Polymerization, anionic styrene

Anionic Styrene Polymerization in a Continuous Stirred-Tank Reactor... 

The aggregates are polymerization inactive their formation is accompanied by a corresponding decrease in the polymerization rate [96], Aggregate formation is not limited to siloxane monomers and polymers. Independently, and in the same year, associate formation during anionic styrene polymerization was described by Worsfold and Bywater... 

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). 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

J.-H. Jiang, Y. Ozaki, M. Kleimann and H.W. Siesler, Resolution of two-way data from on-line Fourier-transform Raman spectroscopic monitoring of the anionic dispersion polymerization of styrene and 1,3-butadiene by parallel vector analysis (PVA) and window factor analysis (WFA), Chemom. Intell. Lab. Syst., 70, 83-92... 

The anionic graft polymerization of vinyl monomers onto carbon fiber or graphite powder initiated by metalized carbon fiber has been investigated. The metalation of polycondensed aromatic rings of a carbon fiber surface was achieved by treatment of the carbon fiber with BuLi in Ar,Ar,Ar, AT -tetramethylethylenediamine. The anionic polymerization of methylmethacrylate and styrene was reported. No grafting was observed when carbon fiber was treated simply with BuLi in THF or toluene [44,45]. 

Only a limited number of monomer pairs form block copolymers in this manner. Examples are conjugated dienes and vinyl aromatics that have similar Q-e values. The nature of the anionic initiator, i.e., the ionic character of the carbon-metal bond plays an important role in both the amount and sequence of block formation. For instance, when potassium or cesium initiators are used, styrene polymerizes first as can be seen in Figure 12. 

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

Tsou, Magee and Malatesta (39) showed the effect of catalyst ratios on steric control m the polymerization of styrene with alkyllithium and titanium tetrachloride. These authors have shown that the isotactic polymer was produced when the butyllithium to titanium ratio was kept within the limits of 3.0 to 1.75. Outside of this critical range, amorphous polymers were produced. In the discussion of this paper, Friedlander (40) pointed out the cationic nature of the low-lithium-to-titanium-ratio-catalysts which also produced considerable rearrangement of the phenyl groups. Above 2.70 lithium to titanium ratio, an anionic type polymerization set in, which produced atactic polymer. At low ratios cationic catalysis also produced atactic polymer. Tsou and co-workers concluded that crystallinity of the catalyst is not important for steric order in the polymer. 

Monomers which can be polymerized with aromatic radical anions include styrenes, dienes, epoxides, and cyelosiloxares. Aromatic radical anions which are too stable do not efficiently initiate polymerization of less reactive monomers thus the anthracene radical anion cannot initiate styrene polymerization. 

The linear polymers, as shown above, can be built up by the sequential addition of monomer or by coupling the living anionic chains using compounds like dichloro dimethylsilane. Hence, the base polymer would have styrene polymerized first, followed by butadiene, and then addition of the coupling agent. If a multifunctional coupling agent such as silicon tetrachloride is used, a radial block or star-branched SBS is formed. 

In the absence of an added electrophile the anions undergo spontaneous electron transfer to the metal with release of Co2, and the nitrile radical, which dimerizes. Alternatively, the coordinated radical anion induces polymerization of methacrylate, styrene, acrylonitrile and methylacrylonitrile monomers under mild non-aqueous conditions.181... 

The styrene/butadiene block polymers were synthesized in tetra-hydrofuran by the Szwarc anionic living polymerization technique (2, 3, 4). Details of the polymerization, as well as extensive characterization... 

Mono- and dianions can, of course, be used for the initiation of linear chain propagation. Gordon and Loftus observed interesting differences in styrene polymerization when initiated with BuLi and with delocalized carbanions. In the latter case, the anions have various counter-ions (Li+ and K + ) at each of these, propagation proceeds at a different rate. When the initiator contains a greater number of delocalized carbanions, insolubility of the product and non-uniformity of charge on the anions cause some problems [199],... 

Lewis acids based on titanium tend to aggregate and form dimers which are usually more reactive than their monomeric precursors (cf., Chapter 2). The degree of aggregation depends on the solvent, temperature, and the ligands attached to titanium no dimerization was detected by cryoscopy at -95° C in CH2CI2 [174], However, kinetic measurements of isobutene and styrene polymerizations indicate that polymerization is second order in titanium chloride [175,176], perhaps due to formation of a low concentration of the more reactive dimer or more stable Ti2Cl9-anions. However, polymerizations performed at lower [TiCl4] were reported to be first order in titanium chloride [105]. 

These KD values are much smaller than those of trityl and benz-hydryl salts (lO-MO-4 mol/L) (cf Tables 7 and 16). The KD values of trityl salts correspond to interionic distances of approximately 5 A. The smaller (Kd 107-10-6 mol/L) dissociation constants calculated from the common ion effect in styrene polymerizations with triflate and perchlorate anions correspond to interionic distances =4 A. However, perchlorate and triflate anions may not be spherical, and their dipole moments should therefore also be considered in calculating their interionic distances and dissociation constants. As discussed in Section II.D, specific interactions of counteranions with the ar-H atoms of the secondary carbenium ions may result in lower dissociation constants [39],... 

Schwab FC, Murray JG (1985) Anionic dispersion polymerization of styrene. In Culbertson BM, McGrath JE (eds) Advances in polymer synthesis. Plenum, New York... 

Continuous solution Anionic Pure styrene monomer Much recycled solvent Anionic initiators Polymerize to completion Low residual monomer High polymerization rate Good for spec, copolymer Sensitivity to impurities Initiator cost Color of product Cannot produce HIPS Not proven for high-volume GP... 

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