Isotactic Polypropylene Compounds

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

Small-angle X-ray scattering (SAXS) data have made it possible to deduce the localisation of organic additives (pigments) in the bulk of isotactic polypropylene (iPP) [344]. This work has confirmed that the additives are located in the amorphous phase, in spite of their crucial influence on the formation of the crystalline phase of iPP. SAXS has also been used to study the 3D structure of different carbon-black aggregates, and silica-filled SBR rubber compounds [345]. 

Hafnium complexes with bis(cyclopentadienyl)s, 4, 731 in carbometallations, overview, 10, 255 in C-C bond formation, 10, 424 in ethylene polymerization, 4, 1139 for isotactic polypropylene, 4, 1126 metal-metal bonds, 4, 755 mixed-valence compounds, 4, 755 with pyridyl amines, for olefin polymerization, 4, 1094 in Ru-Os heterodinuclear compounds, 6, 1046 with silicon, 3, 515... 

The impact of Ziegler-Natta catalysis was enormous. The combination of TiCL as a transition metal compound with Et3Al as a main group metal compound opened the possibility for transmetallation. This is one of the most important unit reactions in transition metal-catalysed reactions. Also, production of isotactic polypropylene is a harbinger of stereocontrolled reactions catalysed by transition metal complexes, leading finally to asymmetric catalysis. 

Therefore, a stereochemical model consisting of a finite chain with equal end-groups was used [17], Isotactic polypropylene is described as a meso-compound with the two half-molecules being a mirror image of each other. 

Hydroperoxides are much more efficient than ketones for initiating photooxidation of ethylene-propylene copolymers [19]. This fact was confirmed by the results from photolysis of low-molecular model compounds and isotactic polypropylene [20]. 

The activity of the 4-alkylpyrocatechols (la), 3-alkylpyro-catechols (lb), 3,5-dialkylpyrocatechols (Ic), 3,6-dialkylpyro-catechols (II), 4,5-dialkylpyrocatechols (III), 3,4,6-trialkyl-pyrocatechol (TV), and 4,5- and 4,6-dialkyl-2-alkoxyphenols (V and VI) was studied in isotactic polypropylene at 180 =t 0.1° C. The relative activities Ar were correlated with substituent constants and redox potentials. In the la series the activity of 4-n-alkyl derivatives exceeded that of 4-tert-alkyl derivatives. The length of the main alkyl chain in la and lb and the steric effects in the latter exerted a specific favorable influence. In agreement with these effects, 3-tert-alkyl-5-methyl derivatives are the most active compounds in Series Ic compounds with other combinations of alkyls are weaker antioxidants as are the isomeric substances II and III or compound IV. Etherification of one hydroxyl group exerts an unfavorable effect influences of substitution in 2-alkoxyphenols (V and VI) are generally the same as in the pyrocatechols Z-ZZZ. 

Polypropylene. Isotactic polypropylene for all experiments was prepared in the Research Institute for Macromolecular Chemistry in Brno. Technical data MW = 510,000 (calculated from viscosity measurements in Tetralin), atactic fraction = 0.68%, stereoblocks = 6.75%, ash = 0.29%, Ti = 0.0172% (no compounds with Ti—Cl bonds), A1 = 0.11% free of stabilizers. The polymer was stored in an inert atmosphere in sealed ampoules at low temperature in the dark. 

Under the conditions used these compounds were the most active derivatives of pyrocatechol. High activity was found also with two derivatives having acyl groups (Table I). The decrease in the rate of oxygen absorption in isotactic polypropylene in the presence of 4-caproyl-, 4-lauroyl-, and 4-stearoylpyrocatechol compared with pyrocatechol is shown by Fic (10). The activity of 4-alkyl and 4-acyl derivatives of pyrocatechol found at high temperatures in polypropylene differs from the published data on the activity of 4-dodecyl- and 4-lauroylpyrocatechol in lard (39) compared with pyrocatechol, the relative activity decreased in the presence of the first substance by 15%, and it decreased in the presence of the second by as much as 68%. Tamura and co-workers... 

The activity of the 2-alkylhydroquinones (la), 2,5-dialkyl-hydroquinones (lb), 2,6-dialkylhydroquinones (Ila), and of 2,5- and 2,6-dialkyl-4-alkoxyphenols (Ic and lib) was studied in isotactic polypropylene at 180 0.1° C. The values obtained were correlated with hydroquinone (Ari) and pyro-catechol (AT2). All compounds studied (la) except the tert-octyl derivative were inferior in activity to hydroquinone. Two alkyls of Types lb and Ha exerted further but nonadditive unfavorable effects, particularly the lb type. Despite over-all low values of Ari, a weak favorable steric effect of the tert-alkyls is apparent in Type la compounds. Etherification even as acetylation of one hydroxyl group (Ic, lib), has a strong favorable effect. These compounds were the most active antioxidants of the entire hydroquinone series studied nevertheless their activity did not reach that of pyrocatechol (At2 < 1). 

The activities of all compounds from the hydroquinone series studied as antioxidants in isotactic polypropylene were lower than that of pyro-catechol. A similar conclusion was reached even when comparing both groups of substances as stabilizers in y-irradiated isotactic polypropylene... 

They are based on various metals. Such as zirconium, complexed with cyclopentadienide anions. This type of compound is called a zirconocene and is used with organoalu-minum to make highly regular polymers. The catalyst has the ability to flip back and forth from making atactic to isotactic polypropylene in the same polymerization. The alternating tacticity of the polymer breaks up the crystallinity of the chains and yields an elastomer. Metallocene catalysts are currently very expensive and cannot yet polymerize dienes such as butadiene, so they have only enjoyed limited commercial success in elastomers. However, this is one of the most intense fields of polymer research and many new product breakthroughs are expected in the near future. 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

Two compounds of this type were placed at our disposal isotactic polypropylene and an alternating erythro-iso-copolymer of butene-2 and ethylene. Looking to the extended chemical formula of the latter (Figure 13), it is indeed immediately evident that this copolymer can be considered as an equivalent to a HH polypropylene. The XPS analysis of the valence band spectrum of this compound reveals that its electronic structure, reflected through the C-C (C2s) molecular orbitals is entirely different from that of polypropylene (Figure 14). 

Highly active MgCl2 based catalysts can be obtained by mechanical treatment of mixtures of MgCl2 and Ti-compound. During such treatments the MgCl2 structure undergoes substantial variations which may affect the catalyst performance considerably. A Lewis base can also be added to the systems. The presence of the base is essential for the synthesis of isotactic polypropylene. Both cases will be considered separately. 

Examples of homogeneous catalytic systems in propylene polymerization are very few. Zambelli et al. obtained syndiotactic polypropylene by working at low temperatures with catalytic systems based on some vanadium compounds and aluminum alkyls, while Giannini et al. were the first to prepare isotactic polypropylene with some benzyl derivatives of titanium or zirconium. 

Table 6.1.8. Compounds identified in the pyrogram of isotactic polypropylene as shown in Figure 6.1.16.

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],... 

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