Polypropylene, atactic isotactic

The rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

New Ziegler-Natta catalysts are homogeneous systems. These are based on a combination of metallocenes and MAO. By appropriate use of these catalysts linear polyethylene, and all the three types of polypropylene (atactic, isotactic and syndiotactic) can be produced in a specific manner. A cationic alkyl-metallocene complex is the active species in the polymerization. MAO helps in alkylating the metallocene (if the metallocene is a metallocene dihalide like Cp2ZrCl2) and also in assisting the removal of a CHa. Further it stabilizes the cationic center and also possibly scavenges impurities. 

The regular syndiotactic and isotactic structures are capable of crystallisation whereas the atactic polymer carmot normally do so. In the case of polypropylene the isotactic material is a crystalline fibre-forming material. It is also an important thermoplastic which can withstand boiling water for prolonged periods. Atactic polypropylene is a dead amorphous material. Polystyrene as commonly encountered is atactic and glass-like but the syndiotactic material... 

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. 

Figure 1,8 Principal types of tacticity found in polypropylene a) isotactic, b) syndiotactic, c) atactic...

The polymerizations occur at much lower pressures, and the polymers that are produced are much higher melting than atactic polypropylene, i) Isotactic polypropylenemelts at 175 °C. 

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

To date, no Raman spectrum of syndiotactic polypropylene has been published although vibrational analyses have been issued by Schacht-schneider and Snyder and also by Peraldo and Cambini during 1965. Recently we have had the opportunity to examine polypropylene in three forms atactic, isotactic and syndiotactic. The results for the last specimen are included in Fig. 6. It will be seen that there is an enormous emission in the 1350—1400 cm-1 region. The nature of this is not known — it may be fluorescence but it cannot be checked, as the anti-Stokes band at v0+ 1350 cm-1 would be vanishingly weak due to the low Boltzmann population 1350 cm-1 above the ground state. A coordinate analysis is available for syndiotactic polypropylene and currently we are working on an assignment of the observed results. 

Polypropylenes with low amounts of ohgomers atactic, isotactic, isoblock, stereoblock, and syndiotactic... 

Figure 6.10 Temperature-dependent equilibrium between chiral and achiral isomers of a metallocene catalyst and the resultant polypropylene with isotactic and atactic polymer-blocks.

Fig. 12a-c AFM pictures of different polypropylenes, a Isotactic polymer from Table 2, entry 4 b elastomeric polymer obtained with Ti benzamidinate complex [41 ] c atactic polymer fraction from Table 2, entry 8... 

Isotactic polypropylene Atactic polystyrene Low density polyethylene High density polyethylene A soft polyurethane Nylon 6... 

According to Eqs. (2.1.68), the function 5o(q) displays a small, sharp peak at q = 0, otherwise being nearly stationary. Calculations performed on isotactic polypropylene, atactic polystyrene, and polyethylene with realistic parameters [27] lead to o(0) of 1.30,1.20, and 1.05, respectively. This square expansion ratio increases with the thickness, or degree of bulkiness, of the chain and decreases with the length of the statistical segment if the two quantities are taken as proportional to v A r and to C(0)/, respectively. 

The feedstock materials used in this work included medium density polyethylene (PE), atactic-polypropylene (aPP), isotactic-polypropylenc (iPP), beech wood, pine wood, cellulose and hydrolytic lignin. The size of the wood biomass and the plastic particles was less 0,1 mm. 

The origin of plastic has more influence on the distribution of products of biomass / plastic co-pyrolysis. According to Table 3 the yield of light liquids from beech-wood / plastic mixtures (1 2 weight ratio) was decreased in the following sequence isotactic-polypropylene > atactic-polypropylene > polyethylene, whilst the yield of heavy liquids was increased in the same order,... 

Chiral titanocenes, zirconocenes, and hafnocenes in combination with methylalu-minoxane [A1(CH3)—0] , can lead to highly isotactic propylene. Nonchiral metallocenes like (Cp)2ZrCl2 or other similar compounds produce only pure atactic polypropylenes. Molecular mass of 590,000 for atactic polypropylenes can be achieved by low polymerization r. The activities of these hydrocarbon soluble catalysts are extremely high. Different structures of polypropylenes are obtained when the rr-bonded ligand of the transition metal is varied (Fig. 1). With no other catalyst can atactic, isotactic, stereoblock, isoblock, and syndiotactic polypropylene of such purity be produced. 

Description of polypropylene in the literature is often accompanied by terms atactic, isotactic, or syndiotactic. The origin of the terms is as follows. Side methyl groups in polypropylene chains can be all on the same side of plane (as it is shown above in a very simplified manner, as in reality carbon atoms in the chain are in zig-zag stereoconfiguration), on alternate sides, or in a random arrangement with respect to plane of carbon atom chain. These forms of PP are called isotactic (fiber-forming), syndiotactic, and atactic, respectively. All these forms are related to homopolymers of polypropylene. PP used for common applications, including WPC, is atactic and will be referred to here as polypropylene. 

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... 

In 1995 Coates and Waymouth ° reported a catalyst that produced polypropylene-containing blocks of atactic polypropylene and isotactic polypropylene. This novel elastomeric material was referred to as elastomeric homopolypropylene (EHPP). The unit cell of the crystal structure of the EHPP catalyst precursor bis(2-phenylindenyl)zirconium dichloride was observed to contain two distinct conformers see Figure 1 for the structures. In one conformer. 8M. the indenyl ligands were syn to one another (a meso stereochemistry), while in the other conformer. 8R. the indenyl ligands were anti to one another (a rac stereochemistry). Production of isotactic blocks of polypropylene could be explained by polymerization from the rac active site and production of atactic blocks by polymerization from the meso active site. In 1996 Pietsch and Rappe published a molecular... 

MAJOR APPLICATIONS The polymers referred to in this chapter include those families of homopolymers of propylene which are known to have elastomeric recovery properties at reasonable molecular weight and for which properties have been attributed to a crystallizable-noncrystallizable (e.g., isotactic-atactic) stereoblock structure, or to a major component with a stereoblock structure, whether or not the compositions are homogeneous by solvent fractionation tests. Copol)rmers and blends are not deliberately included in the data presented, but are described in some of the references. (See also some of the closely related elastomeric polymers presented in the entry on Polypropylene, atactic in this handbook.) The criterion of multiple crystallizable blocks per polymer chain may be met in significant fractions of low-tacticity, low-stereoregularity polymers of very high molecular weight. 

As a rule, technical plastics contain atactic, isotactic, and syndiotactic moieties. The ratio of these moieties to one another is decisive in terms of the plastic material properties. In polypropylene, for example, the atactic moiety is only 2-5 %. This means the degree of crystallization is potentially very high in polypropylene. The more regular the arrangement of the substituents, the greater the probability that crystalline structures can form. 

Fig. 3. The three basic structures for polypropylene (a) isotactic, (b) syndiotactic and (c) atactic.

Another saturated hydrocarbon polymer that has been studied is polypropylene . The isotactic form (with the methyl groups on one side of the chain) crystallizes the atactic form has insufficient configurational order for crystallization. It has been shown that in relatively highly crystalline isotactic material G(sc)/G(cl) 1.5 —1.8, while in material with low crystallinity (atactic as well as isotactic) a value of ca 0.8 is found. 

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