Polymerization, anionic styrene rate constants

A kinetic study for the polymerization of styrene, initiated with n BuLi, was designed to explore the Trommsdorff effect on rate constants of initiation and propagation and polystyryl anion association. Initiator association, initiation rate and propagation rates are essentially independent of solution viscosity, Polystyryl anion association is dependent on media viscosity. Temperature dependency correlates as an Arrhenius relationship. Observations were restricted to viscosities less than 200 centipoise. Population density distribution analysis indicates that rate constants are also independent of degree of polymerization, which is consistent with Flory s principle of equal reactivity. 

Auguste S, Edwards HGM, Johnson AF et al. (1996) Anionic polymerization of styrene and butadiene initiated by n-butyllithium in ethylbenzene determination of the propagation rate constants using Raman spectroscopy and gel permeation chromatography. Polymer 37 3665-3673... 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l -4). For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7.) Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8). 

Alkyllithium-transition metal halide catalysis is completely different from the sodium ketyl and alfin catalysis. Natta, Danusso, Scanesi and Macchi (36) have found that the polymerization of styrene and substituted styrenes by titanium tetrachloride-triethyl aluminum catalysts was different from the above anionic systems. A plot of the log of the rate of the polymerization against Hammett s sigma constant produced a straight line with a rho value of —1.0. Electron releasing groups facilitated this polymerization. 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

The relative reactivity of ions and ion pairs are very different in anionic and cationic polymerizations. In anionic systems, the reactivity of ions (kp ) is similar to that of solvent-separated ion pairs (kp x), but much higher than that of contact ion pairs (kp-r) [12]. For example, the rate constants of propagation of styrene at ambient temperature are kp = 10s moI Lsec 1, kp s 104 mol-1L sec and kp c 101... 

Figure 5 Effect of temperature on rate constants of propagation, depropagation, transfer to monomer, transfer to triflate anion, and indan formation in the carbocat-ionic polymerization of styrene (From Ref. 292).

Polymerization reactions require stringent operating conditions for continuous production of quality resins. In this paper the chain-growth polymerization of styrene initiated with n-butyllithium in the presence of a solvent is described. A perfectly mixed isothermal, constant volume reactor is employed. Coupled kinetic relationships descriptive of the initiator, monomer, polystyryl anion and polymer mass concentration are simulated. Trommsdorff effects (1) are incorporated. Controlled variables include number average molecular weight and production rate of total polymer. Manipulated variables are flow rate, input monomer concentration, and input initiator concentration. The... 

Polymerization of styrene initiated by n-butyl lithium in benzene was investigated by a spectrophotometric technique by Worsfold and Bywater156). Concentration of polysty-ryl anions was monitored by their absorbance at 334 nm, while the concentration of the unreacted styrene was determined by its absorbance at 291 nm. The results are shown graphically in Fig. 23. Concentration of polystyryl anions increases with time and eventually reaches its asymptotic value, being constant afterwards. This observation indicates the stability of these species. On the other hand, the concentration of styrene decays in sigmoidal fashion. This classic study unequivocally demonstrated the living character of the resulting polymers, and therefore it was justified to identify the rate of increase of the absorbance at 334 nm with the rate of initiation. 

The yield of the absorption at 320 m/x was found to rise with concentration in a way suggesting that it is formed by anionic process—e.g., by protonation of a styrene anion by its geminate partner (31). If it is a polymerizing radical, then the rate constant for addition of the benzyl-type radical to styrene must be at least KP-lOW-1 sec.-1. The yield of the observable anion seemed much less dependent on concentration, which is consistent with the view that it is formed by those electrons (G.— 0.2) which escape from the spur. Absorptions with peaks close to 320 m/x are seen for all the other solvents above, but the component at the longer wavelength is seen only with the aliphatic hydrocarbons. For methanol solutions this may be because of rapid protonation. 

Rate Constants for the Transitions between the Different Ionic States of the Active Chain End in the Anionic Polymerization of Styrene. The cases treated in this section demonstrate the high information capacity of MWDs which makes this determination extremely useful for solving kinetic problems. In addition, these examples reveal how it is possible to test experimentally determined MWDs by kinetic measurements. 

The rate constant for propagation of (polystyrene) Na+ in tetrahydrofinan for 25°C was found to be 400 1-mol s. The rate constants for proton transfer to the anion from water and ethanol were 4000 l-moT s and 4 1-mol sr, respectively. Which of these impurities (water or ethanol) is more likely to inhibit high polymer formation in sodium-catalyzed anionic polymerization of styrene in tetrahydrofuran ... 

Both the solvent and gegen ion have a pronounced influence on the rates of anionic polymerizations. The polymerization rate generally increases with increasing polarity of the solvent for example, = 2.0 dm mol s for the anionic polymerization of styrene in benzene, but = 3800 dm mol s whm the solvent is 1,2-dimethoxy-ethane. Unfoitunately, the dielectric constant is not a useful guide to polarity or solvating power in these systems, ask = 550 dm mol s when the solvent is changed to THF, whose dielectric constant e is higher than e for 1,2-dimethoxyethane. 

Additional well-defined side-chain liquid crystalline polymers should be synthesized by controlled polymerizations of mesogen-ic acrylates (anionic or free radical polymerizations), styrenes (anionic, cationic or free radical), vinyl pyridines (anionic), various heterocyclic monomers (anionic, cationic and metalloporphyrin-initiated), cyclobutenes (ROMP), and 7-oxanorbornenes and 7-oxanorbornadienes (ROMP). Ideally, the kinetics of these living polymerizations will be determined by measuring the individual rate constants for termination and... 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

The polymerization rate depends very much on the proportion of free macroanions present. In the anionic polymerization of styrene with sodium as gegenion in THF, kinetic measurements (see below) gave the rate constants for polymerization via free macroanions as /C(-> = 65000 dm mol s" and fc(+) = 80 dm mol s for the polymerization via ion pairs (Table 18-2). According to the relation Vp = kp[P ] [M], the rate of the propagation reaction depends on the active species concentration [P ] as well as on the rate constant kp. But in this system, the dissociation equilibrium constant Kjy for ion pairs into free ions is only 10" mol/dm. If the polymerization is carried out in a solution with 10 mol/dm, then the proportion of free ions is consequently only (10 10 ) = 0.01, that is, 1 %, with 99% of the active species being ion pairs. Thus, despite a much lower rate constant, the polymerization rate is also determined to a considerable extent by the ion pairs. 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

Aromatic radical anions such as sodium naphthalenide react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions as shown in Scheme 5 (R=H, CH3). Although the equilibrium between the radical anion of the monomer and the aromatic radical anion lies far to the left because of the low electron affinity of the monomer compared to the condensed aromatic hydrocarbon, this is an efficient initiation process because the resulting monomer radical anions undergo tail-to-tail dimerization reactions with rate constants that approach diffirsion control (fed=10 -10 Imol" s" )." Aromatic radical anions can be used to initiate the polymerization of styrene and 1,3-diene monomers. 

---