Styrene in benzene

However, the mechanisms by which the initiation and propagation reactions occur are far more complex. Dimeric association of polystyryllithium is reported by Morton, al. ( ) and it is generally accepted that the reactions are first order with respect to monomer concentration. Unfortunately, the existence of associated complexes of initiator and polystyryllithium as well as possible cross association between the two species have negated the determination of the exact polymerization mechanisms (, 10, 11, 12, 13). It is this high degree of complexity which necessitates the use of empirical rate equations. One such empirical rate expression for the auto-catalytic initiation reaction for the anionic polymerization of styrene in benzene solvent as reported by Tanlak (14) is given by ... 

Styrene in benzene using benzoyl peroxide at 60°C Mayo et al. ) ... 

Data illustrating the relationship of the initial rate to the concentration of monomer at fixed initiator concentration are given in Table X for styrene in benzene and for methyl methacrylate polymerized at various concentrations in the same solvent. If the efficiency / of utilization of primary radicals is independent of the monomer concentration, the quantity given in the last column should be... 

Conditions 6.20 mmol styrene in benzene, 0.0031 mmol catalyst, 20 bar ( Oi I. (1 1), 60 °C, 12 hours, conversions to aldehydes >99%. b Hexane as solvent.c Lower cross-linking degree. d Higher cross-linking... 

Another, more indirect but perhaps more efficient method, would be to determine K for a number of typical systems, perhaps by use of model compounds, and then to select for kinetic experiments initiator systems for which K is so great that [Pn+] is effectively equal to c0, so that then the simple Equation (1) with [Pn+] = c0 is applicable. The trouble is that this method will probably only work for fairly polar solvents, because it is to be expected that Kp will be smaller, the less polar the solvent. This effect is probably one of the factors responsible for the improbably low kp value obtained by Higashimura for styrene in benzene solution [7]. In any case, for solvents of low polarity the participation of paired cations must be taken into account, which makes the relevant equations rather more complicated, but does not alter the relevance and importance of equilibrium (i). 

Fig. 3-6 Determination of initiator chain-transfer constants in the t-butyl hydroperoxide initiated polymerization of styrene in benzene solution at 70°C. After Walling and Heaton [1965] (by permission of American Chemical Society, Washington, DC.

Consider the polymerization of styrene initiated by di-t-butyl peroxide at 60°C. For a solution of 0.01 M peroxide and 1.0 M styrene in benzene, the initial rates of initiation and polymerization are 4.0 x 10 11 and 1.5 x 10 7 mol L 1 s 1, respectively. Calculate the values of (jkj), the initial kinetic chain length, and the initial degree of polymerization. Indicate how often on the average chain transfer occurs per each initiating radical from the peroxide. What is the breadth of the molecular weight distribution that is expected, that is, what is the value of Xw/Xnl Use the following chain-transfer constants ... 

Table 3. Field effect on the living anionic polymerization of styrene in benzene-di-...

Fig. 5. Effect of tetrahydrofuran on the propagation rate in the polymerization of styrene in benzene. Concentration of polystyryHithium, o I.l x 10 molar, x 1.4 x 10 molar...

In some but not so rare cases, however, reactivity of macromonomers was found to be apparently reduced by the nature of their polymer chains. For example, p-vinylbenzyl- or methacrylate-ended PEO macromonomers, 26 (m=l) or 27b, were found to copolymerize with styrene (as A) in tetrahydrofuran with increasing difficulty (l/rA is reduced to one half) with increasing chain length of the PEO [41]. Since we are concerned with polymer-polymer reactions, as shown in Fig. 3, the results suggest that any thermodynamically repulsive interaction, which is usually observed between different, incompatible polymer chains, in this case PEO and PSt chains, may retard their approach and hence the reaction between their end groups, polystyryl radical and p-vinylbenzyl or methacrylate group. Such an incompatibility effect was discussed in terms of the degree of interpenetration and the interaction parameters between unlike polymers to support the observed reduction in the macromonomers copolymerization reactivity [31,40]. Similar observations of reduction of the copolymerization reactivity of macromonomers have recently been reported for the PEO macromonomers, 27a (m=ll) with styrene in benzene [42], 27b with acrylamide in water [43], and for poly(L-lactide), 28, with dimethyl acrylamide or N-vinylpyr-rolidone in dioxane [44]. 

A kinetic study has been carried out on the system butyllithium plus styrene in benzene. Although published reports by Tobolsky (85) and Welch (88) differ in their interpretations, it appears that these differences may be due to the fact that Welch examined a wider range of initiator concentrations. Welch showed that no termination occurs in this system and that the molecular weight was a linear function of the ratio monomer/ butyllithium. The rate results showed... 

Sinn, Lundborg and Kirchner (61) have reported that the homogeneous polymerization of styrene in benzene using lithium alkyls at 50° produces a relatively low molecular weight polymer a portion of which is crystallizable by treatment with boiling heptane. 

From such data it is then possible to derive the absolute values of the propagation rate constants representing the interaction of the growing chain with the incoming monomer unit. These are shown for isoprene and styrene in Table II. Obviously these propagation rate constants are all characterized by a low activation energy as well as an extremely low frequency factor. This is true both for the non-stereospecific polymerization of isoprene in THF as well as for the stereospecific case of isoprene in hexane (although it seems somewhat more extreme in the latter and for the styrene in benzene). 

A hexamer (PhSb)6 PhH has been prepared by heating PhSbH2 with styrene in benzene under an argon atmosphere (equation 128 ). 

Figure 5 shows the rate of reaction of n- and sec.-butyllithium with styrene in benzene as measured spectroscopically from initial rates of... 

Because of fhe medicinal relevance of stilbenoids, a solid-phase approach using CM has been recently disclosed [228]. Thus, 4-viriyl phenol (115) was attached to a Merrifield resin and subjected to CM wifh various substituted styrenes in benzene at 80 °C for 12 h in fhe presence of Grubbs rufhenium-carbene catalyst (Scheme 27). 

In the field of ionic polymerization, we should mention the investigators87,114) who calculated the relative chain transfer constants for the polymerization of styrene in benzene solutions and its mixture with 1,2-dichloroethane on Friedel-Crafts catalysts. 

While it is assumed that termination by coupling takes place when maleic anhydride and styrene are copolymerized in a good solvent such as acetone, insoluble macroradicals precipitate when these monomers are copolymerized in a poor solvent such as benzene (7). Insoluble macroradicals obtained by bulk polymerization of acrylonitrile (1, 11) and the solution copolymerization of maleic anhydride and styrene in benzene (7) have been used as seeds for the preparation of block copolymers. 

Also, it has been reported that poly(styrene-co-maleic anhydride-b-styrene) could be obtained either by addition of styrene monomer to the styrene-maleic anhydride macroradicals or by the free-radical-initiated copolymerization of maleic anhydride with more than an equimolar proportion of styrene in benzene solution (7). However, the maximum amount of styrene present in these block copolymers was less than 35% of the weight of the original macroradical. This limitation on the yield of the block copolymer is now assumed to be related to the increased solubility of the styrene block in the benzene solvent, which permits termination of the new macroradicals by coupling. 

Isotactic polystyrene. Isotactic polystyrene was prepared by heating a 10% solution of styrene in benzene at 60° for 24 hours in the presence of a catalyst prepared in the presence of monomer from equimolar amounts of (C2H5)3A1 (heptane solution) and T1CI3-AA (heptane suspension). The reaction mixture was diluted with benzene and poured into an excess of isopropanol. The resulting precipitate was dissolved in warm methylene chloride. The solution was filtered and added to hot methyl ethyl ketone. The solution was concentrated and cooled to obtain the isotactic polymer in the form of a white powder that was washed with methyl ethyl ketone and dried under vacuum. Final purification was achieved by reprecipitation of the polymer from benzene solution into methanol, followed by drying at 60° under vacuum. 

Fig. 27. Typical curves for the appearance of the absorption of poly-styryl anions. Curve A reaction of 1.1 10 3 M sec. -BuLi with 5.3 10 2 M styrene in benzene at 30 °C. Curve B reaction of 1.3 10 3 M sec. -BuLi with 8.7 10-2 M styrene in cycfo-hexane at 40 °C...

The bulk polymerization of DAP has been studied at 60°C with azobisisobutyronittile as initiator (39). Branching of the polymer chains is confirmed by enhanced broadening of the molecular weight distribution imtil gdation occurred at about 25% conversion. In copolymerizations with styrene at 80°C with benzoyl peroxide as initiator the gel time increases with fraction comonomer in the feed. Both the yidd of gd and the styrene units in the gel increase with copolymerization time. Heating DAP prepolymer with styrene in benzene solution at 60—100°C with the initiators gives no gelation, but slow formation of polystyrene and copolymer. 

The opinion has also been expressed that the viscometric technique will overestimate the degree of dissociation in systems where the dissociation constants involving the influence of ethers are studied. This claim can be examined by considering a simple example. Bywater and Worsfold and Meier studied the influence of tetrahydrofuran on the propagation rate of styrene in benzene. Their kinetic results can be interpreted as showing that at an ether/active center ratio of about 10, the poly(styryl)li ium dimers were largely disrupted by solvation with the ether for the process shown in Eq. (11). This joint conclusionis identical to that reached by Morton via the viscometric technique. Thus, at least for the case of poly(styryl)lithium, the viscometric procedure does not appear to overestimate the extent of dimer dissociation. 

Studies on local viscosity effects can be carried out using a free probe molecule, such as naphthacene, in polymer solution. Thus poly(styrene) in benzene solution has been studied with this probe at a concentration of 10 mole S,. Two stepwise increases In local viscosity at polymer concentrations of 20 to 30% and 60 to 70% were taken as indications of internal structure in the polymer solution at these concentrations (4). Similar studies were carried out on melts of n-paraffin homologues and poly(ethylene) at 150°C. The local viscosity increased with increasing molecular weight in the low molecular-weight range, but reached a plateau at around MW 2000. 

Typical kinetics of the initiation reaction of n-butyllithium with styrene in benzene exhibits a first-order dependence on styrene concentration and approximately a one-sixth order dependence on n-butyllithium concentration as shown in Equation 7.14. 

Example 7.1 The following are data for the polymerization of styrene in benzene at 60°C with benzoyl peroxide as the initiator. 

Example 7.5 The transfer constant to the solvent for the polymerization of styrene in benzene at 100°C is0.184x 10" . How much dilution is required to halve the molecular weight given that l/X o = 2.5 x 10 ... 

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. 

Figure 4-19. Change of the z average root mean square radius of gyration ( Rg) with concentration for poly(styrene) in benzene at 20° C and various molar masses (after H. Dautzenberg).

In a benzoyl peroxide initiated polymerization of 2 moles of styrene in benzene at 85 °C ( d = 8.94 X 10 L/mole-s at 85 °C). How much benzoyl peroxide will be required in the polymerization solution to attain an average molecular weight of250,000 Assume that termination occurs only by recombination and no chain transferring takes place. 

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