Polydienes isotactic

In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. Natta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. 

All the possible line repetition groups for cis and trans poly dienes compatible with the isotactic or syndiotactic configurations are reported in Figure 2.15,47,68 In order to consider only the possible conformations assumed in the crystalline state, the torsion angle of the central single bond is assumed to be 180° trans) in both the cis and trans polydienes. This condition produces conformations sufficiently extended to be packed in a crystalline lattice for each value of the torsion angles 0i and 02 (Figure 2.15). 

Natta (67) has studied the effect of two isomeric forms of titanium trichloride on the polymerization of dienes and of isotactic polypropylene. He has found that the alpha isomer, which gave the higher isotactic polypropylene, gave greater amounts of trans 1.4-polydiene than the beta, which gave cis 1.4-diene. 

Synthesis of linear polyethylene and coordinate polymerisation (combination of trialkyl aluminium and titanium chloride as catalyst) by Ziegler Isotactic and syndiotactic polymerisation (polypropylene and polydienes) discovered by Natta... 

By 1,4-polymerization of each of these dienes, a cis or a trans isomer can be obtained. If the dienes are 1,2-polymerized, a chiral carbon atom with a vinyl group arises by introducing a sequence of the asymmetric configurations in the same or alternating orientations, an isotactic or a syndiotactic 1,2-polydiene is formed. In the case of isoprene, a 3,4-polymerization can additionally take place, which also leads to a chiral carbon atom but with an isopropenyl group. Thus, the formation of an isotactic or syndiotactic structure is possible in this case too. 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg= — 108°C) and trans(T = — 18°C) or polyisoprene cis Tg = —73°C) and trans T = —53°C). An example of tacticity variation is polyfmethyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

This group of Ziegler-Natta catalysts is stereospecific for the polymerisation of a-olefins and 1,3-dienes the products are mainly isotactic polyolefins (polypropylene, polystyrene, and so on) and 3 5-l,4-polydienes (polybutadiene, polyisoprene, and so on). 

The commercial polydienes are elastomers. Q s-1,4 polybutadiene has a Tg of -100 °C and has a crystalline melting point of less than 0 °C. Q s-1,4 polyisoprene has a Tg of -70 °C and has a crystalline melting point of 35 C. Both polymers crystallize rather slowly. Trans-1,4 polybutadiene and polyisoprenes are crystalline thermoplastics at room temperature. They are not, however, used commercially because of their poor aging characteristics relative to polyolefins. This is associated with the double bonds in their backbones. Polybutadienes with high atactic 1,2-contents have been widely used in the tire industry. Their Tg is about -15 °C. Isotactic and syndiotactic 1,2-polybutadienes are high melting crystalline thermoplastics, but age poorly compared to polyolefins. The 1,2-polybutadienes have been used as packaging for additives in the rubber industry. 

The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., ataaic) microstructure. The racemic diad fraction (PJ was 0.53 for the butyllithium/toluene system. The effects of cotmterion, solvent, and temperature on polystyrene stereochemistry are shown in Table 12. The principal conclusion from these results is that the stereoregularity of polystyrenes prepared by aniortic polymerization is predominantly syndio-tactic (Pr = 0.56-0.74) and that the stereoregularity is surprisingly independent of the nature of the cation, solvent, and temperature, in contrast to the sensitivity of polydiene stereochemistry to these variables. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium t-butoxide to 58% isotactic triads with potassium t-butoxide. °... 

How would this differ from the isomerism that arises in 1,4-polydienes c. Which of the polymers, if any, could have isotactic and syndiotactic isomers Illustrate. 

Double bonds present along the chains of polymers of 1,3-dienes with 1,4 additions of the monomeric units are stereoisomeric centers that may assume cis or trans configurations. Polidienes may also contain up to two stereoisomeric tetrahedral centers. Stereoregular polydienes can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndiotactic if two stereoisomeric tetrahedral centers are present. In the latter case, erythro and threo structures are defined depending on the relative configurations of two carbon atoms [1]. 

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