Spontaneous polymerization and chain microstructure evolution in high-temperature solution polymerization of n-butyl acrylate

This study concerns understanding of the underlying mechanistic pathways in high temperature solution polymerization of n-butyl acrylate (nBA) in the absence of added thermal initiators. The particular system of interest is the batch polymerization of nBA in xylene at temperatures between 140 and 180 °C with initial monomer content between 20 and 40 wt%. A mechanistic process model is developed to capture the dynamics of the polymerization system. Postulated reaction mechanisms include chain-initiation by monomer (self-initiation), chain-initiation by unknown impurities, chain-propagation by secondary and tertiary radicals, intra-molecular chain-transfer to polymer (back-biting), chain-fragmentation (β-scission), chain-transfer to monomer and solvent, and chain termination by disproportionation and combination. The extent of the reactions is quantified by estimating the reaction rate constants of the initiation and the secondary reactions, based on a set of process measurements. The set of measurements considered in the parameter estimation includes monomer conversion, number- and weight-average molecular weights, and average number of chain-branches per chain (CBC). Effect of temperature on chain microstructures was observed to be most evident when microstructures are expressed in terms of their quantities per chain. The evolution of other microstructural quantities such as average number of terminal double bonds per chain (TDBC) and average number of terminal solvent groups per chain (TSGC) was then also investigated. Microstructural quantities per polymer chain (TDBC, TSGC, CBC) are defined based on combinations of ¹³C, ¹H NMR and chromatographic measurements. This study presents (i) a mechanistic explanation for the competing nature of short-chain-branch and terminal double bond formation (i.e. as temperature increases, number of chain branches per chain decreases and number of terminal double bonds per chain increases), (ii) quantitative insights into dominant modes of chain-initiation and chain-termination reactions, and (iii) mechanistic explanations for the observed spontaneous polymerization. The study also reports estimated Arrhenius parameters for second-order self initiation, tertiary radical propagation, secondary radical backbiting and tertiary radical β-scission reaction rate constants. Validation of the mechanistic process model with the estimated Arrhenius parameters and comparison of estimated parameter values to recently reported estimates are also presented.     

Studies on the radical polymerization of vinyl acetate in benzene,chlorobenzene and ethyl acetate by 1H NMR spectroscopy

Summary Polymerizations of vinyl acetate were carried out with AIBN in benzene, chlorobenzene and ethyl acetate, and the resultant polymers were analyzed for terminal group by using ¹H NMR technique. The results revealed that a part of the polymer molecules prepared in aromatic solvent contained one solvent fragment at the chain end, indicating the incorporation of aromatic molecule through the chain transfer reaction and not by the copolymerization. It was found that there existed at least 0.7 branching per a molecule in the polymer prepared in ethyl acetate, whereas almost no branching in the polymer prepared in aromatic solvents.     

New polyisobutylene-based model ionomers

Strain induced crystallization has been observed at 25°C in low molecular weight three-arm star polyisobutylene ionomers at elongations exceeding about 550%. The sulfonated form of the polymer was neutralized with calcium hydroxide. The number average molecular weight of the polymer was only 9,000 with a dispersity ratio of about 1.7. This molecular weight is at the edge of the critical molecular weight for entanglements. Strain induced crystallization has been reported in the literature for very high molecular weight linear polyisobutylene. However, no such effects have been observed for linear, low molecular weight polyisobutylene and it has been stated it could not be induced. The cause for this strain induced crystallization in our materials is due to the presence of three ionic terminal groups per molecule which results in sufficient coulombic forces for maintenance of molecular orientation with strain without significant relaxation.     

Photocrosslinking of a maleimide functionalized polymethacrylate

A polymethacrylate bearing pendent maleimide (MI) groups has been crosslinked by UV irradiation of the photoinitiator‐free functionalized polymer. The reaction was followed in situ by real‐time IR spectroscopy and shown to proceed with an initial quantum yield of two maleimide double bonds polymerized per photon absorbed, the value expected for a photocycloaddition mechanism. A twofold increase of the reaction rate was achieved by performing the UV irradiation in the presence of a thioxanthone photosensitizer (1 wt%). Insolubilization of the photoresist requires the reaction of at least 18 MI double bonds per polymer chain, thus implying the occurrence of an intramolecular coupling process between neighbouring MI groups located on the same polymer chain. In the presence of an electron donor monomer, like the divinylether of triethyleneglycol used, the cycloaddition reaction gives way to a radical‐induced copolymerization of maleimide and vinyl ether double bonds, which becomes the only crosslinking process in a stoichiometric mixture of the two components. Copyright © 2003 Society of Chemical Industry     

Radiochemical studies of free-radical vinyl polymerizations: 7. Polymerization of methyl acrylate

Termination of poly(methyl acrylate) radicals in benzene solution has been studied by polymerization of the monomer using [¹⁴C] azoisobutyronitrile as initiator. When polymerizations were initiated thermally at 60° and 45°C values for the number of initiator fragments per polymer molecule, n, were substantially less than 2 but were dependent upon the rate of initiation, R_j. As R_j was increased n increased, indicating that chain transfer reactions were important. By correcting n for chain transfer it was shown that the polymer radicals most probably terminated by combination. The results at 25°C were more conclusive. With higher rates of photochemical initiation chain transfer reactions became unimportant, values of n were close to 2 and polymer radicals terminated almost exclusively by combination.     

The structure of polyenoic odd- and even-numbered fatty acids of mullet (Mugil cephalus)

Mullet oil contains more than 25% straight-chain odd-numbered fatty acids. Odd- and even-numbered components of chain lengths C₁₅ to C₂₀ were isolated and their structures determined. The vinylmethane rhythm prevails in all polyun-saturated acids. Numerous homologs have their double bonds in identical positions, relative to the carboxyl group, as for example, 舥^9,12- and 舥^6,9,12-C₁₅, -C₁₆, -C₁₇ and -C₁₈ acids. The terminal structures which are characteristic for oleic, linoleic, etc., families are not found in the unsaturated odd-numbered acids. The results show: that the proximal structure has greater influence than the terminal structure on the biosynthesis of unsaturated odd-numbered acids; that chain lengths of 17 and 18 carbon atoms with double bonds in position 9 are crucial for synthesis of the polyenoic C₁₉ and C₂₀ acids; that chain lengths C₁₅ and C₁₆ with double bonds in position 9 are suitable for desaturation but that they are not suitable for desaturation after elongation. These specifications bring all acids of mullet into a rational order and reflect their possible interconversions. Presently, such classification has only limited predictive value in regard to the physiological properties of polyunsaturated acids. However, the new definitions for grouping the polyunsaturated fatty acids lead to interesting working hypotheses.     

The correlation between electrical properties and structure of substituted polyacetylenes

Summary The electrical conductivity of polyphenylacetylenes and polyhalophenylacetylenes was measured in dependence on the temperature and on the type of catalyst used for polymerization. The electrical conductivity increased with increasing temperature. The energy gap was calculated from the temperature dependence on the electrical conductivity; it decreased with increasing number of conjugated double bonds in the polymer chain and with the regularity of structure of the polymer. The number of double bonds, cyclization and configurations were determined by ¹H and ¹³C-NMR. In polyphenylacetylenes only aliphatic carbons deriving from cyclohexadiene units were found but no such units were found in polyhalophenylacetylenes. By UV-vis measurement the number of conjugated double bonds in segments was estimated. The synthesized polymers were amorphous. Their molecular weight was between 2000 and 74.000.     

13C n.m.r. spectra of polymers made by ring-opening polymerization of (±)- and (+)-exo-5-methylbicyclo [2.2.1] hept-2-ene using metathesis catalysts

A range of olefin metathesis catalysts has been used to prepare ring-opened polymers of (±)- and (+)-exo-5-methylbicyclo [2.2.1] hept-2-ene, (l), having cis double bond contents of 11–100%. The ¹³C n.m.r. spectra of these polymers are interpreted in terms of TH, TT, HH, HT and tt, tc, ct, cc structures (T = tail, H = head, referring to methyl groups; t = trans, c = cis, referring to double bonds). The all-cis polymer has a fully-syndiotactic ring sequence, but polymers with less than 55% cis double bonds have an atactic ring sequence. The substitution shift parameters indicate that the cyclopentane rings in the polymer chain adopt the puckered conformation which minimizes non-bonded repulsion between the cis-1,3-olefinic substituents.     

Pyrrolidides for mass spectrometric determination of the position of the double bond in monounsaturated fatty acids

Mass spectra of pyrrolidides of monounsaturated straight chain fatty acids are presented and discussed. The spectra of pyrrolidides contain mainly ions from the polar part of the molecule. This gives simple spectra from which double bond positions can be deduced directly. If an interval of 12 atomic mass units is observed between the most intense peaks of clusters of fragments containing n and n−1 carbon atoms of the acid moiety, the double bond occurs between carbons n and n+1 in the molecule This rule is valid for double bonds occurring at positions Δ⁵-Δ¹⁵ in an 18-carbon chain and has been applied to acids having 10–24 carbon atoms.     

Preparation of surface initiated polystyrenesulfonate films and PEDOT doped by the films

Various substrates such as glass slides and silicon wafers were modified by styrylethyltrimethoxysilane to attach double bonds to those surfaces. The double bond layer was initiated and capped by benzoyl peroxide (BPO) and 2,2,6,6-tetramethylpiperidirooxy (TEMPO), respectively, to form ‘living’ free radical layer from which polystyrene brushes were grown. The density of double bonds on the surface controlled the orientation of polystyrene brush or film. The polystyrene films were then sulfonated by fuming sulfuric acid (H₂SO₄·xSO₃) to obtain polystyrenesulfonic acid (PSS) films with controlled polymer chain alignment. The lower double bond density led to a lower degree of polymer chain alignment. 3,4-Ethylenedioxythiophene monomer was diffused into PSS film and then polymerized. A conductive polyethylenedioxythiophene (PEDOT)/PSS film was obtained. The films were characterized by four-point probe, AFM and UV-VIS. The conductivity of PSS/PEDOT film measured along the direction which is normal to polymer chain alignment, is lower than that from commercial PSS/PEDOT.     

The incorporation of14C-glycerol into different species of diglycerides and triglycerides in rat liver slices

The relative rates of de novo synthesis of species of diglycerides and triglycerides from¹⁴C-glycerol were examined in rat liver slices. Diglycerides containing one or two double bonds per molecule and triglycerides containing four or more double bonds per molecule represented 70% and 60% respectively of the newly synthesized diglycerides and triglycerides. The newly synthesized triglycerides were more unsaturated than the endogenous triglycerides. Our results suggest that a nonrandom synthesis of species of diglycerides occurred followed by an almost random utilization of the various diglyceride species for the biosynthesis of triglycerides.     

Hydrosilylation of terminal double bonds in polypropylene through reactive processing

The melt-phase hydrosilylation of terminal double bonds in polypropylene (PP) was investigated. The double bonds were generated by peroxide initiated degradation of PP in an extruder or a batch mixer. For this purpose, an organic peroxide, Lupersol 101, was employed in concentrations of 0.5–5 wt%. A hydride-terminated polydimethylsiloxane was employed as a model substance to investigate the feasibility of hydrosilylating the terminal double bonds of the degraded polypropylene. Reactive processing experiments were carried out in a hot press, a batch mixer, and a single screw extruder. Two different reaction mechanisms were used to initiate the hydrosilylation reaction: a radical chain addition mechanism and a platinum catalyzed mechanism using Karstedt's catlyst. For the radical mechanism, it was shown that catalytic amounts of a peroxide could initiate the addition of silanes to the double bonds of the degraded polypropylene. Furthermore, it was found that both reactions, degradation and hydrosilylation, could be performed simultaneously. For the catalytic mechanism, the required stabilization of the platinum colloid formed in this mechanism was accomplished by adding t-butylhydroperoxide as a cocatalyst. This melt-phase hydrosilylation route may be used to produce terminally functionalized polypropylenes, and a case study involving the terminal attachment of a styrene functionality was examined.     

The heats of hydrogenation of unsaturated hydrocarbons

Recent precise experimental work on the heats of hydrogenation of hydrocarbon compounds containing one or more double bonds shows a variation from compound to compound which can be correlated with the structure of the molecule. For compounds containing one double bond or several non-conjugated bonds, the heat of hydrogenation per double bond is nearly constant. However, in compounds containing conjugated double bonds the average heat of hydrogenation per bond is less than in non-conjugated systems due to the interaction energy between the double bonds tending to stabilize the system. This effect is most pronounced in aromatice compounds. Although the hydrogenation of benzene to form cyclohexane (addition of three molecules of hydrogen per molecule of benzene) is an exothermic reaction, the hydrogenation of benzene to form cyclohexadiene −1,3 (addition of one hydrogen molecule per molecule of benzen) is an endothermic reaction. This signifies that the energy associated with the aromatic character of benzene is greater than the energy of hydrogenation of a double bond, so that the resultant of these two effects is an endothermic reaction.     

Concentration effects on polymer coil overlap as perceived from n.m.r. observation on high molecular weight polyisobutylene chains

Chain segment motions observed on a n.m.r. time scale are perceived as non-isotropic ones because of slow isotropic relaxation of chain constraints such as entanglements. This property, observed on protons bound to polyisobutylene chains, is applied to the investigation on a molecular scale of the crossover from the dilute to the semi-dilute regime. It is shown that coil overlap of polyisobutylene chains in CS₂ is perceived from n.m.r. at a monomer unit concentration C_v about eight times greater than the ideal concentration $\text{C1}$ defined from measured radii of gyration. By considering different chain lengths and different solvents, it is shown that the coil overlap concentration C_v corresponds to a virtual partition of the chain into about 15 blobs.     

Copolymerization of ethylene with α-olefins over supported titanium–magnesium catalysts. II. Comonomer as a chain transfer agent

The data on the effect of comonomer (propylene and 1-hexene) on molecular weight (M_w), molecular weight distribution (MWD), and content of terminal double bonds were obtained for ethylene/α-olefin copolymers produced over a supported titanium–magnesium catalyst (TMC) upon polymerization in the absence of hydrogen. The experimental data on the effect of comonomer concentration on M_w of polymers were used to calculate the ratios between the effective rate constants of chain transfer with monomer and the propagation rate constant. It was shown that the effective rate constant of chain transfer with monomers increases in the row of monomers: ethylene < 1-hexene < propylene. Meanwhile, the data on the effect of copolymers on content of terminal double bonds of various types demonstrate that different reactions of chain transfer with comonomer may simultaneously occur during copolymerization. It results in simultaneous formation of terminal vinylidene and trans-vinylene bonds. Therefore, the calculated rate constants of chain transfer with comonomer are complex values, which include the rate constants of chain transfer with comonomer occurring via different mechanisms. The data on MWD, short chain branching (SCB) and terminal double bonds content of different types were obtained by molecular weight fractionation of copolymers followed by the analysis of narrow fractions. The analysis of the data on MWDs of SCB and terminal double bonds shows that active sites of the TMC are considerably heterogeneous with respect to the rates of different chain transfer reactions with monomers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Alanine grafting of ion‐beam‐modified polyethylene

The grafting of a polyethylene (PE) surface layer implanted with 156 keV Xe⁺ ions at fluences of 3.10¹³ to 1.10¹⁵ cm⁻² with alanine was studied with the aim to prepare prospective materials for biological experiments. It was shown by different spectroscopical methods that alanine adds onto double bonds or reacts with radicals created by ion irradiation on the polymer chain. The grafting occurs in the whole ion‐damaged polymer layer. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1144–1148, 2000     

Free radical hydrosilylation of polypropylene

Hydrosilylation of terminal double bonds in polypropylene was investigated via a free radical process in the melt-phase. This process involved the following two reactions occurring in parallel: chain scission, which leads to formation of double bonds, and the addition of silyl radicals to such bonds. Fourier Transform Infrared Spectrometry (FT–IR) was used to follow silane incorporation and terminal double bond formation. The experimental trends were verified using elemental analysis obtained from energy dispersive x-ray analysis (EDX). Using a single screw extruder and a model silane compound, a 2³ full factorial experiment design was implemented with the following investigated factors: residence time, silane concentration, and peroxide concentration. Statistical analysis of the FT–IR results, showed that, for the silane incorporation, residence time, peroxide concentration, and the interaction between the silane and peroxide concentrations had significant effects. The significant effects for the forma-tion of terminal double bonds were residence time, silane concentration, and peroxide concentration. The experimental results have shown that melt-phase hydrosilyla-tion of polypropylene can be accomplished through this noncatalytic free radical method. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:439–447, 1997     

Preparation of polynorbornene with β-diketonate titanium / MAO catalysts

Summary Polynorbornene was synthesized by β -diketonate titanium / MAO (methylaluminoxane) catalysts. The polymerization activity was up to 8 × 10³ g polymer/(mol Ti h). FT-IR, ¹H NMR, ¹³C NMR and WAXD analyses showed that the polynorbornenes contained both ring-opening metathesis (trans and cis) and addition polymer chain structures and they are amorphous. The portions of trans- and cis- double bonds decreased when the polymerization temperature and Al/Ti molar ratio decreased. In addition, using 1,2-dichlorobenzene, instead of toluene, as the polymerization solvent increased the activity and produced the polymer containing more cis-double bonds. The glass transition temperature of the elastic polymers ranged from 330°C ∼ 400°C. Received: 10 September 2001/Revised version: 15 October 2001/Accepted: 10 December 2001     

Infra-red spectrum of the tight (110) fold in n-C198H398

The alkane n-C₁₉₈H₃₉₈ has been crystallised in both extended chain and once-folded forms and annealed to produce materials with low concentrations of gauche bonds. The concentrations of the specific conformers detected by FTIR spectroscopy at −173 °C are calculated, using measurements on liquid n-hexadecane for calibration: values are all generally less than 2.0 per 100 carbon atoms, with extended chain samples showing values less than 1.0 per 100 carbon atoms. A subtraction spectrum (Once-folded chain sample minus Extended chain sample) shows positive bands at 1298, 1340, 1347 and 1369 cm⁻¹, which are predicted in earlier calculations for a (110) fold, while additional positive bands at 1353 and 1363 cm⁻¹ are assigned, respectively, to gg conformers and (tentatively) to strained gtg or gtg′ conformations.     

Isomeric monoethylenic fatty acids in herring oil

Monoethylenic fatty acids from herring oil were concentrated by chromatography by chromatography on silver nitratesilicic acid columns. Examination of consecutive fractions by open tubular gas chromatography confirmed the preferential elution of longer chain length esters and of esters within one chain length with the double bond closer to the terminal methyl group. Isomeric monoethylenic fatty acids with double bonds in the positions closer to the carboxyl group than the approximate midpoint of the even-numbered fatty acid chains could not be adequately separated by gas chromatography and were determined by ozonolysis. The isomers observed are consistent with primary formation from saturated acids through the action of an enzyme specifically removing hydrogen atoms in positions Δ⁹ and Δ¹⁰ relative to the carboxyl group. Chain extension of particular monoethylenic isomers by two carbon atoms in the C₂₀ and longer chain lengths is apparently influenced by the position of the double bond. This work was carried out in partial fulfillment of MSc requirements at Dalhousie University.