behavior in molecular weight changes for both but contrasting trends in rheology.

In Vitro Evolution of Preferred Topoisomerase II DNA Cleavage Sites

Topoisomerase II is an essential enzyme that is the target for several clinically important anticancer drugs. Although this enzyme must create transient double-stranded breaks in the genetic material in order to carry out its indispensable DNA strand passage reaction, the factors that underlie its nucleotide cleavage specificity remain an enigma. Therefore, to address the critical issue of enzyme specificity, a modified systematic evolution of ligands by exponential enrichment (SELEX) protocol was employed to select/evolve DNA sequences that were preferentially cleaved by Drosophila melanogaster topoisomerase II. Levels of DNA scission rose substantially (from 3 to 20%) over 20 rounds of SELEX. In vitro selection/evolution converged on an alternating purine/pyrmidine sequence that was highly AT-rich (TATATATACATATATATA). The preference for this sequence was more pronounced for Drosophila topoisomerase II over other species and was increased in the presence of DNA cleavage-enhancing anticancer drugs. Enhanced cleavage appeared to be based on higher rates of DNA scission rather than increased binding affinity or decreased religation rates. The preferred sequence for topoisomerase II-mediated DNA cleavage is dramatically overrepresented ( approximately 10,000-fold) in the euchromatic genome of D. melanogaster, implying that it may be a site for the physiological action of this enzyme.

The influence of processing induced differences in molecular structure on the biological and non-biological degradation of poly (3-hydroxybutyrate-co-3-hydroxyvalerate), P(3-HB-co-3-HV)

The influence of processing conditions on the degradation rate of melt-extruded poly(3-hydroxybutyrate-co-7%3-hydroxyvalerate) in pure fungal cultures ( Phanerochaete chrysosporium, Penicillium simplicissimum and Aspergillus fumigatus) and by chemical hydrolysis (pH 10.5, 60°C) was investigated. The degradation was monitored by gravimetry, size exclusion chromatography (SEC), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray scattering, infrared spectroscopy (FTIR) and tensile testing. The rate of erosion, measured as weight loss, was highest for the samples processed at high temperature, and lowest for the samples processed at the lowest temperature. The erosion rate during microbial degradation was not controlled by molecular weight or by the lamellar distribution. An increase in the degree of crystallinity reduced the rate of erosion. A correlation was found between the rate of erosion and the intensities of the sharp X-ray reflections (020) and (110). The rate of weight loss increased with increasing peak ratio I (020)/I (110) indicating that the positioning of the oxygen atoms affected the recognition capability of the extra-cellular enzymes. The reproducibility in weight loss of the samples between the different experiments was good. The erosion proceeded without any decrease in molecular weight. The molecular weight of the samples subjected to chemical hydrolysis exhibited a 50% drop within 40 days. Microtomed samples showed higher rates of chain scission in the center than at the surface. The degree of crystallinity increased by 35% as measured by DSC. The initial appearance of the melting endotherm, governed by the processing conditions, was maintained at 60°C until the molecular weight dropped below 50% of the initial value. The embrittlement of the samples measured as the elongation at break (ε b) was controlled by the decrease in molecular weight.

Application of chemical stress relaxation to the degradation of elastomers by heat and radiation

The article studied the following fundamental problems on the degradation of elastomers by heat and/or radiation by using the measurement of chemical stress relaxation. (1) The efficiency of scission and crosslinking of elastomers by radiation. (2) The continuous and intermittent chemical stress relaxation of tetrafluoroethylene-propylene elastomer was measured at 100°C under the irradiation of 60Co γ rays. The G value of crosslinking (Gc) was found to be 0.88 and the G value of scission (Gs) was 1.89 by this method. On the other hand, Gc and Gs values of the polymer, as obtained from a Charlesby-Pinner plot (sol fraction method) were 1.22 and 2.54, respectively. (3) The method how to express the magnitude of synergism between heat and radiation: The rate of continuous chemical stress relaxation under heat and radiation Kh+r is expressed as Kh+r = Kh + Cc Iα exp(–E'/RT), where Kh is the rate of stress relaxation by heat alone, I is dose rate, E' is the coefficient of synergism, R is the gas constant, T is the temperature (in K), and Cc is the constant which refers to the sensitivity for radiation. The exponent α shows the dose rate dependence. (4) The difference between heat aging and radiation induced degradation of elastomers: Pure vulcanized ethylene-propylene copolymer was irradiated with 60Co γ rays in air at room temperature. The rate of chain scission of irradiated samples by heat was independent from the dose and was found to be ten times higher than that of non-irradiated samples. The results suggest that irradiation consumes the antioxidant agents added during the polymer production process by the energy transfer from polymer to antioxidant agents. On the other hand, the rate of stress decay of non-irradiated samples increased with the heat aging time at all temperatures. In this case, heat degrades polymer and antioxidant agent simultaneously. (5) Pure vulcanized tetrafluoroethylene-propylene elastomer was irradiated by 60Co γ rays in air at room temperature. The chemical stress relaxation of the samples was measured at various temperatures ranging from 100°C to 200°C. The rate of scission increased with increasing dose, but the rate leveled off at about 100 kGy. The increase in scission by irradiation might be due to the initiation of autoxidation by thermal decomposition of hydroperoxide accumulated during irradiation.

Retardation of anaerobic degradation of poly( ρ-methoxyacrylophenone)

The effects of the u.v. absorbers 2,4,-dihydroxybenzophenone (DHB), Tinuvin 213 and Tinuvin P on the anaerobic photochemistry of poly(4-methoxyacrylophenone) have been investigated, the additives being incorporated into thin films and to solutions of the polymer. All retard the degradation of the polymer, in particular both rates of coloration and random chain scission are decreased. None of the additives acts simply as a u.v. absorber, although all of their long-wave u.v. absorptions overlap the n → Π * absorption of the polymer. Rather, experimental evidence supports the proposition that they are acting as quenchers of the CO triplet. The chain scission data conform to Stern–Volmer quenching, and the coloration data for the solid state reactions is well represented by the Perrin equation. DHB is by far the most effective retarder.

Topoisomerase IV Catalysis and the Mechanism of Quinolone Action

Topoisomerase IV is a bacterial type II topoisomerase that is essential for proper chromosome segregation and is a target for quinolone-based antimicrobial agents. Despite the importance of this enzyme to the survival of prokaryotic cells and to the treatment of bacterial infections, relatively little is known about the details of its catalytic mechanism or the basis by which quinolones alter its enzymatic functions. Therefore, a series of experiments that analyzed individual steps of the topoisomerase IV catalytic cycle were undertaken to address these critical mechanistic issues. The following conclusions were drawn. First, equilibrium levels of DNA cleavage mediated by the bacterial enzyme were considerably (>10-fold) higher than those observed with its eukaryotic counterparts. To a large extent, this reflected decreased rates of DNA religation. Second, the preference of topoisomerase IV for catalyzing DNA decatenation over relaxation reflects increased rates of strand passage and enzyme recycling rather than a heightened recognition of intermolecular DNA helices. Third, quinolones stimulate topoisomerase IV-mediated DNA cleavage both by increasing rates of DNA scission and by inhibiting religation of cleaved DNA. Finally, quinolones inhibit the overall catalytic activity of topoisomerase IV primarily by interfering with enzyme-ATP interactions.

Oxidative scission of plant cell wall polysaccharides by ascorbate-induced hydroxyl radicals.

Scission of plant cell wall polysaccharides in vivo has generally been assumed to be enzymic. However, in the presence of l-ascorbate, such polysaccharides are shown to undergo non-enzymic scission under physiologically relevant conditions. Scission of xyloglucan by 1 mM ascorbate had a pH optimum of 4.5, and the maximum scission rate was reached after a 10-25-min delay. Catalase prevented the scission, whereas added H2O2 (0.1-10 mM) increased the scission rate and shortened the delay. Ascorbate caused detectable xyloglucan scission above approx. 5 microM. Dehydroascorbate was much less effective. Added Cu2+ (>0.3 microM) also increased the rate of ascorbate-induced scission; EDTA was inhibitory. The rate of scission in the absence of added metals appeared to be attributable to the traces of Cu (2.8 mg.kg-1) present in the xyloglucan. Ascorbate-induced scission of xyloglucan was inhibited by radical scavengers; their effectiveness was proportional to their rate constants for reaction with hydroxyl radicals (.OH). It is proposed that ascorbate non-enzymically reduces O2 to H2O2, and Cu2+ to Cu+, and that H2O2 and Cu+ react to form .OH, which causes oxidative scission of polysaccharide chains. Evidence is reviewed to suggest that, in the wall of a living plant cell, Cu+ and H2O2 are formed by reactions involving ascorbate and its products, dehydroascorbate and oxalate. Systems may thus be in place to produce apoplastic .OH radicals in vivo. Although .OH radicals are often regarded as detrimental, they are so short-lived that they could act as site-specific oxidants targeted to play a useful role in loosening the cell wall, e.g. during cell expansion, fruit ripening and organ abscission.

Change in molecular weight of hyaluronic acid during measurement with a cone-plate rotational viscometer

It was shown using a cone-plate rotational viscometer that the apparent viscosity of a dilute aqueous solution of sodium hyaluronate decreased gradually during the measurement. Hyaluronic acid (HA) forms a characteristic network by entanglements coupling, so two hypotheses could be postulated from the reduction of viscosity due to shearing stress. One was that disentanglement of the temporary network occurred, and the other was that scission of HA chains was responsible. In this study, the reason for the reduction in viscosity was clarified using high-performance gel permeation chromatography with low-angle laser light scattering. It was thus demonstrated that chain scission occurred during the viscometric measurement. On the assumption that the HA degradation was caused by shearing stress, the effects of shearing rate, the initial molecular weight, salt, and polymer concentrations of samples were investigated. High molecular weight HA chains were preferably severed. From the change in polydispersity of the samples it was inferred that due to the viscoelasticity of HA solution scission behavior would differ, depending on the shearing rate. Also, the salt and polymer concentrations were found to exert large influences on the degradation. Thus, it was concluded that the scission rate was related to the expansion of HA molecules in solution. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2199–2206, 1998

Change in molecular weight of hyaluronic acid during measurement with a cone‐plate rotational viscometer

It was shown using a cone-plate rotational viscometer that the apparent viscosity of a dilute aqueous solution of sodium hyaluronate decreased gradually during the measurement. Hyaluronic acid (HA) forms a characteristic network by entanglements coupling, so two hypotheses could be postulated from the reduction of viscosity due to shearing stress. One was that disentanglement of the temporary network occurred, and the other was that scission of HA chains was responsible. In this study, the reason for the reduction in viscosity was clarified using high-performance gel permeation chromatography with low-angle laser light scattering. It was thus demonstrated that chain scission occurred during the viscometric measurement. On the assumption that the HA degradation was caused by shearing stress, the effects of shearing rate, the initial molecular weight, salt, and polymer concentrations of samples were investigated. High molecular weight HA chains were preferably severed. From the change in polydispersity of the samples it was inferred that due to the viscoelasticity of HA solution scission behavior would differ, depending on the shearing rate. Also, the salt and polymer concentrations were found to exert large influences on the degradation. Thus, it was concluded that the scission rate was related to the expansion of HA molecules in solution. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2199–2206, 1998

Polymer degradation with simultaneous scission and crosslinking. Weight-average molecular weight changes in the case where crosslinking rate surpasses scission rate

When the ratio of scissions and crosslinks numbers is less than unity ( n s n x < 1), gel starts to form at an effective crosslinking index equal to 0.25. The weight-average MW diverges in the vicinity of the gel-point: M w M wo = (1 − CI eff/0.25) −s . The scaling exponent, S, has been found as the function of the ratio of scission and crosslinking rates: S = (1 − n s n x ) −1 .

Effects of the molecular weight of molecular chains constituting the reaction medium on the thermal degradation of polyisobutylene

Non-volatile oligomers obtained by the thermal degradation of polyisobutylene were finely fractionated and the distribution of the functional groups of each fraction was characterized by a kinetic approach including the intermolecular hydrogen abstraction of primary and tertiary terminal macroradicals (R p· and R t·) and volatile small radicals (S·), followed by β scission. The observed value of the composition ratio [TTD]/[NTTD], which is related to the ratio of the competitive β scission rates of a secondary on-chain macroradical, increases monotonically with decreasing molecular weight ( M) of the fractions. This result shows clearly that the reactivity for β scission depends on the segmental motion of the reacting radicals. With respect to the composition ratio [ t-Bu]/[iPr], which is related to the concentration ratio [R p·]/[R t·], the observed value is roughly constant with M for M > M c, but decreases with decreasing M for M < M c, where M c corresponds approximately to the characteristic molecular weight for the entanglement of molten polymer. This ratio for M > M c decreases with decreasing average molecular weight of the molten polymer matrix, owing to an increase in the rate of diffusion-controlled termination. For M < M c, the value of the exponent a determined from the slope of the double logarithmic plots of [ t-Bu]/[iPr] against M is about 0.5 in all cases. This suggests that the hydrogen abstraction of S· depends on the chain dimension of unentangled polymer. It is confirmed from these results that the degradation reaction of all the polymer molecules in an homogenous reaction medium is entirely affected by the segmental motion and self-diffusional motion of the degrading polymers, and the effect of the chain dimension is observed only for unentangled polymer molecules during the degradation.

Thermodynamic parameters governing the stereoselective degradation of arylglycerol-B-aryl ether bonds in milled wood lignin under kraft pulping conditions

Abstract During this investigation, softwood (picea mariana) milled wood lignin was subjected to kraft pulping conditions at the temperature range 100-160 oC, for variable lengths of time. After quantitative isolation of the treated lignins, they were subjected to 31PNMR analyses which allowed the absolute rates of scission of the erythro and thewo diastereomers of the arylglycerol-B-aryl ether structures present in lignin to be derived.The plots of the kinetic data revealed that two kinetic regimes operate i.e. an initial fast phase that is followed by another slower phase. In agreement with previous accounts, the rate constants for these scission reactions were found to follow a pseudo-first-order rate law, during both phases. Rate constant data obtained during this effort, invariably indicated that the erythro isomers of the R-0-4 units of softwood milled wood lignin cleave faster than their thr-eo counterparts during both phases. At elevated temperatures (160 "C) the difference was found to be augmented. The general stability of the threo diastereomers toward kraft pulping seems to be the manifestation of a considerably slower reaction of the pulping reagents with the threo diastereomers of the B-aryl ethers. Arrhenius and Eyring plots o i these data supplied the complete profile of thermodynamic parameters governing this commercially important ether scission reaction.