Radiolytic unsaturation decay in polyethylene. Part II—the effect of irradiation temperature, thermal history and orientation

The decay rate of vinyl unsaturation in high-density polyethylenes irradiated at temperatures from about 310 to 450 K, changes significantly in the melting range up to the crystalline melting point as does free radical mobility and the polymer crystallinity. However, orienting the polymer, or slow cooling or quenching from the melt, prior to irradiation, do not alter the decay process or its rate, although they do alter the rate of increase of insoluble gel and of elastic modulus in the molten state. It is suggested that, below 340 K, the marked deviations from a first-order decay result from the limited mobility of polymeric free radicals in the crystalline phase and from scavenging, by vinyl groups, segregated into the amorphous phase, of radiolytic hydrogen atoms (H). In the melting range, the mobility of polymeric free radicals increases as the crystallinity decreases, reducing the importance of scavenging, so vinyl decay approximates more closely to a first-order relation. In the melt, the vinyl decay relation is not changed qualitatively by H atom scavenging, but the effective vinyl concentration is lower, so the decay rate drops sharply.     

Use of reversed-phase HPLC with solid-phase extraction for monitoring of radiolytic degradation of chlorophenols for environmental protection

The efficiency of radiation induced decomposition of chlorophenols depends substantially on the radiation dose used and the presence of specific scavengers in the irradiated samples. Due to the use of HPLC for decomposition control it was shown, for the first time, that the increase of radiation dose results in gradual elimination of chlorine atoms from the chlorophenols molecules. The efficiency of radiolytic degradation of phenol and chlorophenols was monitored by reversed-phase HPLC. Prior to the chromatography the products of radiolytic degradation were preconcentrated using solid-phase extraction with phenyl columns. The most difficult to decompose is the simple phenol, which is also a product of radiolysis of all chlorophenols. Doses up to 2.0 kGy have not decomposed it completely in experimental conditions used. Degradation of chlorophenols in synthetic aqueous solutions takes place at doses from 0.2 to 2.0 kGy at ppm level of substrates depending on the number of chlorine atoms in molecule, however, for river water matrix containing scavengers such as carbonates or oxygen it requires larger doses.     

Antioxidative stabilization of poly(vinyl chloride)

Halogenated resins such as poly(vinyl)chloride undergo rapid degradation when exposed to heat. Such degradation is significantly increased by oxygen, and it is believed that the induction of the process of decomposition is caused by autoxidation of the polymer, followed by extensive dehydrohalogenation. Sulfur-containing peroxide decomposers allow primary stabilization of PVC with organic compounds which are not HCl acceptors but prevent free radical cross-linking of the polymer. This implies a discolored polymer melt (due to loss of HCl and formation of polyenes) which nonetheless exhibits good long term stability (no cross-linking). The stabilization of labile chlorine atoms results from the oxidation of various chain irregularities, and thus the elimination of the early discoloration can be achieved by the use of small quantities of monohydrocarbyltin co-stabilizers. The relative performance of various antioxidant systems are reported and possible stabilization mechanisms are discussed.     

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.     

Oxazoline functionalization of polyethylenes and their blends with polyamides and polyesters

The compatibilization of blends of polyamide‐6 (PA6) with linear low density polyethylene (LLDPE) and of poly(ethylene terephthalate) (PET) with high density polyethylene (HDPE), by functionalization of the polyethylenes with oxazoline groups was investigated. Chemical modification of LLDPE and HDPE was carried out by melt free radical grafting with ricinoloxazoline maleinate. Blends preparation was made either with a two‐steps procedure comprising functionalization and blending, and in a single step in which the chemical modification of polyethylene with the oxazoline monomer was realized in situ, during blending. The characterization of the products was carried out by FTIR spectroscopy and scanning electron microscopy (SEM). The rheological and mechanical properties of the blends were also investigated. The results show that functionalization of the polyethylenes can be achieved by melt blending with ricinoloxazoline maleinate even in the absence of free radical initiators. The compatibilization of the blends enhances the dispersion of the minor phase significantly, increases the melt viscosity, and improves the mechanical properties. The one‐step preparation of the compatibilized blends was also found to be effective, and is thought to be even more promising in view of commercial application.     

4,4,5,5-Tetra(3,5-dibromophenyl)-2,2-diphenyl-1,3-dioxa-2-silole and related compounds as precursors to flame retardant oligomers

Certain strained five-membered heterocycles undergo thermally-induced, carbon-carbon bond homolysis to generate diradicals capable of initiating vinyl polymerization. These compounds usually contain two oxygen atoms and a larger heteroatom (silicon, phosphorus, sulfur) in addition to carbon. If the heterocycle is suitably substituted with halogen (particularly bromine) containing moities it may function as a vehicle to permit incorporation of flame-retarding units into a polymer or oligomer. When an appropriate heterocycle is used as an initiator for radical polymerization each polymer chain formed contains at least one flame-retarding unit. One such heterocycle is 4,4,5,5-tetra(3,5-dibromophenyl)-2,2-diphenyl-1,3-dioxa-2-silole. In instances in which the heterocycle also acts as a monomer, i.e., it is reactive toward chain-propagating radicals, additional flame-retarding activity may be incorporated into the polymer.     

Preparation and structure of chlorinated poly(vinyl flouride)

The low-temperature chlorination of poly(vinyl fluoride) (PVF) proceeds readily in CCl₄ suspension. The rate of chlorination is high initially, but the reaction slows down considerably when the chlorine content of the polymer reaches 40–50%. At long reaction times, polymers containing 62% chlorine (1.88 chlorine atoms per monomer unit) can be obtained. As the degree of chlorination increases, the solubility of PVF in organic solvents increases. Polymer crystallinity and polymer softening point decrease with chlorination. Polymers containing 40% chlorine appear to be completely amorphous by x-ray analysis. In this respect, PVF differs from poly(vinyl chloride) (PVC), where chlorination increases the softening point, and it resembles polyethylene where both crystallinity and softening point decrease with chlorination. ¹⁹F NMR analysis of the polymers indicates that up to a degree of chlorination of 1 chlorine atom per monomer unit, 50% of the substitution occurs on the α-carbon of the PVF molecule. This result is very different from the predominant β-chlorination of PVC reported by several workers. The chemical selectivity observed in the chlorination of PVF is in quantitative agreement with the results of free-radical chlorination of organic compounds and can be rationalized by considering the size and the electronic properties of the fluorine atom. The results of ¹H NMR analysis are also in support of a polymer structure where the chlorine atoms are distributed between α- and β-carbons. Based on a comparison of the ¹⁹F and ¹H NMR data, the average composition of chlorinated PVF at the 1 chlorine atom per monomer unit level can be represented as: C₂₀₀H₂₀₀F₁₀₀Cl₁₀₀ = (CH₂)₆₃(CHF)₅₀(CHCl)₂₄(CClF)₅₀-(CCl₂)₁₃.     

Cyclopolymerization Kinetics of Dimethyl Diallyl Ammonium Chloride

A survey is made of the present knowledge about the kinetics and mechanism of the radical cyclopolymerization of dimethyl diallyl ammonium chloride which results in soluble, strong cationic poly-electrolytes. The kinetic analysis, taking into consideration nearly complete cyclization, a linear increase of kp /kt, 0.5 with [M], and different mechanism of initiation depending on the nature of the initiator, leads to rate equations which fit the experimental data well. Initiation with S₂O₈2- has the following peculiarities: formation of primary radicals by redox reaction with chloride ions and interaction with the monomer cation, additional termination by chlorine atoms, and an experimental chain transfer constant to monomer which includes transfer to monomer and termination by chlorine radicals.     

Reactive EB Processing of Polymer Compounds

Polymer modification with high energy electrons is well-established in polymer industry and used for degradation, cross-linking, grafting, curing, and polymerization. These applications use local and temporal precise input of energy in order to generate excited atoms or molecules as well as ions for subsequent molecule changes via radical induced chemical reactions. Reactive electron beam (EB) processing combines melt mixing process and chemical reaction simultaneously. For this purpose, a 1.5 MeV electron accelerator was directly coupled to an internal mixer in order to induce chemical reactions by energy input via high energy electrons under dynamic conditions of melt mixing of different polymer compounds. In the present study, reactive EB processing was used for the development of a flame retardant polyethylene composite as well as Thermoplastic Vulcanizate. The influence of absorbed dose as well as electron energy and electron treatment time was studied. Increased values of both tensile strength and elongation at break of polymer compounds indicated in-situ compatibilization upon reactive EB processing.     

A model for the plateau in the relaxation modulus for linear chain polymer melts

A simple model is considered for the free energy associated with the relaxation of a linear chain polymer melt. The relaxation of the chain configurations results in an adjustment of the locations of the interchain contacts. The change in the distribution of the positions of the contacts between a pair of relaxing chains is hindered by the presence of other nearby chains in the melt. There is less configuration space available to the relaxing chains, due to the noncrossability of the chain backbones, than would be the case for phantom chains. This results in an increase in the free energy of the relaxing system. The analysis presented indicates that, given the free energy models considered, the extent of relaxation decreases as the length scale for relaxation increases. This results in a plateau in the relaxation modulus. This qualitative prediction of a plateau does not rely on the assumption of a specific mechanism for the chain dynamics, and is relatively insensitive to the form chosen for the terms in the free energy. If reasonable assumptions are made concerning the form of the free energy, then it is shown that the plateau which results depends on monomer length, Kuhn length, the monomer density for the melt, and, for solutions, the polymer concentration in a manner consistent with experimental data.     

Theoretical study on the electronic,structural, properties and reactivity of a series of mono-, di-, tri- and tetrachlorothiophenes as well as corresponding radical cation forms as monomers for conducting polymers

In this paper, electrical and structural properties of mono-, di-, tri- and tetrachlorothiophenes and their radical cations have been studied using the density functional theory and B3LYP method with 6-311++G** basis set. The effects of the number and position of the substituent of chlorine atoms on the properties of the thiophene ring for all chlorothiophenes and their radical cations have been studied. Vibrational frequencies, nuclear chemical shielding constants, spin-density distribution, size and direction of dipole moment vector, ionization potential, electric polarizabilities and NICS values of these compounds have been calculated as well. The analysis of these data showed that double bonds in 3-chlorothiophene are more delocalized and it is the best possible candidate monomer among all chlorothiophenes for the synthesis of corresponding conducting polymers with modified characteristics.     

Poly α-ester degradation studies. IV. Rheological studies of polymer degradation

The Farol-Weissenberg rheogoniometer has been used to follow molecular weight changes during the degradation of certain poly-α-esters in the melt state. By observing the change in melt viscosity at low shear rates it had been shown that the decomposition of the poly(isopropylidene carboxylate) is substantially first-order with respect to the molecular weight of the residual polymer. The derived kinetic parameters are in good agreement with those previously obtained by other techniques. This provides a substantial piece of supporting evidence for the view that degradation takes place predominantly by intramolecular ester interchange involving the formation of 1,1,4,4,-tetramethylglycollide. The introduction of β-chlorine atoms into the polymer structure leads to a more complex degradation pattern. Thus the presence of a single β chlorine atom per repeat unit, as in poly(3-chloro-2-methyl-2-hydroxypropionic acid) leads to a substantially similar dependence on molecular weight with the added complication of progressive crosslinking which becomes more apparent in later stages of the reaction. This crosslinking reaction plays an increasingly important part as the extent of chlorination of the polymer is increased. In addition, the presence of chlorine leads to an increased sensitivity of the degradation reaction to the presence of oxygen.     

Cationic polymerization of α-methylstyrene from polydienes. I. Synthesis and characterization of poly(butadiene-g-α-methylstyrene) copolymers

The preparation of poly(butadiene-g-α-methylstyrene) copolymers was investigated with three different alkylaluminum coinitiators. The alkylaluminum compounds in conjunction with polybutadiene which contained a low concentration of labile chlorine atoms initiated the polymerization of α-methylstyrene to produce graft copolymers. Trimethylaluminum gave higher grafting efficiencies than diethylaluminum chloride at comparable monomer conversions. Triethylaluminum produced only very low monomer conversions (<5%), even at long reaction times, and for this reason was not studied extensively. The number of grafts per polybutadiene backbone was determined for a number of copolymers and found to increase slightly as the allylic chlorine concentration in the polybutadiene backbone was increased. In all cases, however, only a low percentage of the available labile chlorine sites along the polybutadiene backbone resulted in grafted α-methylstyrene side chains. The addition of small quantities of water to the polymerization solvent greatly enhanced the grafting rate and ultimate monomer conversion during the synthesis of these poly(butadiene-g-α-methylstyrene) copolymers. The mechanistic role of water during these grafting reactions is unknown at the present time.     

N-(monohalogenphenyl) maleimides and their copolymers with styrene and butadiene

N-(Monohalogenphenyl)maleamic acids (I) and N-(monohalogenphenyl) maleimides (II) which contained bromide or chlorine atoms in the 2-, 3-, or 4-position of the phenyl ring and their respective isoimides (III) were prepared. The radical copolymerization of Pairs II + styrene and II + butadiene in benzene solution initiated with 2,2′-azobisisobutyronitrile at 50°C was used to determine the monomer reactivity ratios of II. Their values, which are close to zero, indicated an alternating addition of the two monomers on the polymer radical. The thermal stability of the copolymers was characterized by thermogravimetric analysis; their flammability was determined by the method of limiting oxygen index. The copolymerizability of III with styrene and isobutylene was verified at 30°C.     

Quenching of metal‐catalyzed living radical polymerization with silyl enol ethers

Various silyl enol ethers were employed as quenchers for the living radical polymerization of methyl methacrylate with the R Cl/RuCl₂(PPh₃)₃/Al(Oi–Pr)₃ initiating system. The most effective quencher was a silyl enol ether with an electron‐donating phenyl group conjugated with its double bond [CH₂C(OSiMe₃)(4‐MeOPh) ( 2a )] that afforded a halogen‐free polymer with a ketone terminal at a high end functionality [F̄_n ∼ 1]. Such silyl compounds reacted with the growing radical generated from the dormant chloride terminal and the ruthenium complex to give the ketone terminal via the release of the silyl group along with the chlorine that originated from the dormant terminal. In contrast, less conjugated silyl enol ethers such as CH₂C(OSiMe₃)Me were less effective in quenching the polymerization. The reactivity of the silyl compounds to the poly(methyl methacrylate) radical can be explained by the reactivity of their double bonds, namely, the monomer reactivity ratios of their model vinyl monomers without the silyloxyl groups. The lifetime of the living polymer terminal was also estimated by the quenching reaction mediated with 2a . © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4735–4748, 2000     

Chlorine substitution in poly(arylamine)s during synthesis and protonation in hydrochloric acid

The effects of varying the oxidant/monomer ratio in the polymerization of aniline, N-phenyl-p-phenylenediamine and N,N′-diphenylbenzidine in a hydrochloric acid solution of (NH₄)₂S₂O₈ were investigated. With the first two monomers, increasing the oxidant/monomer ratio from 0·3 to 3 results in a substantial increase in polymer yield but the extent of covalent bond formation between chlorine and the polymer is also increased. In addition, differences in the N/C and imine/amine ratios, and in thermal stability, are evident in the polymers synthesized at different oxidant/monomer ratios. The degree of polymerization of N,N′-diphenylbenzidine is low and it exhibits a very high susceptibility to chlorine substitution in the reaction mixture. A comparison of the extent of chlorine substitution is made among polymers synthesized in (NH₄)₂S₂O₈/HCl, and polyaniline base and aromatic amine monomers treated with HCl of the same concentration.     

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.     

Determination of chlorine distribution in chlorosulfonated polyethylenes by high-resolution NMR spectroscopy

High-resolution proton magnetic resonance (NMR) spectroscopy was used to determine the sequence distribution of chlorines in elastomeric chlorosulfonated polyethylenes. The determination is based on measuring the relative amounts of methylene groups that are α, β, and γ (or greater) from chlorine containing groups (CHCl groups) in chlorosulfonated polyethylenes. The results obtained from the NMR examination at 220 MHz were compared with the theoretical predictions based on the statistics of substitution polymers. The comparison showed that polyethylenes chlorosulfonated by a solution reaction with gaseous chlorine and sulfur dioxide show a random chlorine distribution.     

Radiolytic unsaturation decay in polyethylene. Part I—general review and analysis with additional new work

Previously published mechanisms for radiolytic vinyl decay in polyethylene lead to first-order, second-order and modified first-order relations. Such equations are fitted using regression analysis to the results of unsaturation measurements on irradiated polyethylenes carried out by the present author (and colleagues) or made available to him by another. It is found that the modified first-order relation best represents the radiolytic decay of vinyl, vinylidene and added allyl unsaturation as triallylisocyanurate or triallylcyanurate in polyethylene at room temperature.     

Hydrochlorinated derivatives of syndiotactic 1,2-polybutadiene

Chlorine-containing polymeric products with a degree of functionalization of up to 70% are synthesized through the hydrochlorination of syndiotactic 1,2-polybutadiene via carbon-carbon double bonds. The introduction of chlorine atoms into polydiene units causes substantial changes in the viscosity of polymer solutions, the flowability of the polymer melt, the glass-transition and flow temperatures, and the thermal stability of polymers. The hydrochlorinated derivatives of syndiotactic 1,2-polybutadiene demonstrate good adhesion with respect to steel and can be used in adhesive compositions.