Polymer structure syndiotactic

Polypropylene was not developed until the 1950s when Ziegler and Natta invented coordination catalysts. The structural difference between polyethylene and polypropylene is the methyl group in the propylene unit. Its presence makes a difference because it makes possible three different polymer structures Isotactic, with all methyl groups in the same plane makes the best plastic syndiotactic, in which the methyl groups alternate in the same plane and atactic, with the methyl groups randomly in and out of the plane is soft and rubbery. Polypropylene is used as film and in many structural forms. It is also used as fibers for carpet manufacture and for thermal clothing. 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Stereoselective polymerizations yielding isotactic and syndiotactic polymers are termed isoselective and syndioselective polymerizations, respectively. The polymer structures are termed stereoregular polymers. The terms isotactic and syndiotactic are placed before the name of a polymer to indicate the respective tactic structures, such as isotactic polypro-pene and syndiotactic polypropene. The absence of these terms denotes the atactic structure polypropene means atactic polypropene. The prefixes it- and st- together with the formula of the polymer, have been suggested for the same purpose it-[CH2CH(CH3)] and st-[CH2 CH(CH3)] [IUPAC, 1966],... 

It should be noted that other polymer structures can be postulated—those where one substituent is atactic while the other is either isotactic or syndiotactic or those where one substituent is isotactic while the other is syndiotactic. However, these possibilities are rarely observed since the factors that lead to ordering or disordering of one substituent during polymerization generally have the same effect on the other substituent. An exception is the formation of hemiisotactic polypropene where isotactic placements alternate with atactic placements [Coates, 2000]. 

Several other types of monomers are capable of yielding stereoisomeric polymer structures. Ordered structures are possible in the polymerization of carbonyl monomers (RCHO and RCOR ) and the ring-opening polymerizations of certain monomers. Thus, for example, the polymers from acetaldehyde and propylene oxide can have isotactic and syndiotactic structures as shown in Figs. 8-3 and 8-4. 

The situation is exactly analogous to the polymerization of monosubstituted alkenes the various polymer structures would be those in Fig. 8-1 with R = — CH=CH2. With chloroprene and isoprene, the possibilities are enlarged since the two double bonds are substituted differently. Polymerizations through the 1,2- and 3,4-double bonds do not yield the same product as they would in 1,3-butadiene polymerization. There are, therefore, a total of six structures possible—corresponding to isotactic, syndiotactic, and atactic structures for both 1,2- and... 

Infrared spectroscopy has been used for quantitatively measuring the amounts of 1,2-, 3,4-, cis-1,4-, and trans-1,4-polymers in the polymerization of 1,3-dienes its use for analysis of isotactic and syndiotactic polymer structures is very limited [Coleman et al., 1978 Tosi and Ciampelli, 1973]. Nuclear magnetic resonance spectroscopy is the most powerful tool for detecting both types of stereoisomerism in polymers. High-resolution proton NMR and especially 13C NMR allow one to obtain considerable detail about the sequence distribution of stereoisomeric units within the polymer chain [Bovey, 1972, 1982 Bovey and Mirau, 1996 Tonelli, 1989 Zambelli and Gatti, 1978],... 

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 ... 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis structures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer structures with pendent vinyl groups (Fig. 2). Commercial production of these polymers started in 1960 in the United States. Firestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

An analysis of the ionic factors for the polymerization of dienes to cis and trans structures is possible in the same way as for isotactic mono-enes. The mechanism which controls the steric structure of poly 1,4 dienes is parallel to that we have already seen for the mono-olefins. Roha (2) listed the catalysts which polymerize dienes according to the polymer structures produced. It was shown that the highly anionic as well as the highly cationic catalyst systems with increasing ionic separation produced trans-poly-1,4-dienes. This is analogous to the production of syndiotactic polyolefins. 

Propylene content in EPM rubber can be determined with the help of IR spectra. A propylene band near 1155 cm 1 has been widely used [79] for EPM analysis, frequently in combination with the polyethylene band at 721 cm"1. Tacticity is important in EPM rubber, and the bands at 1229 and 1252 cm"1 are characteristic of syndiotactic and isotactic structures, respectively, (both bands are present in atactic polypropylene as well). Polymer structure may vary in the relative tactic placement of adjacent head to tail propylene units and in the sequence distribution of base units along the chain. Some of them can be identified [80] by infrared spectra, such as isolated or head to tail propylene units ... 

A further breakthrough was the synthesis of enantiomeric sterorigid ansa-metallocenes by Brintzinger and co-workers [33] and the discovery by Ewen [34] that such racemic metallocene/methylalumoxane systems generate isotactic polypropylene. It was further found that the metallocene structure determines the polymer structure [35-37]. Again, with these compounds polyolefins such as syndiotactic polypropylene become available on a large scale [38]. Indeed, metallocene/methylalumoxane catalysts offer new prospects for olefin oligomers and polymers [39 2],... 

S-type) cyclobutane ring Thus, the polymer structure should have a unique repeating unit in which three types of cyclobutane structures are incorporated in a sequence of [aPEPaP eP ] (or [Q SjSa/3 S/3 ]), where /3 and are of opposite absolute configuration. Considering the stereochemistry of the cyclobutane rings, the polymer should be double syndiotactic ... 

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

Explain why it is possible to synthesize two diisotactic polymer structures, but only one syndiotactic polymer structure. 

Depending on the type of catalyst and other polymerization conditions, the molecular structure of PP homopolymer can consist of any of the three different types of stereochemical configurations for vinyl polymers isotactic, syndiotactic, and atactic. 

Distinguish between isotactic, syndiotactic, and atactic polymer structures. 

Syndiotactic polypropene has a regular alternation of 50% of hydrogen/methyl groups in front of/ behind the —C—C—C—chain viewing plane as shown in Figure 1.13. Its properties are similar to isotactic polypropene rather than the atactic form, i.e., the regular polymer structure produces stronger intermolecular forces and a more crystalline form than the atactic polypropene. 

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