Polymerization in hydrocarbon solvents

The stereochemistry of the polymerization of dienes is most conveniently discussed in two sections (a) polymerization in hydrocarbon solvents and (b) polymerization in the presence of amines, ethers and other electron donors. 

The conjugated dienes can polymerize in four modes cis 1,4-, trans 1,4-, 1,2- and 3,4-, the latter pair being equivalent in the absence of appropriate substitution. Early workers relied entirely upon IR spectroscopy to analyze the concatenation in their polymers. There are a number of problems associated with the technique correct assignment of peaks, the additivity and the inherent insensitivity arising from the smallness of the extinction coefficients of double bonds bearing more than one substituent (such as arises from 1,4-enchainment). In consequence, the reliability of much of the early work is uncertain the advant of NMR spectrometers has, 

However, the absence of a simple correlation need not be totally surprising if propagation is only through the intermediacy of a minute proportion of chains present in a non-aggregated form with the NMR spectrum reflecting the structure in the predominant aggregates. 

Worsfold and Bywater 212) have proposed that propagation through non-aggregated chains is kinetically (and not thermodynamically) controlled and yields only the carbanion having the cis conformation this can isomerize to the trans form unless the geometry is locked in by a further act of propagation  

The rate of conformational isomerization in heptane was determined for model compounds I, II and III. 

Worsfold and Bywater have proposed that propagation through non-aggregated 

---

Reaction and Heat-Transfer Solvents. Many industrial production processes use solvents as reaction media. Ethylene and propylene are polymerized in hydrocarbon solvents, which dissolves the gaseous reactant and also removes the heat of reaction. Because the polymer is not soluble in the hydrocarbon solvent, polymer recovery is a simple physical operation. Ethylene oxide production is exothermic and the catalyst-filled reaction tubes are surrounded by hydrocarbon heat-transfer duid. 

Fig. 3. Comparison of the rates of polymerization by one-component and two-component catalysts (75°C, ethylene pressure 5.5 kg/cm2, polymerization in hydrocarbon solvent). Curve 1—TiCh (specific surface 24 m /g). Curve 2—TiCfi + AlEtjCl (Al Ti = 4, specific surface of TiClj 20 m2/g the same sample of TiClj was used for the preparation of TiCls). Curve 3—TiCfi + AlEtjCl (Al Ti = 4).

It is useful to understand the reasons for the faster reaction rates encountered in many anionic polymerizations compared to their radical counterparts. This can be done by comparing the kinetic parameters in appropriate rate equations Eq. 3-22 for radical polymerization and Eq. 5-84 for anionic polymerization. The kp values in radical polymerization are similar to the fc pp values in anionic polymerization. Anionic fc pp values may be 10-100-fold lower than in radical polymerization for polymerization in hydrocarbon solvents, while they may be... 

Here [Pf ] is the concentration of growing centres ending in monomer x and kx y is the absolute rate coefficient of reaction of P with monomer y. Two difficulties arise in anionic polymerization. In hydrocarbon solvents with lithium and sodium based initiators, [Pf ] is not the total concentration of polymer units ending in unit x but, due to self-association phenomena, only that part in an active form. The reactivity ratios determined are, however, unaffected by the association phenomena. As each ratio refers to a common active centre, the effective concentration of active species is reduced equally to both monomers. In polar solvents such as tetrahydrofuran, this difficulty does not arise, but there will be two types of each reactive centre Pf, one an anion and the other an ion-pair. Application of eqn. (22) will give apparent rate coefficients as discussed in Section 4 if total concentrations of Pf are used. Reactivities can change with concentration if defined on this basis. 

C2Hs)2N end groups on the polymer, and the fact that styrene, incapable of this type of complexation, is not polymerized in hydrocarbon solvents tend to confirm this mechanism. 

IV.13. Propagation of Styrene and the Dienes Polymerization in Hydrocarbon Solvents with Li+ Counter-Ions... 

Nizhnekamskneftekhim" in 1995-1996 [78, 79]. Polymerization in hydrocarbons solvents is carried out at 20-40°C and pressure 0,2-1,4 MPa. Polymerization duration is 0,5-l,5 hours. Disadvantages of existent method of SKEP(T) production are problem of effective heat removal ... 

The <7-bonded lithium chain can be expected to predominate. In highly solvating solvents, such as ethers, the 7r-allyl structure is dominant leading to high 1,2 placements. Because the 2,3-bond is maintained, the above-shown equilibria should not be expected to lead to cis-trans isomerization. Such isomerizations do not take place for butadiene or for isoprene when they are polymerized in hydrocarbon solvents. They do occur, however, in polar solvents at high temperatures. This suggests that additional equilibria exist between the r-allylic structures and the covalent 1,2 chain ends. Table 3.3 shows the manner in which different polymerization initiators and solvents affect the microstructures of polyisoprenes. 

It would be expected that the kinetics of organolithium-initiated polymerization in hydrocarbon solvents would be simplified because of the expected correspondence between the initiator concentration and the concentration of propagating anionic species, resulting from the lack of termination and chain transfer reactions. However, in spite of intensive study, there is... 

Here the driving force is provided by the formation of a carbanion adjacent to a stabilizing group (RjSi— or dithioacetal). Organolithium reagents, however, add to conjugated dienes and hence initiate their polymerization. In hydrocarbon solvents n-butyl lithium and butadiene result in mostly 1,4-addition. The polymer contains a mixture of cis and trans stereochemistry about the double bonds. 

A notable exception to the emphasis on free-radical polymerization studies was provided by Karl Ziegler and his co-workers who extended the study of the alkali metal polymerization of dienes to include metals other than sodium and various metal alkyls. Of particular interest were the results obtained with the simplest Group I alkali metal, lithium. It was found that when lithium metal was used as a polymerization initiator 1,4- structures predominated over 1,2-polymers. It was also found that polymerization in hydrocarbon solvents further favoured production 1,4- structures whilst polymerization in polar liquids such as ethers and amines often favoured the formation of 1,2- units. It was also found that reaction of lithium with monomer led to the production of an organo-lithium compound which made feasible homogeneous polymerization—a discovery which eventually led to commercial exploitation. 

Tab. 7.S. Literature values for propagation rate coefficient = kp(ff/2) for Li-initiated styrene polymerization in hydrocarbon solvents.

Nonpolar hydrocarbon monomers such as styrene, isoprene, and butadiene are polymerized in hydrocarbon solvents such as benzene or cyclohexane. Initiation is achieved with the use of alkyllithiums such as sec-butyllithium and molecular mass is controlled by the ratio of initiator to monomer. The living nature of anionic polymerization allows the syntheses of block copolymers by sequential addition of the monomers. After one monomer is exhausted, the chain remains reactive, or living. The addition of the second monomer then continues the polymerization to form a block copolymer. Such techniques are used to synthesize polystyrene-polyisoprene or polystyrene-polybutadiene copolymers (PS-PI or PS-PB, respectively). 

In a recent patent, a preparation of powdered PA 12 by dispersion polymerization in hydrocarbon solvents was described where the polymerization mixture contained, in addition to the initiation system, a mineral filler and bisamide [88]. Powdered polyester-amides were prepared in a similar way, namely by the copolymerization of a lactam and a lactone [89]. 

---