Long-chain branching in free-radical polymerization due to chain transfer to polymer

The Monte Carlo sampling technique is used to investigate the branched structure formation during free-radical polymerization that involves chain transfer to polymer. This method accounts for the history of the generated branched structure and can provide virtually any structural information, because one can observe each polymer molecule directly. In this paper, we investigate the whole molecular weight distribution (MWD) for both pre- and postgelation periods, the MWDs for polymer molecules containing 0, 1, 2, 3, … branch points, the branching density of polymer molecules as functions of both size and the number of branch points, the spatial distribution of the branched chains at the theta state, etc. Contrary to the term ‘long-chain’ branching, many branch chains are relatively small, and the branched structures formed are significantly different from those usually depicted to introduce ‘branched polymers’ in many introductory textbooks. The radii of gyration at the theta state can be approximated by the Zimm-Stockmayer equation for random branching, in spite of various violations against the assumptions used in deriving the equation © 1995 John Wiley & Sons, Inc.     

Transfer and propagation reactions in free-radical copolymerization

The contribution of entropic factors to the penultimate unit effect in free-radical copolymerizations is discussed and exemplified. In addition, significant penultimate unit effects on radical selectivity in transfer reactions are demonstrated and are shown to have a significant polar component. Further, ring-opening copolymerization studies are presented and describe surprising results that seem to originate from strong solvent effects in copolymerization. These results could not have been predicted with current knowledge, prior to the experiment. The present contribution demonstrates in detail that radical reactions are highly complex and there are significant dangers and drawbacks in employing simplified kinetic models when in search of fundamental understanding.     

Post-irradiation free-radical reactions in poly(ethylene terephthalate)

Exposure of poly(ethylene terephthalate) to γ-rays results in the formation of radical I, radical II (tentatively), and to an unassigned radical (III) which is responsible for a central peak in the ESR spectrum. It is believed that completely amorphous samples of polymer contain radicals II and III. On heating, the radicals decay, and the relative proportion of radical III increases. The kinetics of the overall decay process were followed by measuring the decrease in peak height with time. After an initially rapid reaction the decay of the radical population conformed to second order kinetics. An Arrhenius plot of the logarithm of specific rate versus 1/T indicated two lines which intersected at 72°C, which is close to the glass transition temperature. The activation energies were 112 kcal/mole above 72° and roughly 25 kcal/mole below 72°C. Reference to reports in the literature suggests that this decay can be explained by long-range movement of the polymer molecules, even in the glassy solid. The decay of radical I in the crystalline regions of an oriented sample was shown to follow first-order kinetics. As the decay occurs at temperatures as low as 100°C (the melting point is about 260°C), it seems that decay by normal physical movement is unlikely. The results might be explained by invoking the hypothesis of chemical migration of free radical sites by hydrogen atom hopping.     

Chain transfer in ethylene polymerization

Chain transfer constants in the homogeneous free-radical polymerization of ethylene at 1360 atm. and 130°C. have been determined for over 50 compounds, including nearly 300 hydrocarbons. The effects of substitution, unsaturation, and ring strain in the transfer agent molecule on the reactivity of its C? H bonds in chain-transfer reactions with a polyethylene growing chain are analyzed. Qualitatively, these trends parallel those found for simple radicals attacking simple molecules. However, the principle that the reactivity of a compound is the sum of the reactivities of all reactive bonds, which is well established for simple radicals and transfer agents, is found not to be true in ethylene polymerization. It is postulated that the deviations from this principle are due to steric factors which become very important when the free radical is bulky. The transfer constants measured in ethylene polymerization are also compared with transfer constants in other systems. A strong correlation is found between the transfer constants in ethylene and published data on rates of abstraction of hydrogen atoms by methyl radicals.     

Pressure and temperature dependence of the propagation rate coefficient in free-radical polymerization of butyl methacrylate

The propagation rate coefficient in free-radical bulk polymerization of butyl methacrylate has been studied at temperatures between −10°C and 60°C up to a maximum pressure of 2 000 bar. The data were obtained by means of the PLP-GPC technique which combines pulsed-laser polymerization with product analysis by gel-permeation chromatography.     

Short-chain branching in γ-radiation-induced polymerization of ethylene

In an attempt to elucidate the mechanism of chain-branch formation in the polymerization of ethylene, the effect of reaction conditions on short-chain branching in γ-radiationinduced polymerization of ethylene was investigated by using infrared spectroscopy. The concentration of methyl groups, i.e., the frequency of short-chain branching, increases with temperature and pressure and is independent of ethylene conversion to polymer and radiation intensity. The number of methyl groups per polymer molecule increases almost proportionally with the degree of polymerization. These facts indicate that short-chain branching occurs mainly by the mechanism of intramolecular hydrogen transfer. The effect of pressure on the rate of chain branching can be postulated by considering the transition state to be six-membered rings in hydrogen transfer reactions. The activation energy of chain branching is found to exceed that of propagation by 6 kcal./mole.     

Diphenylnitrenium ion: cyclization, electron transfer, and polymerization reactions

Reactions of diphenylnitrenium ion were examined using laser flash photolysis (LFP), product analysis, and computational modeling using density functional theory (DFT). In the absence of trapping agents, diphenylnitrenium ion cyclizes to form carbazole. On the basis of laser flash photolysis experiments and DFT calculations it is argued that this process is a concerted cyclization/proton transfer that forms the H-4a tautomer of carbazole. Additional LFP experiments and product studies show that diphenylnitrenium ion reacts with electron-rich arenes (e.g., N,N-dimethylaniline, diphenylamine, and carbazole) through an initial one-electron transfer. The radical intermediates formed in this step then couple to form dimeric products. Secondary reactions between the diphenylnitrenium ion and these dimers results in the formation of oligomeric materials.     

Role of chain transfer in heterogeneous polymerization of ethylene

The role of chain transfer was studied for the radiation-induced polymerization of ethylene in precipitating media, namely n-butyl alcohol, tert-butyl alcohol and their mixtures. The affinities of those solvents for polyethylene are similar, but the chain-transfer coefficient of n-butyl alcohol is larger than that of tert-butyl alcohol. The polymerizations were carried out in a reactor of 100 ml under a pressure of 300 kg/cm², at 60°C, dose rate of 3.07 × 10⁴–1.75 × 10⁵ rad/hr in the presence of 50 ml of solvents. The polymerization in tert-butyl alcohol shows the kinetic behavior characteristic of a heterogeneous polymerization, such as rate acceleration, high dose rate dependence of polymerization rate, and low dose rate dependence of polymer molecular weight, whereas the polymerization in n-butyl alcohol does not exhibit such behavior and gives polymer having a molecular weight much lower than that of polymer obtained in tert-butyl alcohol. The polymer formed in tert-butyl alcohol exhibits a bimodal molecular weight distribution measured by gel permeation chromatography. In mixed tert-butyl alcohol and n-butyl alcohol solvent, with increasing fraction of n-butyl alcohol, the two peaks not only shift to lower molecular weight but the higher molecular weight peak becomes relatively small. Eventually, the polymer formed in n-butyl alcohol exhibits a unimodal distribution. Those results are well explained on the basis of the proposed scheme for heterogeneous polymerization.     

Accurate ab initio prediction of propagation rate coefficients in free-radical polymerization: Acrylonitrile and vinyl chloride

A systematic methodology for calculating accurate propagation rate coefficients in free-radical polymerization was designed and tested for vinyl chloride and acrylonitrile polymerization. For small to medium-sized polymer systems, theoretical reaction barriers are calculated using G3(MP2)-RAD. For larger systems, G3(MP2)-RAD barriers can be approximated (to within 1 kJ mol⁻¹) via an ONIOM-based approach in which the core is studied at G3(MP2)-RAD and the substituent effects are modeled with ROMP2/6-311+G(3df,2p). DFT methods (including BLYP, B3LYP, MPWB195, BB1K and MPWB1K) failed to reproduce the correct trends in the reaction barriers and enthalpies with molecular size, though KMLYP showed some promise as a low cost option for very large systems. Reaction rates are calculated via standard transition state theory in conjunction with the one-dimensional hindered rotor model. The harmonic oscillator approximation was shown to introduce an error of a factor of 2–3, and would be suitable for “order-of-magnitude” estimates. A systematic study of chain length effects indicated that rate coefficients had largely converged to their long chain limit at the dimer radical stage, and the inclusion of the primary substituent of the penultimate unit was sufficient for practical purposes. Solvent effects, as calculated using the COSMO model, were found to be relatively minor. The overall methodology reproduced the available experimental data for both of these monomers within a factor of 2.     

Gel formation in free radical polymerization via chain transfer and terminal branching

Gel formation in free-radical polymerization via chain transfer to polymer, recombination termination, and terminal branching due to either chain transfer to monomer or disproportionation termination is investigated using the method of moments. It is found that no gel can possibly form in the systems consisting of initiation, propagation, and one of the above reactions. However, systems with the following combination of reactions are found to be capable of gelling. They are: chain transfer to polymer + recombination termination; chain transfer to polymer + terminal branching due to disproportionation termination; and terminal branching due to transfer to monomer + recombination termination. Systems with the following combination of reactions are incapable of gelling; transfer to polymer + terminal branching due to transfer to monomer; and terminal branching due to disproportionation termination + recombination termination. An examination of the gelation mechanisms reveals that the formation of multivinyl macromonomers during the course of polymerization is the reason that systems involving terminal branching gel. Sol/gel diagrams are generated to give critical kinetic parameters required for gelation. It is found that terminal branching does not always promote gelation due to the adverse effect on chain length through chain transfer to monomer and termination by disproportionation, reactions which generate terminal double bonds. © 1994 John Wiley & Sons, Inc.     

Kinetics of ethylene polymerization reactions with chromium oxide catalysts

Principal kinetic data are presented for ethylene homopolymerization and ethylene/1‐hexene copolymerization reactions with two types of chromium oxide catalyst. The reaction rate of the homopolymerization reaction is first order with respect to ethylene concentration (both for gas‐phase and slurry reactions); its effective activation energy is 10.2 kcal/mol (42.8 kJ/mol). The r₁ value for ethylene/1‐hexene copolymerization reactions with the catalysts is ～30, which places these catalysts in terms of efficiency of α‐olefin copolymerization with ethylene between metallocene catalysts (r₁ ～ 20) and Ti‐based Ziegler‐Natta catalysts (r₁ in the 80–120 range). GPC, DSC, and Crystaf data for ethylene/1‐hexene copolymers of different compositions produced with the catalysts show that the reaction products have broad molecular weight and compositional distributions. A combination of kinetic data and structural data for the copolymers provided detailed information about the frequency of chain transfer reactions for several types of active centers present in the catalysts, their copolymerization efficiency, and stability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5315–5329, 2008     

Synthesis and reactivity of a fluorinated N-alkylmaleimide towards free-radical grafting and polymerization reactions

A new fluorinated N-alkylmaleimide, N-(2-pentadecafluoro-n-octanoyloxy)-ethyl)maleimide (FOMI), was synthesized from perfluorooctyl carboxylic acid (FOCA) by reaction of the furan adduct of maleic anhydride (MAH) with ethanolamine and esterification of the resulting alcohol-imide. The reactivity of FOMI in free-radical reactions such as copolymerization with butyl vinyl ether (BVE) and grafting onto olefin copolymers (OCP) was investigated. Copolymerization with 2/1 molar excess BVE yields a copolymer with 63 mol% FOMI in moderate conversion, indicating that homopropagation of the maleimide prevails over comonomer alternation. The thermal stability of FOMI is quite poor, with onset of the pyrolytic loss of the N-perfluoroalkylcarboxyethyl moiety just above 100 °C, similarly to that of FOCA. This restricts the possibilities for direct melt functionalization of OCP with FOMI. To bypass this limitation three distinct approaches were preliminarly investigated. These were respectively based on either direct grafting of FOMI at low temperature, or reaction of a MAH-grafted polyolefin with a low molecular weight amine or amino-terminated oligomer bearing the perfluoroheptyl group. A functionalization degree FD=2% was achieved by solution grafting of a linear very low density polyethylene (VLDPE) with FOMI and Perkadox 16 as the free-radical initiator. The relatively high grafting efficiency was attributed to the growth of some oligomeric FOMI grafts onto the OCP. The alternative routes of post-modification of VLDPE-g-MAH by imidization with an amidoamine obtained by amidation of FOCA with a diamine or an amino-terminated FOMI/BVE oligomer, respectively, were also preliminarly investigated.