Gel-gel reaction in RAf and RA2+RBf polymerisations

Summary The amount of gel-gel reaction in RA_f and RA₂+RB_f random polymerisations is evaluated. For a given functionality f and stoichmetric amounts of reactants, half as much gel-gel reaction occurs in RA₂+RB_f polymerisations by complete reaction as in RA_f polymerisations. This difference is readily understandable in that the formation of a loop requirestwice as many pairs of groups to react in the former type of polymerisation as in the latter. Gel-gel reaction is necessary for the formation of the network structure at complete reaction. However, it also leads to network imperfections, viz. inelastic loops.Dedicated to Prof. Walther Burchard on the occasion of his 60th birthday     

Effects of functionality and temperature on gel points in random polymerisation

First, the relationship between gel point and average functionality for an RA₂+R′B_f type polymerisation has been investigated by measuring the product of extents of reaction at gelation (α_c) for sebacoyl chloride (SC)/polyoxyproplyene (POP)diol/POP triol mixtures reacting at various initial dilutions in diglyme as solvent at 60°C. Intramolecular reaction always delays gelation and a generalisation of the Ahmad-Stepto gel point expression has been used to interpret the gelation data in terms of the ring-forming parameter λ′_ab. From the variation of λ′_ab with initial dilution, values of b, the effective bond length of the chain forming the smallest ring structure, have been derived. It is found that b decreases with average polyol functionality (f_w). This decrease apparently compensates for the increase in λ′_ab with f_w, so that ring formation is less sensitive to functionality than may have been expected. It is found that the Ahmad-Stepto expression does not predict a consistent relationship between λ′_ab, f_w and the gel dilution of reactive groups. Second, the effect of temperature on the gel point has been studied by measuring α_c for SC/POP triol mixtures reacting at various initial dilutions in diglyme as solvent at 27° C, 40° C and 60° C. At a given temperature, b decreases as triol molar mass increases, indicating that the POP residue of the chain forming the smallest ring structure is more flexible than the SC residue. This result is in keeping with those from previous investigations,^1,2 For a given triol, α_c decreases as temperature increases, indicating an increase in chain stiffness with temperature. Values of d In <r²>/dT are derived and found to be larger than those for linear chains.     

Computational study of network formation of phenolic resins

The network formation of phenol-formaldehyde resin was investigated by the Monte Carlo (MC) simulation technique. The observed value of the gel point, p_c, was exactly determined as a critical extent of reaction, where the Mw/Mn in soluble parts is maximum. The MC simulation with the cubic percolation theory was applied to the gelation of phenolic resins and gave an exact gel point, which was in excellent agreement with the observed value of p_c. The simulation showed that the intramolecular reaction occurred frequently with increasing the gel fraction beyond the gel point. The structural analysis of the maximum cluster with the computer strongly supported the contribution of the intramolecular reaction to the network formation of the gel.     

Theoretical and experimental studies of network formation and properties

A survey of recent work on pre-gel intramolecular reaction and gelation in oxypropylene-triol and tetrol-based polyester and polyurethane network-forming systems is presented, and relationships between the properties of the networks formed at complete reaction and the product of extents of reaction at gelation (α_c) are discussed. In contrast to that in corresponding linear reaction systems, the amount of pre-gel intramolecular reaction in the triol-based polyurethanes is never negligible, even in bulk. Kilb's model of RA₂ + RB_f type gelling systems has been reanalysed and a more accurate condition for gelation evaluated. The shear moduli and T_gs of dry networks are found to increase as α_c increases, indicating that these properties are strongly dependent on the amount of pre-gel loop formation. Extrapolated values of the shear moduli indicate that perfect, affine rather than phantom networks would be formed by reaction systems having the ideal gel point [α_c = (f ? 1)^?1].     

Directional polymerisation and electronic properties of a cyanosubstituted dialkylpolyphenylenevinylene

Summary The synthesis of a monomer allowing for directional polymerisation based on the Knoevenagel condensation reaction is presented. The free running polymerisation gave a molecular weight (M _w = 24650 g mol^-1) polymer product. The polymerisation reaction could be followed by size exclusion chromatography (SEC) and the molecular weight of the polymer product could be controlled by termination of the polymerisation reaction at a suitable time. When terminated before completion a lower molecular weight product was obtained that could be separated into oligomers using preparative SEC. The polymer product and the oligomers were found to have very similar physical properties in terms of the optical band gaps, electronic energy levels and charge carrier mobilities as studied by UV-vis and pulse radiolysis time resolved microwave conductivity (PR-TRMC). Finally the positions of the filled energy levels were determined using ultraviolet photoelectron spectroscopy (UPS).This revised version was published online in September 2004. Due to technical problems, the previous version contained an incomplete PDF.     

Sodium metabisulphite initiated aqueous polymerisation of methyl methacrylate

Sodium metabisulphite (Na₂S₂O₅) has been found to initiate the aqueous polymerisation of methyl methacrylate (MMA) at 35°C in phosphate buffer solutions of pH 6.85 and a constant ionic strength (μ) of the media in an inert atmosphere of pure nitrogen. The reaction has a well defined induction period which is a function of the concentrations of the initiator, of the monomer and also of temperature. The polymerisation is signalled by the sudden appearance of a haze at the end of the induction period in a given run, and the polymer separates out as a coarse precipitate during the progress of the polymerisation reactions. When the conversion is over 50 per cent complete, the polymerisation media looks like a thick curd if the monomer concentration is relatively high. The rate of polymerisation is found to decrease with conversion or time in a given run, and the initial rate (v_p), obtained by extrapolating the linear yield/time versus time curves to zero time, keeping the conversion below 10 percent, is found as where (I) = initiator concentration in the range, (0.26 to 3.94) × 10^?3 (mol dm^?3), and (M) = monomer concentration in the range 0.019 to 0.141 (mol dm^?3). At high initiator concentrations, the rate of polymerisation is found to decrease. In a given run, the viscosity average molecular weights (M _v) of the polymers is found to increase quickly with a conversion of up to 25 to 30 percent, and then slowly with the further increase in conversion. (M_v) however is found to decrease with the increase of the initiator concentrations at a given conversion but increases with the increase of the monomer concentrations. Hydroquinone inhibits the polymerisation reactions, whereas air is found to increase the induction period, but later enhances the polymerisation rate in the same run. It has been shown that the bisulphite addition reaction of MMA is not important under the experimental conditions, and the polymerisation occurs by the free radical mechanism. The rate constant (k₂) of the reaction, has been estimated from the analytical data as, k₂ = 14.62 × 10^?2 (dm³ mol^?1 s^?1) at 35°C.     

A kinetic approach to multifunctional polymerization with cyclization

A kinetic model for the step growth polymerization of multifunctional monomers RA_f valid up to the gel point has been presented which accounts for the intramolecular reactions. From the Jacobson-Stockmeyer equation, the average rats constant for this step has been derived and is found to be chain length dependent. A computation scheme has been evolved by which the molecular weight distribution of the linear, branched as well as cyclized polymer can be determined up to the gel point. It is found that even though the molecular weight distribution of cyclic oligomers is little affected for the range of the values of the intramolecular rate constant (k_c) studied, the average chain length μ_n and the polydispersity index p of the polymer formed are extremely sensitive to k_c near the gel point. Numerical computations confirm that as the gel point is approached, larger polymer chains are formed which have a higher tendency to react intramolecularly. However, even at high conversions the number of the cyclic rings per polymer chain remains small, which is consistent with experimentally observed results.     

State multiplicity in CSTR-separator-recycle polymerisation systems

This article continues earlier work (Comput. Chem. Eng. 24 (2000) 209) concerning the design and control of isothermal reactor-separator-recycle systems. The multiplicity behaviour of six reaction systems of increasing complexity, from one-reactant, first-order reaction to chain-growth polymerisation, is investigated. Below a critical value of the plant Damkohler number, Da<Da^cr, the only steady state involves infinite flow rates. Feasible steady states become possible if the critical value is exceeded, Da>Da^cr. For one-reaction systems, one stable steady state is born at a transcritical bifurcation. For consecutive-reaction systems, including polymerisation, a fold bifurcation can lead to two feasible steady states. Moreover, the transcritical bifurcation is destroyed when two reactants are involved. If the gel-effect is included, a maximum of four steady states are possible. When multiple steady states exist, the achievable conversion is constrained by the instability of the low-conversion branch. This has practical importance for polymerisation systems when the radicals’ quasi-steady state assumption is not valid or the gel effect is significant.     

Calculation of heat of polymerisation: group-contribution method

Heat of Polymerisation of a polymeric system is largely dependent on its monomer. So depending upon the structure of the monomer, the prediction of the heat of polymerisation can be done. In the present study the structural features of the monomer, that is undergoing polymerisation reaction, has been assumed to be responsible for the heat of polymerisation. To calculate the heat of polymerisation of a particular reaction, Group-Contribution method (GC-method) has been used. Certain values (S_R value) have been assigned for the different structural units, present in the monomers. The S_R values have been obtained by correlating of the heat of polymerisation of a large number of polymers. Addition of the S_R values corresponding to the structural units of the monomer will give the heat of polymerisation of that particular reaction. This GC-method is applicable where liquid monomers form condensed polymers. This empirical method gives quite satisfactory results in calculating the heat of polymerisation for a large number of polymeric systems. Received: 23 October 1998/Revised version: 9 December 1998/Accepted: 15 December 1998     

Determination of initiator decomposition constant in a heterogeneous polymerisation system

A new method for determination of initiator decomposition constant k_d is described based on the use of instantaneous polymerisation rates for two similar experiments run with different initiator concentrations. The constant found will be specific for the polymerisation system used. If the propagation and termination rate constants for the monomer are known from other studies the initiator efficiency factor f can then be obtained. To verify the validity of the method, poly(vinyl chloride) latexes were prepared by potassium persulphate (K₂S₂O₈) initiated polymerisation of submicron-sized monomer droplets. The monomer droplets were obtained by diffusion of vinyl chloride monomer into a preformed emulsion of water-insoluble oil droplets. The polymerisation was performed in an isothermal calorimeter. From the measured heat evolution, the instantaneous polymerisation rates were calculated. The thermal decomposition constant of potassium persulphate k_d was determined to be 2·5×10^-6s^-1 at 51°C. In the same system the order of reaction with respect to the initiator concentration was found to be 0·50. © 1998 SCI.     

Effect of intramolecular cycles on the polycyclotrimerization of aromatic dicyanates

The gel conversions (α_gel) for the polycyclotrimerization of aromatic dicyanates are significantly higher than the classical mean-field value of 0.5. The reasons for the higher gel conversion, which is consistent with all experimental results of different structures of monomers, were inductively attributed to the accessibility effect of the functional group and the substitution effect, as well as the effect of the intramolecular cyclization. Nevertheless, the former two effects on the gel conversion can be quantitatively represented in terms of the extent of the intramolecular cyclization. Some theoretical expressions (including gel conversion and crosslink density with respect to the conversion) were derived by use of the recursive method with due consideration of the intramolecular cyclization. These expressions (with only one experimental parameter, α_gel) were found to be effective in describing gel fraction–conversion data for various polycyanurates. A dramatic change in the product value of ΔC_p · T_g was also found in the vicinity of the gel point for all different structures of aromatic dicyanate systems. The dramatic change in ΔC_p · T_g occurs at the gel point rather than the expected mean-field gel conversion of 0.5, presumably due to the intramolecular cyclization. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1927–1938, 1999     

Studies of thermal oxidative degradation of polypropylene impact copolymer using the temperature rising elution fractionation method

Thermal oxidative degradation of polypropylene impact copolymer has been studied with its fractions obtained using the temperature rising elution fractionation method. The fractions were analyzed using ¹³C NMR, Fourier transform infrared and differential scanning calorimetry measurements, and the chemical structure, isotacticity, conformation and melting point were investigated. It is found that these fractions are composed of a homopolymer or a polymer blend of polypropylene, polyethylene and ethylene–propylene copolymer. The thermal oxidative degradation of each fraction was carried out at 130 °C, and the degradation progress was estimated by the change of molecular weight distribution (from gel permeation chromatography curves). The rate of degradation is found to be dominated by the content of tertiary C? H bonds (propylene unit) and the existence of 3₁ helix conformation corresponding to a crystalline polypropylene part in each fraction. The latter is more evident leading to the degradation reaction path with a lower activation energy. Copyright © 2007 Society of Chemical Industry     

Redox polymerisation—kinetics of polymerisation of acrylamide initiated by the Mn3+-citric acid redox system

The kinetics and mechanism of polymerisation of acrylamide initiated by manganese(III) acetate-citric acid (CA) redox system in aqueous acetic acid and sulphuric acid medium were studied over the temperature range 30-50° C. The overall rates of polymerisation (R_p), disappearance of manganic ion (—R_m), and degree of polymerisation (X?_n) were measured with variation in [monomer], [Mn³⁺], [CA], [H⁺], μ and temperature. The polymerisation is initiated by the organic free radical arising from the Mn³⁺-CA reaction and the termination is by metal ions. The various rate parameters were evaluated and discussed.     

The ROMP catalysts Mo(CO)5PPh3/alkylaluminum with unstrained cyclic and linear dienes as initiators of the addition polymerisation

Summary We study the polymerisation of a series of unstrained cyclic and acyclic dienes with the catalytic system of Mo(CO)₅PPh₃/alkylaluminum. The Mo(CO)₅PPh₃/alkylaluminum catalyse the nonmetathesis polymerisation, through a pair of ionic complexes similar to that of Ziegler-Natta mechanism, and not through the ROMP (Ring-Opening-Metathesis-Polymerisation) mechanism followed by strained olefins. Because of the nature of these complexes the reactivity is lower with respect to that of a common Ziegler-Natta catalyst, such as Cp₂TiCl₂/Et₃Al₂Cl₃, as it is demonstrated, by means of kinetics and ESR measurements, in the case of the polymerisation of of 1,5-cyclooctadiene. Similarly to the Ziegler-Natta catalysts, they give cyclised products in the polymerisation of 1,5-hexadiene and 1,7-octadiene, and the 1,4-cis unit in the case of the polypentadienes. Finally we find that they are active catalysts for the polymerisation of styrene. Received: 12 March 1998/Revised version: 20 April 1998/Accepted: 22 April 1998     

The reactivity of 2-furaldehyde and its derivatives in polymerisation reactions involving the carbonyl bond

The homopolymerisation of 2-furaldehyde, 5-methyl-2-furaldehyde and 2-furyl methyl ketone and their cationic copolymerisation (BF₃. Et₂) with olefins were investigated. All attempts to obtain homopolymers failed because of thermodynamic restrictions. 2-Furaldehyde and α-methylstyrene gave an alternating copolymer of low degree of polymerisation, 2-methyl-2-furaldehyde and α-methylstyrene gave copolymers rich in 5-methyl-2-furaldehyde, indicating a penultimate effect in the growth mechanism. The use of less basic comonomers (cyclopentadiene, styrene, isoprene, etc.) resulted in inhibition or strong retardation of the polymerisation. The use of more basic comonomers (acenaphthy-lene, N-vinylcarbazole) resulted in their homopolymerisation. Furyl methyl ketone inhibited the polymerisation of all comonomers tried. A complex between BF₃. Et₂O and 2-furaldehyde was isolated and characterised. A general mechanism is offered to explain the behaviour of these systems.     

PEO-functionalized polystyrene as polymeric support in metallocene catalysed olefin polymerisation

Summary A new catalyst system based on easily accessible cross-linked (by Diels-Alder reaction) polymeric supports, functionalized with nucleophilic polyethyleneoxide-monomethylether (PEO-M) is presented. The metallocene-MAO complex is noncovalently bonded to the support, avoiding a complicated polymer analogous metallocene synthesis. The polymerisation of propene and ethene with Me₂Si(2MeBenzInd)₂ZrCl₂ as metallocene is performed using this support. The resulting polymers have high molecular weights and melting points and narrow molecular weight distribution. Polypropene is produced with 95% isotacticity similar to that from homogeneous catalysis. The productivities of up to 8600 for polypropene and 1300 for polyethene ( kg polymer / mol Zr h bar ) are comparably high. The product beads exhibit a good morphology, which can be explained by fragmentation processes of the support due to the reversibility of the network formation. Received: 21 February 2001/Revised version: 2 November 2001/ Accepted: 6 November 2001     

Reaction kinetics and product formation in the heterogeneous polymerisation of 2,6-dimethylphenol

In the oxidative condensation of 2,6-dimethylphenol catalysed by copper compounds, poly(2,6-dimethylphenylene oxide) was formed as a particulate solid in both pyridine and pyridine-methanol mixtures. No quinone was produced when the volume fraction of pyridine exceeded 15%. The rate of polymerisation was independent of monomer concentration and first order with respect to catalyst concentration. This first-order dependence ceased when catalyst concentrations exceeded 4 × 10^?2 molar. Above this concentration the polymerisation rate is probably controlled by mass transfer of oxygen from the gas to the liquid phase. The rate of polymerisation increased as the volume fraction of methanol increased. A reaction mechanism is proposed to account for the kinetics; this involves a reversible step. Particle sizes of the dispersed polymers have been measured. Heterogeneous systems can be used to produce polymers of higher molecular weights than those obtained in solution systems.     

The determination of active centres in olefin polymerisation

The kinetics of Ziegler-Natta polymerisations can be described in terms of a spectrum of active sites, each with differing propagation rates and stereospecificity. It is therefore necessary to use complementary methods of measuring active centres to get the whole picture. Higher activity in polymerisation of propylene using TiCl₃ - Et₂AlCl systems can be achieved by varying the catalyst preparation so that the effective surface area is increased or by removing inhibitors. When Et₃Al is used as activatior in place of Et₂ AlCl higher activity is due to a combination of increase in number of centres per mole TiCl₃ and of increase in their propagation rate.     

Synthesis and properties of polyester-based TPUs prepared by solution polymerisation

Abstract

The polyester based thermoplastic polyurethane elastomers (TPUs) were prepared by one-step solution polymerisation to obtain a product with completely linear molecular structure and with excellent solubility. The synthesis technique was determined by investigation of the influence of solvent and catalyst on the viscosity of the reaction system and the mechanical properties of the resulting product. The effects of the molecular structure on the mechanical, damping and crystallisation properties of TPUs were studied in terms of the type of diisocyanate, mass fraction of hard segment, type and molecular weight of soft segment, and type of chain extender. The structure and morphology of TPUs were characterised by FTIR, GPC and AFM analysis. There was microphase separation in the molecules of TPU, in which the hard-segment phase was dispersed in the soft-segment phase.