Structure of Polypropene

Polymerization of propene gives polypropene (52), with creation of one stereogenic centre per monomer unit. Three possibilities exist for the structure of 52 and these are shown in the stereo drawings 53-55. Compound 53 has a random distribution of configurations at carbons along the chain, and this type of polymer is known as atactic. 

In polymer 54, all methyl groups are on the same side of the polymer backbone, which is drawn in the plane of the page this type of polymer is known as isotactic. The methyl groups alternate regularly from one side of the chain to the other in polymer 55, which is known as syndiotactic 

In general, atactic polymers tend to be soft, amorphous material, whereas the more regular structures of isotactic 54 and syndiotactic 55 polymers permit chains to lie closer together, and accordingly these polymers have a greater tendency to be crystalline. The terms atactic, isotactic and syndiotactic are quite general and are used for other similarly substituted polymers. 

Assign configuration to each of the stereogenic centres in 56. 57 and 58. Which of the compounds is/are meso  

These answers demonstrate that compounds with appropriate stereochemistry, i.e. 56 and 58, can be meso even though the stereogenic centres are not adjacent. One can see this more clearly in the rigid structure 56 than in the acyclic diol 58, However, to demonstrate that these two molecules are conceptually related, proceed as follows break the C( 1 )-C(7), C(3)-C(4) and C(4)-C(5) bonds in 56 to leave a diol, 58, that derives from C( 1). C(2). C 3), C(5) and C(6) of 56, with, of course, hydrogens added to satisfy the tetravalency of carbon. 

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However, there are subtle differences in the ways to draw this structure. Figure 1.11 shows a longer structure of polypropene and also some three-dimensional structure. This structure shows how some bonds (the dotted lines) are behind the plane of the paper and others stick out of the paper (the ones on the ends of the little triangular wedges). In this structure, some of the methyl (—CH3) groups are above the paper plane and others are behind the paper plane. This is called atactic polypropene. 

The stereoselectivity of polymerization depends on the transition metal and the structure of the initiator. Syndioselective polymerization is more common than isoselective polymerization. Some titanium phenoxy-imine initiators yield highly syndioselective polymerization by chain end control. For example the initiator with R2 = R3 = t-butyl yields polypropene with (rr) = 0.92 [Tian and Coates, 2000]. The initiator with R2 = t-butyl and R1 = C6Fs yields polypropene with (rr) = 0.98 [Saito et al., 2001 Tian et al., 2001], Moderately isoselective polymerization is obtained with some zirconium and hafnium phenoxy-imine initiators [Saito et al., 2002]. 

Metallocenes are very versatile catalysts for the production of polyolefins, polystyrene and copolymers. Some polymers such as syndiotaetic polypropene, syndiotactic polystyrene, cycloolefin copolymers, optically active oligomers, and polymethylenecycloalkenes can be produced only by metallocene catalysts. It is possible to tailor the microstructure of polymers by changing the ligand structure of the metallocene. The effect and influence of the ligands can more and more be predicted and understood by molecular modeling and other calculations. 

The combination, VCl4/AlEt2Cl(Al/V = 2—20), at low temperatures (—78°C) gives the syndiotactic form of polypropene [130] the rate is proportional to vanadium concentration. Addition of AlEt3 to the catalyst changes the structure of the polymer-to the isotactic form. 

Every active metal atom has two available coordination sites (the two locks), which can both insert the olefin, and that can be different in either shape or chirality. In the framework of the chain migratory insertion mechanism, the monomer has to be inserted alternately on each site, and the structure of the resulting polypropene depends on the combination of the regio- and enantioselectivity of the two... 

The most important mechanism of stereospecific polymerization is isospecific enantiomorphic site control, which allows today the production of more than 25 million tons per year of isotactic polypropene and its copolymers, in a wide range of molecular weights and crystallinities. As already mentioned in section II, the molecular architecture of polypropenes obtained from ansa-zirconocenes is strongly dependent on the biscyclopentadienyl ligand structure. 

Broad melting transitions are observed in the DSC analyses of the more isotactic polymers, indicating a broad distribution of tacticity in the polymer. Similar melting points (Tm = 137 °C) but very different enthalpy of fusion (0.2 < AH< 17.4 J/g) are observed for polymer obtained using different catalysts. The enthalpy of fusion is proportional to the mmmm content. The high melting points in the presence of a low mmmm pentad content speak for a blocky structure of the polymers. As a matter of fact, these polypropylenes contain isotactic, isotactic-Woc/c-atac-tic and atactic polypropene chains. A polypropene sample obtained with the (2-Ph-Ind)2ZrCl2/MAO catalyst (7(, = 20 °C in liquid propene) was fractionated... 

Isospecific Ci-Symmetric Metallocenes. Perfect hemiisotacticity requires that mmmm = 18.75%. Deviating from the structure of Ci-I-6 in general makes the complexes more isospecific. Fink studied the series Ci-I-6—Ci-I-9 where the R substituent on Cp increases from methyl to terf-butyl. While the ethyl derivative produces a polypropene very similar to that (prevailingly hemiisotactic) made with the methyl derivative Ci-I-6, the 3-i-PrCp derivative C -... 

Another most important source of variability in the molecular architecture of polypropenes obtained from ansa-zirconocenes, besides the biscyclopentadienyl ligand structure and monomer concentration, is the polymerization temperature, Tp. Unfortunately, most of the earlier catalytic studies on the performance of metallocene catalysts have been carried out in solution at largely different propene concentrations, so changes in the latter due to lower propene concentrations at the higher Tp become the primary cause for changes on both polymer properties and polymerization kinetics, rather than Tj, itself (see previous section). It is therefore of the utmost importance, when comparing the polymerization performance of different zirconocene catalysts, to perform the experiments under high and identical monomer concentrations, and preferably in liquid propene, to minimize the extent of chain-end epimerization. 

As already discussed in the section dedicated to stereoregularity, besides the biscyclopentadienyl ligand structure and the concentration of propene, the polymerization temperature is another important source of variability in the microstructure of polypropenes obtained from ansa-zirconocenes. In liquid monomer, both the amount of secondary insertions and the rate of 2,1 3,1 isomerization increase with increasing Tp. For example, for rac-C- iil-lndlzLrCXzl MAO and rac-C2H4(4,7-Me2-l-Ind)2ZrCl2/MAO, total... 

Using micro-Raman imaging three blends consisting of polypropene/polyethene/ethene-propene copolymer, PBTP/polycarbonate/LDPE, and styrene-acrylonitrile copolymer/styrene-maleic anhydride copolymer/ polydimethylphenylene oxide were studied with regard to compositional and morphological heterogeneities. The general structure of PE fibres in an epoxy resin matrix was also studied. 59 refs. 

PPs all have the same simphfied structural polymer formula of polypropene as shown in Figure 1.10. 

Schupfner, G Kaminsky, W. Microstructure of polypropene samples produced with different homogeneous bridged indenyl zirconium catalysts. Clues on the structure and reactivity relation. J. Mol. Catal. A Chem. 1995,102, 59-65. 

This construction principle is mandatory for the stereoregular structure of the polypropene molecule. In addition, head to head... 

While the structural description of low molecular weight compounds with asymmetric carbon atoms is explicit, there are no similarly accurate rules for the description of polymers. Tertiary carbon atoms in polyolefin chains are not asymmetric in a general chemical sense. Even with one of the substituents bearing a double bond at its end and the other terminated by an ethyl group, they are very similar. Therefore, these carbon atoms are often called pseudo asymmetric. The differences between the three forms of polypropene with identical molecular weight distribution and branching percentage are considerable (Table 13). 

With ansa(chiral) titanocenes, zirconocenes, and hafnocenes in combination with methylalumoxane (MAO) it is possible to obtain highly isotactic polypropene [366-374]. When changing the symmetry of the complex, different structures of the polypropene are yielded. The activity of these hydrocarbon soluble catalysts are extremely high. 

Figure 47 A, 22.6 MHz CP/HPHD/MAS spectrum of syndiotactic polypropene [10] B, projection of the helical chain in the crystal structure of syndiotactic polypropene [12]...

Figure 3.14 3D ball-and-stick structure of isotactic polypropene [4]. 

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