Polypropylene isotactic/syndiotactic

Metallocene catalysts have been applied to a number of key product families across the chemical industry. Today, polyolefins and styrenics are either commercial or in development with additional polymers and specialty chemicals in the R D pipeline. The work in polyolefins has focussed on polyethylene, polypropylene (isotactic, syndiotactic, and atactic), and cyclic olefins. 

Polypropylene. There is an added dimension to the catalytic polymerization of propylene, since in addition to the requirement that the catalyst be sufficiently active to allow minute amounts of catalyst to yield large quantities of polymer, it must also give predominantly polypropylene with high tacticity that is, a highly ordered molecular stmcture with high crystallinity. The three stmctures for polypropylene are the isotactic, syndiotactic, and atactic forms (90) (see Olefin polya rs, polypropylene). 

Examine three different strands ofpolypropylene. For each strand, assign R/S stereochemistry to each stereocenter. (All three strands have as their terminal monomer perfluoropropane in order to facilitate assignment of stereochemistry.) Which of the three strands corresponds to atactic polypropylene, isotactic polypropylene and syndiotactic polypropylene ... 

The three different stereochemical forms of polypropylene all have somewhat different properties, and all can be made by using the right polymerization catalyst. Propylene polymerization using radical initiators does not work well, but polymerization using Ziegler-Natta catalysts allows preparation of isotactic, syndiotactic, and atactic polypropylene. 

Problem 31.5 Polymers such as polypropylene contain a large number of chirality centers. Would you therefore expect samples of isotactic, syndiotactic, or atactic polypropylene to rotate plane-polarized light Explain. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

Polymers that incorporate steric centers into their backbones can display various types of tacticity. The three principal types of tacticity are isotactic, syndiotactic, and atactic, as illustrated in Fig. 1.8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins,... 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

The model with different end-groups is not realistic in all cases isotactic polypropylene and syndiotactic polypropylene are chiral, or more precisely, their structure is cryptochiral. This model is to be chosen when examining oligomers, and especially when studying the polymerisation mechanism where the structure of the reactive chain end is of extreme importance [16]. 

In which of the three groups (isotactic, syndiotactic or hemiisotactic) one may include syndioisostereoblock polypropylene ... 

Some properties of isotactic, syndiotactic, and atactic PP are listed in Table 12.2.17 The insolubility of iPP in hydrocarbon solvents at room temperature often is used to separate iPP from atactic polypropylene (aPP). 

TABLE 12.2 Properties of Isotactic, Syndiotactic, and Atactic Polypropylene... 

Scheme 5.7 Three examples of ansa-zirconocene dichlorides directing the polymerization of propene, in the presence of MAO, to isotactic, syndiotactic and atactic polypropylene (PP)...

Polypropylene (PP) with the formula [-CH2CH(CH3)-] , CAS 9003-07-0, is a common polymer that is usually obtained by coordination catalyst polymerization. This polymer can be made in isotactic, syndiotactic or atactic forms, typically head to tail. Most practical uses are known for isotactic polypropylene, which is linear and highly crystalline. As indicated in Section 1.3, the linearity of the isotactic polymer implies in fact that the backbone forms a regular spiral that in case of polypropylene has three units per turn, as schematically shown below ... 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Fig. 6. Polypropylene isotactic arrangement of methyl groups along the helix (described by a tape) the structure of syndiotactic polypropylene (drawing in the middle) is also regular the chain in atactic polypropylene (below) is statistically irregular ... 

From the 1,2-polybutadiene, analogous to polypropylene, there are three structurally different polymers the isotactic, syndiotactic, and atactic form. All have been isolated -. ... 

Their potential to polymerize prochiral olefins, such as propylene, to give stereospecific polymers (isotactic. syndiotactic. hemitactic polypropylene). This variety opens the door to polyolefin elastomers. 

High density polyethylene (HOPE) Linear low density polyethylene (LLDPE) Isotactic polypropylene (iPP) Syndiotactic polypropylene (sPP) tram-1,4-Polyisoprene Syndiotactic polystyrene (sPS) Cyclooleflns Ethylene-propylene copolymers Styrene-ethylene copolymers cw -1,4-polybutadiene rrarw -1,4-Poly isoprene Random ethylene-a-olefin copolymers Ethylene-propylene rubber (EPR) Ethylene-propylene-diene copolymers (EPDM)... 

Stereoregular polymers synthesized with (A and B) and without (C) the Ziegler-Natta catalyst A) isotactic polypropylene B) syndiotactic polypropylene C) atactic polypropylene... 

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