Vinyl Unsaturation Research Articles

Novel mechanism for the formation of long-chain branching in polyethylene

Polyethylene produced by a vanadium-based polymerization catalyst contains long-chain branching as determined by NMR and rheology, even though the polymer has very low levels of vinyl unsaturation. A new mechanism is proposed for the formation of the long-chain branching, which involves CH bond activation of the polyethylene backbone through a σ-bond metathesis reaction, followed by ethylene insertion at the new VC bond. Consistent with the proposed CH bond activation mechanism, the polymerization catalyst was also found to insert ethylene into the CH bonds of alkanes such as heptane. A bridged metallocene catalyst was also found to activate CH bonds of alkanes suggesting this new mechanism may explain the formation of long-chain branching in some metallocene-produced polyethylene. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2889–2898, 1998

Crosslinking of polybutadiene rubber without any vulcanization agent

AbstractPolybutadiene rubber with 10% vinyl unsaturation has been crosslinked without the addition of any vulcanization chemicals. The crosslinks have been created through treatment at high temperature (240–250°C) and elevated pressure (<293 MPa) in a plunger type mold. The treatment time needed to achieve a crosslinked material is of the order of a few minutes and the reaction rate is shown to be dependent upon the temperature as well as upon the pressure applied. As a result of the severe treatment, some degradation reactions may be expected but careful FTIR analysis shows no increased oxidation. There is some consumption of the vinyl unsaturation, to an amount comparable to the number of crosslinks formed. The presence of carbon black increases the measured crosslink density, as expected, but at higher pressures the amount of extra network material, because of the carbon black, is much larger than at lower pressures, suggesting that the carbon black plays a critical role in the network formation process. The stress relaxation properties in compression of the rubber material were investigated and found to be comparable to those of a peroxide vulcanizate.

Catalytic Chain Transfer for Molecular Weight Control in the Emulsion Polymerization of Methyl Methacrylate and Methyl Methacrylate−Styrene

The application of catalytic chain transfer in the emulsion homopolymerization of methyl methacrylate and copolymerization of methyl methacrylate and styrene has been examined. [Bis[μ-[(2,3-butanedione dioximato)(2−)-O:O‘]] tetrafluorodiborato(2−)-N, N‘,N‘‘,N‘‘‘]cobalt (COBF, 1) has been used as a low-spin cobalt(II) catalytic chain transfer agent. The number-average molecular weight of poly(methyl methacrylate) is decreased from over 120 000 in the absence of catalytic chain transfer agent to less than 3000 on addition of 36 ppm of 1 remaining constant throughout the reaction while the polydispersity index was maintained at approximately equal to 2. The mechanism is essentially the same as in solution or bulk with very small amounts of primary radical initiation, undetectable by matrix-assisted laser desorption ionization time of flight mass spectrometry, and vinyl unsaturation at the terminal end of each polymer molecule. Copolymerization of methyl methacrylate with styrene has also been demonstrated. Catalytic chain transfer polymerization has been shown to be an extremely effective method of controlling molecular weight under emulsion conditions. The effects of catalytic chain transfer agent level, feed conditions, and styrene level are presented. The behavior of catalytic chain transfer in emulsion proves to be more complicated than in solution, due to partitioning of the catalytic chain transfer agent in the compartmentalized system. COBF partitions almost equally between the organic and aqueous phases. The feed profile of each reaction ingredient is important, with the best results being obtained when catalyst is fed continually as a solution in the monomer feed. A critical level of catalyst is apparent which is shown to be related to the number of catalyst molecules per particle.

Improved ozone resistance of styrene-butadiene rubber cured by a combination of sulfur and ionizing radiation

Fourier Transform Infrared (FTIR) studies performed in this work indicate that high ozone resistance of Styrene-Butadiene Rubber (SBR) formulations cured by a combination of sulfur and ionizing radiation is associated with unusually high vinyl concentration. On the other hand, sulfur cured SBR formulations with low vinyl concentration have poor ozone resistance. Curing with peroxides which involves chemistry similar to that of radiation curing, also leads to high vinyl concentration (relative to sulfur curing) and high ozone resistance. Increasing the absorbed dose in sulfur-radiation cured samples decreased the high vinyl content to a point where the ozone resistance declined greatly. Carbon black was shown to reduce the absorption of both the transvinylene and the vinyl unsaturation groups, but not to the same extent in all formulations. Also, The carbon black seems to play a greater role in the absorption of the unsaturation as sulfur increases.

Processing stabilization of HDPE: a complex study of an additive package

Response surface methodology (RSM) has been applied for optimizing an additive formulation in HDPE. This experimental approach allows a more precise description of the whole complexity of the polymer stabilization. In contrast to the generally used single parameter variations, RSM gives access to the mutual interactions of the additives and their temperature dependence. Several responses (MFI, melt pressure, vinyl unsaturation content) are shown to be sensitive to the composition of the additive system. The data emphasize the importance of the phosphorus based stabilizer for HDPE melt processing. The contribution of the phenolic antioxidant is temperature dependent passing through a maximum. Some interaction terms, including the effects of processing aids, gain importance under certain conditions, and their influence can even surpass the direct contributions of the single additives.

Crosslinking reactions in pigmented olefinic polymers

AbstractThermally initiated crosslinking reactions have been studied in a linear low‐density polyethylene, and an ethylene—propylene rubber. The polymers contained conventional thermal stabilizers or one of three surface‐coated rutile pigments. Enthalpies of crosslinking and reaction kinetics depended on the degree of terminal vinyl unsaturation in the polymers, that characteristic being greater in the rubber than in the polyethylene. The presence of thermal stabilizers did not exert any measurable influence on measured parameters of the crosslinking processes. The effects of rutile varied, depending on their surface acidity or basicity, as determined from chromatographic measurements. Basic rutile was found to reduce heats and to slow rates of reaction, while acidic rutiles did not affect the reactions. These observations may be attributed to interaction between the basic pigment surface and acidic moieties involved in the crosslinking reaction. © 1992 John Wiley & Sons, Inc.

Effects of polyethylene molecular structure on peroxide crosslinking of low density polyethylene

AbstractThe effects of molecular structure on the dicumyl peroxide crosslinking of two low density polyethylenes have been studied. Peroxide efficiency was determined largely by the content of terminal vinyls in the two polymers. Differences in molecular weight distribution and branch content had comparatively little influence on the gel content but hot creep resistance of the vulcanized materials was affected. Scorching was affected, also, by the presence of vinyl unsaturations in the polymer.

Plasma polymerization using pulsed microwave power

AbstractPulsed microwave power was used to polymerize a variety of monomers containing different functional groups. We examined the effects of pulse frequency and duty cycle on the deposition rates and the composition of the polymers. For monomers that do not contain oxygen we find that there is an increase in deposition rate with increasing pulse frequency and that the pulsed deposition rate is always less than the continuous power deposition rate. For monomers that contain oxygen, or for co‐depositions of hydrocarbon monomers with O2 or CO, we find that there is a decrease in deposition rate with increasing frequency, however the deposition rate using pulsed power is greater than the rate using continuous power. This result is shown to be related to the amount of etching that takes place during the deposition process. Infrared studies reveal that pulsed power can alter the composition of plasma polymers of some oxygen‐containing monomers. The presence or absence of vinyl unsaturation, nitrile groups, or a cyclic structure have no effect on the polymerization process.

The effect of molecular orientation on gamma-radiation induced crosslinking in high density polyethylene

The effects of molecular orientation on γ-radiation induced crosslinking in high density polyethylene is studied using two different polymer grades. It is found that transfer of the initial isotropic semi-crystalline polymer structure to fully oriented fibrillar structure results in a large increase in crosslinking efficiency at relatively low radiation doses. However, in the ultra-oriented samples drawn at high temperatures, crosslinking efficiency approaches that of the undrawn polymer. A similar trend is also found for the decay of the inherent vinyl unsaturation while the elastic modulus of the ultra-oriented polymer initially increases with the radiation dose, followed by a decay period. These results are analysed in view of the morphology of the oriented polymer and the direct and catalytic actions of vinyl unsaturation in the crosslinking process.

Imaging with Functional Group Maps Using Infrared Microspectroscopy

The interface of a two-dimensional computer-controlled stepper-motor-driven microscope stage with an infrared microspectroscopy system is explained in terms of its usefulness in performing microscopic imaging. Images are obtained by reducing spatially specific infrared spectra to functional group maps. The maps are constructed at a given infrared absorption frequency and are a function of the intensity versus x and y spatial coordinates. The use of the technique is demonstrated through two examples of polymer characterization. Transmittance infrared spectra reduced to five images based on absorptions at 1735, 1241, 1016 (ester functionalities), 1892 (crystalline band), and 908 (vinyl unsaturation) cm−1 show that a polyethylene film contains an inclusion of an acetate-containing material. Reflectance infrared spectra reduced to an image based on a ratio of the absorptions at 1738 cm−1 (ester carbonyl) and 1705 cm−1 (acid carbonyl) show that the streak in a film of an ethylene-acrylic acid/ethylene-methacrylate copolymer blend on aluminum foil contains more of the ester component of the blend.

Radiation chemistry of linear low‐density polyethylene. I. Gel formation and unsaturation effects

AbstractQuenched and annealed samples of linear low‐density polyethylene (LLDPE) were irradiated with 60Co γ rays in vacuo at room temperature. The data follow rather accurately Charlesby's equation s = k/r, where s is the soluble fraction, r the dose, and k a constant from which G(X), the G‐value for crosslinks, was calculated. Crosslinking in the LLDPE is about twice as extensive at equal doses as in LHDPE. Production of vinylene unsaturation was approximately the same in the two types of polyethylene.

Melt spinning of syndiotactic 1, 2‐polybutadiene for preparation of carbon fibers

AbstractThe thermal crosslinking and loss of vinyl unsaturation of syndiotactic 1, 2‐polybutadiene(s‐PB) at 180–230°C were prevented by stabilizers with 3, 5‐di‐t‐butyl‐4‐hydroxybenzyloxy group. The s‐PB samples (mp 140–198°C and MW 20,000–70,000) that contained the stabilizers could be melt‐spun at a temperature below 220°C into 1‐denier fibers to be used for the preparation of carbon fibers. The s‐PB fibers with higher mp and/or higher MW could be obtained by the addition of a high boiling solvent such as tetralin. The relationship between the molecular structures of s‐PB and the properties of resulting s‐PB fibers, including the degree of molecular orientation measured by birefringence and x‐ray diffraction, is presented. Spun fibers showed small swellings here and there along the fiber axis, which would have resulted from the inhomogeneity of the melt of s‐PB spun at a temperature slightly above the melting point. The gelation was unlikely to occur.

The effect of bromination and inherent vinyl unsaturation on the drawing behavior of high‐density polyethylene

AbstractVarious high‐density polyethylenes were brominated in order to eliminate the inherent vinyl Unsaturation in the polymer. They were then drawn uniaxially at three different temperatures, and the resulting fibrous materials were studied under a scanning electron microscope. It was found that the brominated samples yielded transparent fibrous materials that had more regular fibrillar structure and fewer surface overgrowths, cracks, and voids compared with the unbrominated polymers drawn under identical conditions. Bromination also appeared to improve the mechanical properties of the drawn and the undrawn polymer. High‐density polyethylene that had no inherent vinyl Unsaturation also yielded transparent fibrous material with fibrillar structure similar to that of brominated polymers.

Photoradiation crosslinking of polyethylene

Abstract Crosslinking of PE with the combined action of ionizing radiation and UV light was examined by EPR, IR spectroscopy, mass spectrometry and determination of gel fractions. The effect of crosslinking of PE at 300°K increases 5-fold with the simultaneous action, and 1·3-fold, with the sequential action of ionizing radiation and exposure to UV light. The yield of hydrogen and rans -vinylene unsaturation increases at the same time. With the sequential action of γ-radiation and exposure to UV light at 77°K the gel fraction increases ≈ 2·5-fold, hydrogen, ≈ 14-fold and trans -vinylene double bonds, ≈ 23-fold. These products are formed during photolysis as a result of photoradical chain reactions, the continuation of which is the rupture of the CH bond of adjacent polymer macromolecules, a reaction photo-sensitized with alkyl radicals.

Radiation-Induced Crosslinking of Chlorobutyl and Polydiene Elastomers. Promotion by Polythiols

Abstract The response of Chlorobutyl (CIIR), an elastomer with 1,1-disubstituted carbon atoms along the polymer chain, to ionizing radiation has been directed toward a predominantly crosslinking reaction by addition of 1–4 phr of thioetherpolythiols (TEPT) prepared by reaction of trivinylcyclohexane or cyclododecatriene with H2S or low-molecular-weight dithiols. From a fundamental viewpoint, the unique feature of the CIIR-TEPT system is the reversal of expected behavior for a polymer molecule containing 1,1-disubstituted carbon atoms when exposed to high energy radiation: instead of chain scission, cross-linking is the predominant physical change to an extent comparable to that obtained from thermal curing processes. The TEPT becomes part of the radiation-cured network, for, as the radiation dose is increased, there is progressive addition of thiol sulfur to the crosslinked network. Concomitantly, there is a progressive reduction in the chlorine content as radiation-induced vulcanization proceeds. These polythiols are also powerful accelerators of radiation-induced cross-linking of polydiene elastomers, especially those with appreciable amounts of pendent vinyl unsaturation, such as conventional SBR. Evidence exists that a blend of CIIR and SBR can be covulcanized by radiation-induced curing in the presence of TEPT. Experiments were conducted with gamma radiation from a 60Co source and with a 1.4 MeV electron beam at dose rates of 20–120 kJ/kg (2–12 megarads). Physical properties of fully compounded polythiol-promoted CIIR systems crosslinked by radiation are lower than comparable thermally cured compounds, but the strengths would be adequate for many applications. Compounding ingredients, especially common accelerators of sulfur vulcanization, provide a wide range of inhibitory action toward radiation precure. However, with selected types, especially organometallic derivatives which are less soluble in the compound, radiation cure can proceed with a minimum of inhibition.

Effect of double bonds on the γ-radiation-induced crosslinking of polyethylene

Various low-density polyethylenes ranging in initial weight-average molecular weight (Mw) from 7600 to 589000 having a ratio of Mw to number-average molecular weight (Mn) of about 5 were irradiated by γ-rays in vacuo at 30°C. Gel fractions were determined and analyzed by using the equation derived by Charlesby and Pinner. The following relationships were obtained when Mw was used as the molecular weight: where rg represents the gel point dosage (Mrad), [CC]0 is the sum of the initial contents of terminal vinyl and vinylidene unsaturations (mole/g polyethylene), and q0 and p0 are the probabilities of crosslinking and main-chain scission per monomer unit for a unit radiation dose in Mrad, respectively. Similar relationships to the equations described above were also obtained when Mn was used. From the results, it was concluded that terminal vinyl and vinylidene unsaturations play an important role for the gel formation in the γ-radiation-induced crosslinking of polyethylene in vacuo at room temperature.

Chromocene‐based catalysts for ethylene polymerization: Thermal removal of the cyclopentadienyl ligand

AbstractThermal aging of a chromocene catalyst, (C5H5)2Cr/SiO2, in an inert atmosphere leads to a modified catalyst which shows poor response to hydrogen as a transfer agent. Polyethylenes prepared at a polymerization temperature of 90°C with this modified catalyst have a low melt index and high vinyl unsaturation level. By thermogravimetry the weight loss of the catalyst, relative to dehydrated silica, was equivalent to loss of one cyclopentadienyl ligand per chromium site. Pyrolytic gas chromatography showed cyclopentadiene was liberated in the thermal process. These overall studies provide strong evidence that loss of a cyclopentadienyl ligand in supported chromium catalysts has a profound effect on overall polymerization behavior.

Accelerating Effect of Acetylene on the γ-Radiation-Induced Cross-Linking of Polyethylene

The γ-radiation-induced cross-linking of polyethylene was investigated in the presence of acetylene under an absolute pressure of 3 kg/cm2 at 30—200°C with a dose rate of 1.1×105 rad/hr. The weight of the polyethylene increased almost proportionally to the radiation dose, and the rate of the increase was larger at 30 and 90°C than at 150 and 200°C. The gel fraction increased with rising irradiation temperature, passed through a maximum at around 105°C, and thereafter decreased sharply to become almost constant above 150°C. At temperatures below the melting temperature of polyethylene, the gel fraction was much higher than in vacuo. As a characteristic structural change, the formation of terminal vinyl unsaturation was observed, and the rate of the formation increased with the temperature. The formation of hydrogen and trans-vinylene unsaturation was little affected by acetylene. On the basis of these results, the role of acetylene in the γ-radiation-induced cross-linking of polyethylene was discussed from the view-point of the reaction mechanism.

Direct enthalpimetric microdetermination of allylic and vinylic unsaturation

This paper described an extension of the authors' method for direct enthalpimetric determination of olefins to six compounds containing allylic or vinylic unsaturation. Solutions of allylbenzene, allylcyclohexane, allylcyclopentane, methallylbenzene, vinylcyclohexane and vinylcyclopentane in concentrations of 26.4–184μmol have been analyzed with a standard deviation of 1 to 3 %. Under the conditions described, allylic and vinylic unsaturation in these compounds can be determined in the presence of aromatic unsaturation with no interference. Preliminary results for two compounds for which the enthalpimetric method is unsuccessful or at least suspect, allyl acetate and styrene, are also reported.

The reduction of bile pigments by faecal and intestinal bacteria

1. 1. The action of intestinal and faecal bacteria on bile pigments was studied for elucidation of the mode of intestinal conversion of bilirubin to urobilinogen. 2. 2. Biliverdin, free and conjugated bilirubin, mesobilirubin, mesobiliviolin, natural and “synthetic” i-urobilins, (+)-urobilin and (−)-stercobilin were incubated with the bacteria. 3. 3. Mixed organisms from human faeces had no effect on biliverdin but reduced free and conjugated bilirubin, natural (+)- and i-urobilin and (−)-stercobilin to (−)-stercobilinogen but mesobilirubin to a mixture of i- and (+)-urobilins. They reduced mesobiliviolin to the chromogen of a non-urobilinoid pigment of unknown structure. 4. 4. Natural G62 Clostridia reduced free and conjugated bilirubin to stercobilinogen, this action being synergised by Escherichia coli. Total sonicate of these organisms reduced free bilirubin but not conjugated bilirubin to stercobilinogen. The particulate fraction of the sonicate was ineffective; the supernatant converted free bilirubin to a non-urobilinoid chromogen but had no effect on conjugated bilirubin or mesobilirubin. 5. 5. These results are discussed with relevance to complexity of the urobilins and the effect of vinylic unsaturation on bile pigment reduction.