Polymerisation, rate

The unsaturation present at the end of the polyether chain acts as a chain terminator ia the polyurethane reaction and reduces some of the desired physical properties. Much work has been done ia iadustry to reduce unsaturation while continuing to use the same reactors and hoi ding down the cost. In a study (102) usiag 18-crown-6 ether with potassium hydroxide to polymerise PO, a rate enhancement of approximately 10 was found at 110°C and slightly higher at lower temperature. The activation energy for this process was found to be 65 kj/mol (mol ratio, r = 1.5 crown ether/KOH) compared to 78 kj/mol for the KOH-catalysed polymerisation of PO. It was also feasible to prepare a PPO with 10, 000 having narrow distribution at 40°C with added crown ether (r = 1.5) (103). The polymerisation rate under these conditions is about the same as that without crown ether at 80°C. 

Chain transfer also occurs to the emulsifying agents, leading to their permanent iacorporation iato the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerisation rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly(vinyl alcohol) impurity strongly retards polymerisation (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents and lower the molecular weight of the product. 

It is seen from equations (2.5) and (2.6) that while an increase in concentration of initiator increases the polymerisation rate it decreases the molecular weight. 

The polymer may be prepared readily in bulk, emulsion and suspension, the latter technique apparently being preferred on an industrial scale. The monomer must be free from oxygen and metallic impurities. Peroxide such as benzoyl peroxide are used in suspension polymerisations which may be carried out at room temperature or at slightly elevated temperatures. Persulphate initiators and the conventional emulsifying soaps may be used in emulsion polymerisation. The polymerisation rate for vinylidene chloride-vinyl chloride copolymers is markedly less than for either monomer polymerised alone. 

One of the basic assumptions of this theory is that the polymerisation rate can be computed from the transition rate from an initial electronic state E to a final one Ef of the crystal at a given polymerisation state. The energies of these states depend on the nuclear configuration and their changes around the equilibrium positions for the initial and final electronic states can be expressed (43) in terms of vibrational oscillators which at a given temperature are either classical 1ui)c[Pg.181]

Figure 1 Schematic representation of the variation of the polymerisation rate, R, with monomer concentration A, EVE in benzene B, EVE in diglyme C, EVE in CH2C12 D, styrene in CH2C12 E, isobutene in CS2...

It does seem curious that no p+ between the extremes of ca. 104 and 108 1 mol"1 s 1 have been reported for chemically initiated polymerisations. The most likely reason is the sheer technical difficulty of measuring polymerisation rates for such systems with [M] in the range of 4-8 mol/1 and of determining reliably the nature and the concentration of the propagating species and the ratio of unpaired to paired ions. This the more so, since nothing less than the high-vacuum techniques used by Sigwalt, Pepper, Plesch, et al. [4] which have fallen into disuse would be required for such an enterprise. 

It follows from Equation (5.14) that the apparent activation energy, ER, of the polymerisation rate, is given by... 

What we require now in order to consolidate our understanding is more work with a variety of monomers and solvents, with the simplest possible initiator systems, aimed at obtaining the maximum amount of information on the polymerisations (rates, equilibria, conductivities, constituents of the reaction mixtures before and after neutralisation) and on the polymers (existence, nature and concentration of end-groups, DP distributions). There are unfortunately still too many who think they can base a valid theory on the determination of only one or two features of a polymerisation system. 

For example, Melville [26] studied the ultrasonically induced polymerisation of monomers such as styrene, methyl methacrylate and vinyl acetate in the presence and absence of polymethyl methacrylate and found that the polymerisation rates ( 1 % conversion/h) were not substantially increased by the presence of polymer. He concluded, in contrast to Driscoll, that the degradation of polymer was not the major source of radical production. Using hydroquinone as an inhibitor, he was able to deduce, from retardation times, that the rate of radical production was 2 X 10 mol dm s. A typical value for radical production using as an example the thermal initiation of AZBN (10 mol dm ) at 60 °C is 2 x 10" mol dm s" ... 

For the thermally initiated (potassium persulphate) emulsion polymerisation of styrene, we have observed [73] a twofold increase in the initial polymerisation rate in the presence of ultrasound (20 kHz), the increase being dependent upon the level of surfactant employed. Several workers have suggested that possible explanations for the observed increase in rate are ... 

To explain his observed variations of polymerisation rate with time, reaction volume and acoustic intensity, Kruus adopted the following reaction mechanism in which he regarded cavitation bubbles as a reactant and represented their concentration as [C]. 

Fig. 5.37. Polymerisation rate of PMMA as a function of the square root of acoustic...

By sonically inducing the polymerisation of methyl methacrylate (MMA), Price [65] has extended the work of Kruus and studied the effect of the absence and presence of the initiator azobisisobutyronitrUe (AIBN). Similar conversions to Kruus (2-3% per hour) were obtained in the absence of initiator at 25 °C. However considerable improvements in the polymerisation rate were observed when 0.1% of initiator were used (Fig. 5.40), the reaction appearing to become autocatalytic. This no doubt is due to the faster production of polymer in the initiated system (faster initiation due to enhanced initiator breakdown) which is then available for degradation to produce more free radical entities. 

Stoessel [87] has reported using ultrasound to promote the ring opening of polycarbonate rings (Fig. 5.44) whilst Price [88] has studied the ring opening polymerisation of poly(dimethylsiloxane) (Fig. 5.45). Price found that in the presence of ultrasound, and the presence of sulphuric acid at room temperature, faster polymerisation rates and higher molar masses were obtained. 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

L-monomer. The observed effect was, however, even greater than anticipated. In fact, Idelson and Blout (50), who repeated these experiments in sodium methoxide initiated polymerisation that yields a high-molecular weight polypeptide, found enormous effects which are shown in Fig. 18. Addition of even 5% of D-isomer to an L-monomer reduced the polymerisation rate by a factor of 3, and the 50 50 mixture polymerised 17-times more slowly than either isomer. The co-operative effect of this... 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5] ... 

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

Curves presented in Figure 3.13 testify to the large specificity of supported Ziegler-Natta catalysts regarding the kind of monomer some centres that polymerise ethylene do not polymerise propylene (or higher a-olefins, which may also be differentiated by particular catalyst centres, depending on the structure of the oc-olefin, e.g. branched as in 3-methyl-1-butene or not branched). Therefore, no hints about the monomer reactivity can be obtained by simple comparison of polymerisation rates without simultaneous estimation of the concentration of active sites [241]. 

Many supported highly active catalysts show behaviour similar to case B in Figure 3.13 the polymerisation rate may also start at a maximum value and then decrease more or less rapidly with time. Such kinetic behaviour is also characteristic of some homogeneous catalysts. Other polymerisation systems show no acceleration period but have a polymerisation rate that remains almost constant with time this is a rare case and relates, for instance, to 4-methyl-l-pentene polymerisation with MgCl2-supported catalysts containing phthalate esters as well as to ethylene polymerisation with the Cp2TiCl2—[Al(Mc)0]x catalyst (apart from a short settling period in the latter case) [240],... 

Although the polymerisation rate increases with increasing temperature, ethylene and 7-olefin polymerisations in the presence of most Ziegler-Natta catalysts are carried out at moderately elevated temperature, usually not exceeding 100 °C. This is due to destabilisation of the system, which occurs when temperature is raised beyond a certain critical value. There are, however, few catalysts that operate in industrial polymerisation processes at temperatures above 200 °C [51,240]. 

For the majority of olefin polymerisations with heterogeneous Ziegler Natta catalysts, the polymerisation rates, Rv, are proportional to the concentrations of procatalyst (MtX ) and monomer (M), but do not depend on the concentration of alkylaluminium activator (A) as long as a threshold concentration is maintained [37] ... 

This means that there is practically no dependence of the olefin polymerisation rate on the activator/procatalyst molar ratio over a wide range. In some... 

The equation describing the polymerisation rate is simply expressed in terms of the dependence on the concentration of propagation active sites, Cp, and the monomer concentration, [M], where Cp is assumed to be of constant value (dCp /dt = 0), i.e. the polymerisation system is under steady state conditions ... 

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