Styrene polymerization mechanism

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Addition of styrene to a green solution of naphthalene" Na+ in tetrahydrofuran leads to an instantaneous change of color from green to red. Styrene polymerizes rapidly and quantitatively within a few seconds, and when the reaction is completed, addition of water converts the red solution of polystyryl carbanions into colorless solution of polystyrene. After precipitation of the polymer it was shown spectroscopically25 that the residual solution contains an amount of naphthalene equal to that used in the preparation of the initiating catalyst. This observation confirms the proposed mechanism of initiation of the polymerization. 

IUPAC recommendations suggest that a copolymer structure, in this case poly(methyl methacrylate-co-styrene) or copoly(methyl methacrylate/slyrene), should be represented as 1. The most substituted carbon of the configurational repeat unit should appear first. This same rule would apply to the copolymer segments shown in Section 7.1. However, as was mentioned in Chapter I, in this book, because of the focus on mechanism, we have adopted the more traditional depiction 2 which follows more readily from the polymerization mechanism. 

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 ... 

Boodhoo, K.V.K., Jachuck, R.J., and Ramshaw, C. (1997) Spinning disc reactor for the intensification of styrene polymerization, in Process Intensification in Practice Applications and Opportunities (ed. J. Semel) Mechanical Engineering Publications, Ltd, Bury St. Edmunds, pp. 125-133. 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

Polymerization inhibitors miscellaneous, 23 383 in styrene manufacture, 23 338 Polymerization initiators alkyllithiums as, 74 251 cerium application, 5 687 peroxydicarbonates as, 74 290 Polymerization kinetics, in PVC polymerization, 25 666-667 Polymerization mechanism, for low density polyethylene, 20 218 Polymerization methods, choice of,... 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

Styrene (see Fig. 3.4 and Example 3-35). Conversely, monomer pairs whose copolymerization behavior is well known can be used to test new initiator systems in order to draw conclusions about the polymerization mechanism. 

These early works have been reviewed by Fioshin (4) and well summarized by Bbeitenbach (5). Besides, Breitenbach has made a study of the polymerization mechanism using the copolymerization method and has shown that the reaction mechanism depends on the ions used in the electrolytic discharge and on the monomer present in the system. Cationic processes were also found to be initiated in a nitrobenzene solution of styrene by the anodic discharge of perchlorate and borotetrafluoride ions. The possibility that the three different mechanisms could occur simultaneously was demonstrated in the same system of acrylonitrile-styrene using a divided electrolytic cell. 

For instance, in cases where the mechanism of the propagation of the polymer chain is by means of cationic polymerization, the rate increases with the polarity of the solvent. Thus, when the boron trifluoride-diethyl ether complex is used as the catalyst for styrene polymerization, then at 0°C the rate equation for a series of solvents takes the simple form of dependence on the solvent polarity (Heublein 1985) ... 

Oligomer formation by a mechanism different from the polymerization mechanism has been proposed by Kern (7) for the tetramer formation from a number of epoxides and by Pasika (10) for styrene oxide dimer formation, although in the last case the initiation reactions are probably the same. However, in both cases degradation of polymer would be a possible alternative mechanism. 

Dainton and coworkers (11) investigated the polymerization of various monomers with solutions of potassium in dimethoxyethane. Acrylonitrile gave low irreproducible yields of polymer, whereas methyl methacrylate and acrylamide gave no polymer. Styrene polymerized in a reproducible manner to give "living polystyrene. Their data support an ion-radical mechanism with a slow dimerization of the radicals. A 100% yield of polymer was obtained in < 3 seconds, but the optimum molecular weight was not obtained for > 8—9 seconds. 

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

Not many initiators belong to this class even though the halogenoacetic, fluorosulphonic, and other acids are included. A detailed analysis of their polymerization mechanism is obscured by complex formation with monomer and with solvent, by the occurrence of aggregates, and by anion reactions in acids with an unstable anion. In spite of its apparent simplicity, initiation by Bronsted acids has not yet been investigated in detail. The pseudo-cationic polymerization of styrene is an instructive example. 

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