Scission Efficiencies of Short Chain Branches in the γ-Radiolysis of Ethylene-α-Olefin Copolymers

Abstract

γ-Irradiation of copolymers of ethylene with propene, 1-butene, and 1-hexene, containing from 1 to 6 alkyl short chain branches per 1000 carbon atoms, at 25°C in vacuum, produced small amounts of n-alkanes with a maximum yield for the alkane corresponding to the alkyl branch of the α-olefin unit. A multilinear regression analysis showed a highly significant dependence of G(C_n alkane) on the frequency of alkyl branches containing n carbon atoms, determined by ¹³C-NMR. The corrections to the G(C_n alkane) yields from other fragmentation processes were substantial but no dependence for G(C_n alkane) on fragmentation of chain ends or fragmentation of the chain following branch elimination could be deduced from the data. The scission efficiencies = G(alkane) divided by the branch frequency per 1000 carbon atoms ± 95% confidence limits were (0.7 ± 0.7) × 10^?3, (2.7 ± 0.8) × 10^?3, and (1.5 ± 0.3) × 10^?3, for methyl, ethyl, and butyl branches, respectively. These factors can be used to determine the short-chain branch frequencies in similar polymers from n-alkane yields on γ-irradiation.     

Characterization of the short-chain branches in poly(vinyl chloride)s by γ irradiation of the reduced polymers

The volatile products from the γ irradiation of samples of poly(vinyl chloride) prepared under different conditions and reduced to the corresponding polyethylenes have been measured quantitatively and compared with those for low- and high-density polyethylenes and copolymers of ethylene with small amounts of α-olefins. The presence of methyl branches is clearly demonstrated and there is also evidence for ethyl and butyl branches, although these had not been considered significant in previous [¹³C] NMR studies. The method is shown to be extremely sensitive to small quantities of residual trialkyl tin hydride reductant occluded in, and possibly also reacted with, the polymer, and to residual chlorine (<0.5%). The yields of alkanes are higher than expected from the branch frequencies determined by i.r. and [¹³C] NMR and results for ethylene-α-olefin copolymers. This difference is apparently due to sensitisation of the radiation degradation by residual chlorine and reductant. Quantitative determinations of branch frequencies by the radiolytic method are unlikely to be obtained until these problems are overcome.     

Carbon-13 NMR of ethylene-1–olefin copolymers: Extension to the short-chain branch distribution in a low-density polyethylene

As model polymers for isolated short-chain branches in low-density polyethylene, a series of ethylene–1-olefin copolymers was examined by use of ¹³C NMR at 25.2 MHz. An array of ¹³C resonances was observed that could be associated independently with methyl through amyl branches. The ¹³C chemical shifts became insensitive to branch length with hexyl and longer branches. Assignments of the various carbon resonances associated with branching were accomplished by using off-resonance decoupling techniques and the behavior of alkane chemical shifts previously observed by other investigators. The ratio of certain backbone and branch resonances could be used to establish the short-chain branch distribution in a low-density polyethylene.     

Rubber elasticity of branched polyethylenes

Two grades of ethylene/1-hexene copolymer, containing about 0.4 and 1.3 butyl branches per 1000 carbons, were subjected to electron beam irradiation in vacuum and in an acetylene atmosphere. The resulting networks were characterized by gel fraction determination and melt elasticity behaviour. The rubber elastic force-extension data were analysed in terms of the Edwards-Vilgis slip-link model. This theory provided good fits to the experimental data, and the calculated parameters agreed well with the gel fraction determinations. The results indicate that an increase in branch concentration causes an increased susceptibility to chain scission. Irradiation in acetylene is shown to enhance greatly the crosslinking process, without affecting the chain scission, leading to a more perfect network.     

Co-oligomerization of ethylene and higher linear alpha olefins. I. Co-oligomerization with the sulfonated nickel ylide-based catalytic system

Co-oligomers of ethylene and a series of linear α-olefins (propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, and 1-decene) were synthesized with a homogeneous catalyst consisting of sulfonated nickel ylide and diethylaluminum ethoxide at 90°C. GC analysis of the co-oligomerization products allowed complete structural identification of all reaction products, α-olefins with linear and branched chains, vinylidene olefins, and linear olefins with internal double bonds. The article describes the reaction scheme of ethylene–olefin co-oligomerization. The scheme includes chain initiation reactions (insertion of ethylene or an olefin into the Ni? H bond), chain propagation reactions, and chain termination reactions via β-hydride elimination. Primary and secondary inertions of α-olefins into the Ni? H bond in the initiation stage proceed with nearly equal probabilities. Higher olefins participate in the chain growth reactions (insertion into the Ni? C bond) also both in primary and secondary insertion modes. The primary insertion of an α-olefin molecule into the Ni? C bond produces the β-branched Ni? CH₂? CR₁R₂ group. This group is susceptible to β-hydride elimination with the formation of vinylidene olefins. However, the Ni? CH₂? CR₁R₂ groups can participate in further ethylene insertion reactions and thus form vinyl oligomerization products with branched alkyl groups. On the other hand, the secondary insertion of an α-olefin molecule into the Ni? C bond produces the α-branched Ni? CR₁R₂ bond which does not participate in further chain growth reactions and undergoes the β-hydride elimination reaction with the formation of linear reaction products with internal double bonds. Most co-oligomer molecules contain only one α-olefin fragment. However, the analysis of ethylene-propylene and ethylene-1-heptene co-oligomers allowed identification of products with two olefinic fragments which are also formed in the copolymerization reactions with small yields.     

Memory Effect of Crystallization in 1-Butene/α-olefin Copolymers

The macromolecular architecture is the crucial factor in determining the arrangement of the ordering structures, which, because of the multiscale feature, may exhibit distinct melting behaviors and induce the so-called memory effect to affect the following recrystallization. Until present, the correlation between the occurrence of memory effect and the intrinsic molecular structure is still far from the comprehensive understanding. In this work, four kinds of 1-butene/α-olefin random copolymers were designed and synthesized using the (pyridyl-amino) hafnium catalyst to introduce the different branches. The branch length was precisely controlled by the specific α-olefin comonomers, which include 1-hexene, 1-decene, 1-tetradecene, and 1-octadecene, while the branch density was tuned by the incorporation. As expected, the incorporation of α-olefin co-units to poly(1-butene) backbone decreases the non-isothermal crystallization kinetics and the degree of crystallinity. More interestingly, the resulting linear branch can induce the occurrence of memory effect and the threshold concentration of co-units (i.e., branch density) decreases with increasing the branch length. Based on the results of these 1-butene/α-olefin copolymers with designable branches, a direct correlation with the occurrence of memory effect and the fraction of amorphous region was established, which quantitatively indicates the degree of local segregation of the crystallized poly(1-butene) sequences by the α-olefin co-units.

     

Following the Crystallization Process of Polyethylene Single Chain by Molecular Dynamics: The Role of Lateral Chain Defects

Summary: Langevin molecular dynamics (LMD) simulations have been performed in order to understand the role of the short chain branches (SCB) on the formation of ordered domains by cooling ethylene/α-olefins single chain models. Different long single-chain models (C₂₀₀₀) with 0, 5 and 10 branches each 1000 carbons were selected. The branches were randomly distributed along the backbone chain. Furthermore, C₁ (methyl) and C₄ (butyl) branches were taken into account. These models mimic the molecular architecture of ethylene/1-butene and ethylene/1-hexene random copolymers. The simulations are performed according to the following protocol: 20 random chain conformations for each model were equilibrated at high temperature (T* = 13.3) and then they were cooled in steps of 0.45 until the final temperature (T* = 6.2) by running a total of 35 × 10⁶ LMD steps. The distribution peaks of crystallization for each model were calculated by differentiating the global order parameter with respect to the temperature. The Tc* (crystallization temperature) decrease as the number of branches increases as it is experimentally observed. The formation of order in the copolymers is affected by the type and amount of the SCB in the backbone of the polymer chain. The stem lenght and crystallization fraction (α) were defined using the local-bond order parameter. Both parameters decrease as the number of branches increase. In all cases here shown, the C₄ branches are excluded from the ordered domains. However, we have observed that the methyl branch can be incorporated into the ordered regions. These facts satisfactorily agree with experimental data available in the literature.     

Cationic β-Scission of C−H and C−C Bonds for Selective Dimerization and Subsequent Sulfur-Free RAFT Polymerization of α-Methylstyrene and Isobutylene

A series of exo-olefin compounds ((CH₃)₂C(PhY)−CH₂C(=CH₂)PhY) were prepared by selective cationic dimerization of α-methylstyrene (αMS) derivatives (CH₂=C(CH₃)PhY) with p-toluenesulfonic acid (TsOH) via β-C−H scission. They were subsequently used as reversible chain transfer agents for sulfur-free cationic RAFT polymerization of αMS via β-C−C scission in the presence of Lewis acid catalysts such as SnCl₄. In particular, exo-olefin compounds with electron-donating substituents, such as a 4-MeO group (Y) on the aromatic ring, worked as efficient cationic RAFT agents for αMS to produce poly(αMS) with controlled molecular weights and exo-olefin terminals. Other exo-olefin compounds (R−CH₂C(=CH₂)(4-MeOPh)) with various R groups were prepared by different methods to examine the effects of R groups on the cationic RAFT polymerization. A sulfur-free cationic RAFT polymerization also proceeded for isobutylene (IB) with the exo-olefin αMS dimer ((CH₃)₂C(Ph)−CH₂C(=CH₂)Ph). Furthermore, telechelic poly(IB) with exo-olefins at both terminals was obtained with a bifunctional RAFT agent containing two exo-olefins. Finally, block copolymers of αMS and methyl methacrylate (MMA) were prepared via mechanistic transformation from cationic to radical RAFT polymerization using exo-olefin terminals containing 4-MeOPh groups as common sulfur-free RAFT groups for both cationic and radical polymerizations.     

Fonctionnalisation du polypropylene isotactique par le melange oxygene-ozone

Ozonisation of polypropylene in bulk causes chain scission through autooxidation processes at room temperature and produces mainly α-ω diketone polypropylene sequences. With poly(vinylchloride) or polyethylene, formation of α-ω diacid sequences has been observed. Hydroperoxides parallel ketone formation but they are grafted onto the polymer backbone. They may be used to initiate the polymerisation of vinyl monomers during the processing of the polypropylene.     

Some newer concepts on ethylene/α-olefin copolymerization on heterogeneous Ziegler-Natta catalysts

Unsteady diffusion kinetics, recently advanced by this laboratory, is applied to the examination of some polymerization and molecular chain structure problems. Hitherto deemed “anomalous” phenomena, such as the faster rate of copolymerization of ethylene/α-olefin than the homopolymerization of ethylene and the enrichment in the incorporation of a higher α-olefin in its copolymerization with ethylene by a lower α-olefin, are reasonably explained by unsteady diffusion of monomers. Molecular chain structure of copolymers, such as compositional heterogeneity and its dependence on comonomer incorporation originates from the difference in diffusion coefficients of the monomers. A copolymer composition equation taking into consideration the unsteady diffusion was developed. In cases where simulated curves were compared with experimental curves, good agreements were found.     

Radiation-induced oxidation of solid poly(ethylene oxide). II. Mechanism

The mechanism of γ-initiated oxidation of solid poly(ethylene oxide) (PEO) has been investigated, and an overall reaction scheme has been developed which accounts for most of the experimental observations. Data are correlated on the basis that the oxidation process is the sum of two reactions that are first-order and half-order in rate of initiation. They provide evidence that a significant fraction of the interactions of α-alkoxyalkylperoxy radicals is nonterminating at ambient temperature and yield free alkoxy radicals that are very subject to β scission. The unimolecular decomposition of secondary peroxy radicals, which has been invoked previously for the oxidation of PEO in solution, is not needed to explain the products of the oxidation of PEO in the solid phase. Approximately 90% of the total oxygen consumption has been accounted for by the observed products of oxidation. The radiochemical yield for backbone radical formation in irradiated PEO was estimated to be 6.5 ± 1.5.     

Study of the side-chain relaxation of methacrylate homopolymers and copolymers by the dielectric method

Dielectric constant and dielectric loss of copolymers of methyl methacrylate (MMA) with n-butyl methacrylate (nBMA) and isobutyl methacrylate (iBMA) have been measured in the frequency range 30 cps to 1 Mcps at temperatures from 70°K to 370°K. Results lead, together with those of previously published investigations on copolymers of MMA, to the following conclusions. (1) The loss-peak temperature attributed to side-chain relaxation (β peak) of PMMA varies with the comonomer ratio when the comonomer does not have an α-methyl group, but remains almost unchanged for comonomers having an α-methyl group. (2) In both cases, the β peak height of PMMA decreases with increasing ratio of comonomer B and completely vanishes for poly-B, and the loss peak temperature plotted against the fraction of B does not extrapolate to the β peak of poly-B. It is suggested on the basis of the above facts that the moving unit in the side-chain relaxation consists of a single side chain with a segment of the backbone chain and that the change in mobility of the side chain upon copolymerization results from the distortion of the helical structure of the backbone chain due to random distribution of α-methyl groups. Dielectric studies of the low-temperature side-chain relaxation (β₂ peak) in PnBMA, poly(n-octyl methacrylate), and poly(n-dodecyl methacrylate) (130°K at 1 kcps) have been made and an interpretation is offered for the molecular nature of this relaxation.     

Ethylene polymerization reactions with Ziegler–Natta catalysts. III. Chain-end structures and polymerization mechanism

Ethylene polymerization reactions with many Ziegler–Natta catalysts exhibit a number of features that differentiate them from polymerization reactions of α olefins: (1) a relatively low ethylene reactivity, (2) markedly higher polymerization rates in the presence of α olefins, (3) a high reaction order with respect to ethylene concentration, and (4) a strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions¹ provided the basis for a new kinetic scheme that postulates the equilibrium formation of Ti C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. This mechanism predicts several new features of ethylene polymerization reactions, one being that chain initiation via insertion of any α-olefin molecule into the Ti H bond should proceed with an increased probability compared to that via ethylene insertion into the same bond. As a result, a significant fraction of ethylene/α-olefin copolymer chains should contain α-olefin units as the starting units. This article provides experimental data supporting this prediction on the basis of both a detailed structural analysis of co-oligomers formed in ethylene/1-pentene and ethylene/4-methyl-1-pentene copolymerization reactions and a spectroscopic analysis of chain ends in the copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4281–4294, 1999     

Radiolyse des hydrocarbures. 21e communication. Mécanismes radicalaires et non-radicalaires dans les n-alcanes en phase liquide

Contributions of radical and non-radical processes have been determined in the formation of radiolysis products of n-heptane, n-octane, n-nonane and n-decane in a large range of temperature. Calculations are based on the combination and the dismutation of radicals, both reactions having nearly the same importance. Hydrogen abstraction reactions become important above – 25°. Intermediate molecular weight products and dimers are formed by statistical combination of the various radicals resulting from C? C and C? H scission. At low temperature, low molecular weight products are formed by both radical and non-radical processes, the second one being more important (3/4 for alcanes and 2/3 for olefins). The yield of radicals increases with the chain length of the irradiated n-alkane and amounts to 4.5 for n-heptane and 6.8 for n-decane at – 25°. This increase is due only to radicals from C? H scission, while the yield of radicals from C? C scission remains constant. Scission of CH₂? CH₂ bonds is favored for bonds inside the molecule, but this affect diminishes with chain length and CH₂? CH₂ rupture is equally probable at all positions for n-alcanes heavier than decane. Methyl C? H scission is 2.7 times less probable than methylene C? H scission. The radiolysis of mixtures of protonated and deuterated n-alcanes is shown to be able to give information concerning basic processes in radiation chemistry.     

Mesomorphic phase formation of poly (macromonomer)s of polystyrene macromonomers

The liquid crystalline phase formation of poly(macromonomer)s associated with the specific multibranched architecture of high branch density was investigated. The poly(macromonomer)s were prepared by radical chain polymerizations of ω‐methacryloyloxyethyl polystyrene macromonomers. It was confirmed that the mesomorphic phase formation depended on the branching architecture, where sufficient length of the branch chains as well as the backbone chain is crucial for the formation of the mesomorphic phase. Formation of the optically anisotropic mesophase also depended on the nature of solvent. The mesophase was observed in the cast films prepared from p‐xylene, toluene, tetrahydrofuran, carbon disulfide and chloroform but not observed for cyclohexane. The effects of the branched structure and the solvent nature were explained by repulsive interaction between the polystyrene branch chains of high branch density. The repulsive interaction increases the chain stiffness of the central backbone and also prevents the interpenetration of the polystyrene branches of different molecules in solution, which allow poly(macromonomer) molecules to arrange with the orientational order. Copyright © 2000 John Wiley & Sons, Ltd.     

The rôle of chain ends in the thermal degradation of anionic polystyrene

Direct evidence is given of the initiating rôle played in the thermal degradation of anionic polystyrene by chain ends, either present originally or formed during the degradation. In the early stages of degradation, the most likely bond scission in polystyrenes with benzylic type units (CH₂(C₆H₅)) at both chain ends involves the formation of toluene and an unsaturated terminal unit (CH₂C(C₆H₅)CH₂). The depolymerisation of polystyrene to a mixture of monomer and dimeric, trimeric, etc., fragments is then initiated by further scission at such unsaturated chain ends, giving α-methyl styrene and a depolymerising macroradical.After the early stage of degradation, a further overwhelming contribution to the formation of unsaturated chain ends is derived from chain transfer which occurs during depolymerisation. The concentration of unsaturated chain ends increases throughout the degradation process, thus accelerating the formation of the volatile products of depolymerisation. According to this mechanism of initiation, a constant ratio is found between rates of weight loss and of α-methyl styrene evolution throughout the degradation, independently of the original molecular weight of the polymer.     

The mechanism of radiation degradation of polypeptides

It is proposed that γ-irradiation of polypeptides causes ionisation to give the polymeric radical cation, which undergoes chain fragmentation to produce an amino cation and an alkyl radical. This is followed by fragmentation of the cation with elimination of small chain segments and substituent branches. Thus, in poly(alanine) methane and acetamide are major products. The variety of products formed is influenced by the presence of hydrogen-bonded water in the polymer. In particular, when the polymer is carefully dried prior to radiolysis no aliphatic carboxylic acids are formed.     

Ultrahigh Branching of Main-Chain-Functionalized Polyethylenes by Inverted Insertion Selectivity

Branched polyolefin microstructures resulting from so-called “chain walking” are a fascinating feature of late transition metal catalysts; however, to date it has not been demonstrated how desirable branched polyolefin microstructures can be generated thereby. We demonstrate how highly branched polyethylenes with methyl branches (220 Me/1000 C) exclusively and very high molecular weights (ca. 10⁶ g mol⁻¹), reaching the branch density and microstructure of commercial ethylene–propylene elastomers, can be generated from ethylene alone. At the same time, polar groups on the main chain can be generated by in-chain incorporation of methyl acrylate. Key to this strategy is a novel rigid environment in an α-diimine Pd^II catalyst with a steric constraint that allows for excessive chain walking and branching, but restricts branch formation to methyl branches, hinders chain transfer to afford a living polymerization, and inverts the regioselectivity of acrylate insertion to a 1,2-mode.     

铁系亚胺基吡啶复配催化乙烯原位共聚产物的表征

Branched polyethylene from ethylene as single monomer was prepared by the tandem catalyst system of {2-[2-Me C6 H4 N=Me)]2 C5H3N} FeCl2 (1) and {2,6-[1-(2,6-Me2-4-Br-C6H4N=(Me)]2C5H3N} FeCl2 (2) activated with methylaluminoxane (MAO) . The products of polymerization were characterized by DSC, GPC and ^13C-NMR. The results revealed that the copolymer produced by in situ copolymerization of ethylene was a mixture of branched polyethylene and α-olefin. The content of α-olefin in the mixture was increased with increasing the molar ratio of catalysts 1/2. The MWD paramelers of polyethylene and copolymer were 28.6 and 7.9, respectively. ^13C-NMR spectra showed that there were ethyl groups, butyl groups and long chain alkyl groups in the copolymer. The average degree of branching of such branched polyethylene was less than 5C/1000C.     

Radiation Degradation of Poly(olefin Sulfone)s: A Volatile Product Study

The predominant degradation reaction in the γ-irradiation of nine poly(olefin sulfone)s was found to be C-S bond scission with elimination of SO₂ and olefin. The extent of depolymerization, measured by the yields of the two comonomers, increased over five irradiation temperatures from 0 to 150° C and could be correlated with the ceiling temperature. Thus G (total volatile products) increased from 10 to 10,000 over this temperature range. Minor radiolysis products included the alkanes corresponding to (1) loss of the side chain group and (2) scavenging of the side chain radical by monomer olefin. There was a deficiency of olefin relative to SO₂, except at high temperatures, and isomerization of the product olefin in some cases. These observations are attributed to reactions of radiation-induced polymeric cations.