Ziegler polymerizations chains

Anionic polymerization of conjugated dienes and olefins retains its lithium on the chain ends as being active moities and capable of propagating additional monomer. This distinguishing feature has an advantage over other methods of polymerization such as radical, cationic and Ziegler polymerization. Many attempts have been made to prepare block copolymers by the above methods, but they were not successful in preparing the clear characterized block copolymer produced by anionic technique. 

The straight-chain alcohol used for this ester is known commercially as Alfol 610 and is produced by the Ziegler polymerization of ethylene. The composition of the alcohol is 20% Ce. 35% Cs. and 44% C10. 

Tracer studies have been used in an attempt to determine the nature of the ends of the chain but these were as unsatisfactory as for propylene. Feldman and Perry (83) used triterated methanol to react the polyethylene from a titanium tetrachloridetrialkylaluminum catalyst. They found a continual increase in the number of polymeric chain ends which react with the tritium. This agrees with the results of Roha and Beears (84) who showed the very rapid exchange of alkyls which took place when ethylene was grown on a Ziegler catalyst in the presence of excess alkylaluminum chloride. In these experiments only an extremely small... 

Derivation Benzene is alkylated with dodecene, to which it attaches itself in any secondary position the resulting dodecylbenzene is sulfonated with sulfuric acid and neutralized with caustic soda. For ABS (branched-chain alkyl) the dodecene is usually a propylene tetramer, made by catalytic polymerization of propylene. For LAS (straight-chain alkyl), the dodecene may be removed from kerosene or crudes by molecular sieve, may be formed by Ziegler polymerization of ethylene, or by cracking wax paraffins to a-olefins. 

The versatility of polymerization resides not only in the different types of polymerization reactions and types of reactants that can be polymerized, but also in variations allowed by step-growth synthesis, copolymerization, and stereospecific polymerization. Chain polymerization is the most important kind of copolymerization process and is considered separately in Chapter 7, while Chapter 9 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of polymerization conditions, including the more recent metallocene-based Ziegler-Natta systems. Synthetic approaches to starburst and hyperbranched polymers which promise to open up new applications in the future are considered in an earlier chapter dealing with step-growth polymerization. 

When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the ditacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threodiisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythrodiisotactic polymer. For instance, cis and /ra/i -l- Z-propylenes give erythro and threo diisotactic polymers, respectively. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bond in the monomer units rotate after the addition of the monomer to the polymeric chain. 

The mechanism of the Ziegler polymerization has been the subject of many experiments and much speculation. It is certainly not of free radical nature, since hydrogen acts as a chain transfer agent. When tritiated alcohols are added as chain-terminating agents, tritium is found in the polymer. If ( C2H5)3A1 is used as a starter, the polymer is radioactive. Therefore the overall reaction can be formulated only in terms of metal-carbon bond participation ... 

Various termination reactions are possible in Ziegler polymerizations. At polymerization temperatures below 60° C, every polymer chain contains a metal atom. Thus, no thermal termination takes place at this temperature, although it does at higher temperatures, where vinyl and vinylidene groups have been found ... 

Ziegler-Natta chain-growth polymerization involves the formation of an alkyl-transition metal compound and then the repeated insertion of alkene monomers into the transition metal-to-carbon bond to yield a saturated polymer chain. 

Ziegler-Natta chain-growth polymerization of ethylene and substituted ethylenes (Section 16.5B)... 

Titanium-Mediated (Ziegler-Natta) Chain-Growth Polymerization of Ethylene and Substituted Ethylenes (Section 29.6B)... 

PP, a semi-crystalline thermoplastic, is made in its homopol5mier form by pol5mierising propylene monomer using stereospecific Ziegler-Natta catalysts. The catalyst system is termed stereospecific because it controls the position of the side (methyl) group in each propylene unit in the polymeric chain. A t3 ical catalyst system may be prepared by combining titanium trichloride with tributyl aluminium or its variants. Most commercial PP is isotactic. 

Typical heterogeneous Ziegler catalysts operate at temperatures of 70— 100°C and pressures of 0.1—2 MPa (15—300 psi). The polymerization reactions are carried out ia an iaert Hquid medium (eg, hexane, isobutane) or ia the gas phase. Molecular weights of LLDPE resias are coatroUed by usiag hydrogea as a chain-transfer ageat. 

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