Stereo-specific polymerization

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

After the Natta s discovery of highly stereospecific polymerization processes, the interest in the preparation and properties of optically active polymers has greatly increased. In fact, the use of asymmetric catalysts or monomers to obtain optically active polymers may supply interesting informations on the mechanism of steric control in stereo-specific polymerization furthermore optical activity is an useful tool to study the polymer stereoregularity and the chain conformations of polymers in the molten state or in solution. 

Butler, Thomas, and Tyler (9) have reported on the stereo-specific polymerization of N,N-disubstituted acrylamides by alkyl lithiums. Overberger and Schiller (51) reported on the preparation of crystalline poly-/-butyl vinyl ketone by anionic catalysts in toluene at room temperature. 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

Fig. 2 Model for stereo specific polymerization of propylene. The orientation of the growing chain is influenced by the chlorine atom marked with an asterisk.

Natta (194) discovered the phenomenon of stereospecific polymerization responsible for the formation of valuable isotactic polymers of a-olefins in the presence of heterogeneous catalysts and of activators and explained it by oriented adsorption. However, the latter is an obligatory condition of the multiplet theory, according to which the reacting atoms come into contact with the sui face, and the substituents must be oriented in one direction, namely, off the surface (195). According to Natta, the electronic properties also play an important part in stereo-specific polymerization. 

Aggarwal, S. L., Marker, Leon, Kollar, W. L., and Geroch, R., in Advances in Chemistry Series, Number 52, "Elastomer Stereo-specific Polymerization", pp 88-104, American Chemical Society (1966). 

In Volumes 3 and 4, there are chapters describing stereo-specific polymerization of vinyl and cyclic monomers, formation of isotactic and syndiotactic polymers, and polymerization of racemic (as well as meso) cyclic monomers. 

A useful group of rubbers are the stereo specific poly(butadiene) rubbers formed by the polymerization of 1,3-butadiene. These rubbers have a ris-isomer content of more than 30%. They contain at least about 85% of poly(butadiene) formed by 1,4 addition. Further, the rubber should have a second order transition temperature of preferably not higher than -20°C (8). 

Coordination polymerization is yet another variation on the same theme. Here, polymerization is initiated by attachment of a monomer molecule to a metal complex. The polymer grows by successive insertion of monomer molecules at the metal. Growth stops when the metal complex detaches itself or the reactive center becomes deactivated by some intended or inadvertent event. Stereo-specific polymers can be produced. 

By far the most important industrial coordination polymerization processes are Ziegler-Natta polymerizations of 1-olefins [107-110], most notably the production of high-density polyethene [111] and stereo-specific olefin polymers and copolymers [108], However, these processes employ solid catalysts, and the complex kinetics on their surfaces have no place in a book on homogeneous reactions. 

Titanium and zirconium tetrabenzyl and the mixed metal—benzyl halides are soluble in hydrocarbon solvents and will polymerize ethylene and a-olefins, the latter to stereo-specific polymers [64], The structures of the true initiators are not known but they are unlikely to be the simple organo-metal compounds. Catalysts of higher activity are obtained when they are used in combination with aluminium alkyls. It is of interest to note that titanium tetra(dimethyl amide) reacts with acrylonitrile to form an active species, which then forms high molecular weight polymer by coordination polymerization [65]. 

Metallocenes with diastereotopic sites for monomer coordination show quite an interesting polymerization behavior introduction of a methyl group in position 3 of the cyclopentadienyl ring in (21) disturbs the stereospecificity at this site, giving rise to hemiisotactic polypropylene [53], while a f-butyl group at the same position inverts the preferred mode of coordination (22) thus an isotactic polymer is generated [54]. Metallocene (8) has one nonspecific and one stereo-specific site, too at low temperature, hemiisotactic polypropylene is produced while at high temperatures site isomerization without insertion facilitates the formation of isoblock polypropylene. 

Polymerization of cis, cis-1,4-dideuterio-l, 3-butadiene by several transition metal catalysts has been studied. The existence of non-stereo-specific bond forming events is postulated to signal the involvement of allyl isomerization in the polymerization mechanism. Trans-1,4-polymers are accompanied by complete scrambling of deuterium stereochemistry, contrasting with a more specific process to form cis polymers. Allyl isomerization is thus implicated as a key event in the formation of trans, but not cis, polymer. 

Butlerov found out that in alkaline medium (calcium hydroxide), formaldehyde HCHO polymerizes to form about 20 different sugars as racemic mixtures, Butlerov 1861. The reaction requires a divalent metal ion. Breslow found a detailed mechanism of reaction that explains the reaction products, (Breslow 1959). He found that glycol-aldehyde is the first product that is subsequently converted into glyceral-dehyde (a triose), di-hydroxy-acetone, and then into various other sugars, tetrose, pentose, and hexose. The formose reaction advances in an autocatalytic way in which the reaction product is itself the catalyst for that reaction with a long induction period. The intermediary steps proceed via aldol and retro-aldol condensations and, in addition, keto-enol tautomerizations. It remains unexplained how the phosphorylation of 3-glyceraldehyde leads to glycral-3-phosphate (Fig. 3.6). Future work should study whether or not ribozymes exist that can carry out this reaction in a stereo-specific way. 

In 343 (R = H, R = COOEt) the ring opening takes place and gives stereo-specifically the Z-compound 344 even in solution.382 2-Thienylcarbenes also ring open, but the products polymerize.381... 

Later a group of researchers from Leningrad126) turned to the analysis of stereo-specific butadiene polymerization, and in the latest study1291, based on an analysis of their own data and those taken from the literature1301, they made an attempt to create a mathematical model of this process. 

The presence of a comonomer in the polymerization process, when a stereo-specific catalyst is used, results in the production of a rather linear polymer with very short branch-like pendant groups. This polymer is called linear low density polyethylene (LLDPE) or ultra low density polyethylene (ULDPE), depending on the density achieved by the addition of the comonomer. The larger the amount of comonomer added, the lower is the density of the copolymer. For example, if hexene is used, the pendant groups are as follows ... 

SOLUTION POLYMERIZATION Solution SBR typically made in hydrocarbon solution with alkyl lithium-based inihator. In this stereo-specific catalyst system, in principle, every polymer molecule remains live until a deactivator or some other agent capable of reacting with the anion intervenes. Able to control molecular weight, molecular weight distribution, and branching. Able to make random and block copolymers with designed chain sequence. Able to make copolymer with controlled styrene content. Able to control the butadiene structure of vinyl/ ds/ trans. Higher purity due to no addition of soap. 

Living polymerization and stereo-specific catalysts led to novel polymer structures such as dendrimers, star and comb (Figure 1.10). Living polymerization techniques are used to produce. These polymers are finding applications as catalysis and light amplifiers. 

The ability of a polymeric material to crystallise is determined by its molecular structure, that is its structural regularity and flexibility. A regular structure has the potential to exhibit crystallinity, while an irregular structure will tend to be amorphous. The general structure of polyester resin is very complex as these resins are obtained from a mixture of fatty acids with different structures and compositions. In addition, no stereo-specific catalyst is used in the resinification reaction, so the product obtained is random in nature. Thus crystallinity in polyester resins is rarely obtained, most being rather amorphous and highly flexible. 

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