Atactic materials

Whilst completely atactic material would be amorphous, commercial materials have a small measure of crystallinity. This is often assessed in terms of insolubility in n-heptane which is usually of the order of 5-10%. Viseosity average moleeular weights are in the range 20 000-80 000 and specific gravities are about 0.86 g/cm. ... 

Since only a small amount of atactic material is available as a by-product from the manufacture of isotactic polybut-l-ene, atactic polybut-l-ene is normally produced directly. 

Poly(vinyl acetate) is an atactic material and is amorphous. Whilst the structure of polyfvinyl alcohol) is also atactic the polymer exhibits crystallinity and has essentially the same crystal lattice as polyethylene. This is because the hydroxyl groups are small enough to fit into the lattice without disrupting it. 

In case of polypropylene some atactic polymer also gets formed in addition to the required isotactic polymer but much of this atactic material is soluble in the diluent so that the product isolated would be largely isotactic polymer. 

PVA is produced by the hydrolysis of PVAc (Equation 6.44). Because of the ability to hydrogen bond and small size of the hydroxyl grouping, PVA is a crystalline atactic material. 

There are striking differences in physical properties between the atactic and isotactic forms. The atactic material is soft, elastic, somewhat sticky, and rather soluble in solvents such as 1,1,2,2-tetrachloroethane. Isotactic polypropene is a hard, clear, strong crystalline polymer that melts at 175°. It is practically insoluble in all organic solvents at room temperature, but will dissolve to the extent of a few percent in hot 1,1,2,2-tetrachloroethane. That the difference between the atactic and isotactic polymers arises from differences in the configurations of the methyl groups on the chains is shown in a... 

In a typical 80,000 tons/year plant, capital costs were about 220 per metric ton in 1974. To produce 1000 kg of polymer, 1030 kg of monomer is needed, together with 1 kg of hydrogen and 25 kg of diluent. Catalyst and miscellaneous chemicals cost about 4 per 1000 kg of polyethylene pellets produced. For production, 300 kg of medium-pressure steam, 800 kg of low-pressure steam, 530 kWh of electrical energy, 200 m3 of water, 30 m3 of nitrogen, and 600 m3 of air are also required. To polymerize propylene in suspension, the same technology can be used. Catalysts now available [based on TiCl3 (see Table II)] make it unnecessary to separate isotactic from atactic materials. 

The similar, older slurry process uses a less active catalyst. The monomer is dissolved in isooctane, the titanium catalyst and aluminium cocatalyst are added and this mixture is fed to the reactor which is maintained at 70°C. The inorganic corrosive (Cl) residues are removed in a washing step with alcohols. The atactic material is removed by extraction. A third process employs propene as the liquid in combination with a high activity catalyst. The Himont Spheripol process, which uses spherical catalyst particles, gives spherical polymer beads of millimetre size that need no extrusion for certain purposes. A more recent development is the gas-phase polymerization using an agitated bed. All processes are continuous processes, where the product is continuously removed from the reactor. Over the years we have seen a reduction of the number of process steps. The process costs are very low nowadays, propene feed costs amounting to more than 60% of the total cost. 

It is in the stereospecific polymerization of propylene that metallocene complexes display their astonishing versatility. Commercial Ziegler-Natta catalysts for isotactic polypropylene - based on combinations of TiCU, MgCl2, Lewis bases and aluminum alkyls - depend on a metal-centered chirality which exists at specific edge and defect sites on the crystal lattice to direct the incoming monomer in a particular orientation. These catalysts produce small amounts of undesirable atactic material due to the presence of achiral active sites. 

Elastomeric polypropylenes with thermoplastic behavior can be prepared from conformationally dynamic metallocenes such as bis(2-arylindenyl)zirconium dichlorides. These can exist in two conformations in the course of the chain lifetime a chiral rac isomer which is stereodirecting and an achiral meso isomer which is aspeciflc. The resulting polymer consists of blocks of isotactic polypropylene alternating with runs of atactic material (equation 12). 

The existence of such associated organolithium compounds has been estabhshed in various cases (19, 20, 24), In addition to isotactic polystyrene, a considerable amoimt of atactic material is always present it is formed by starting the polymerization on the nonassociated part of the organolithium compounds which probably promote a nonstereospecific anionic polymerization. The stereoregulation of the polymerization of styrene by heterogeneous alkali metal aUcyl initiators is limited by the forces on the surface of the catalyst while the dissolved organolithium initiators in their associated form cause the stereospecific polymerization. 

It was noted in Section 1.1.3 that, when one moves to polymers more complex than polyethylene, the likelihood of the polymer being able to crystallize depends on the chemical composition, in particular whether the repeat unit has an asymmetric centre. When it does, then the ability to crystallize rapidly diminishes with the amount of atactic material in the polymer. Fully atactic polymers will generally be amorphous and the properties of the glass resulting from the cooling of an atactic polymer from the melt are discussed in the following section. 

Three different polypropylene (PP) modifications can be distinguished the atactic, the syndiotactic and the isotactic modification. The atactic modification is an amorphous polymer with a Tg(onset)-value of -21°C. The syndiotactic modification, made with a stereospecific homogeneous metallocene catalyst, is a semi-crystalline polymer (crystallinity about 25 %wt.) with a Tm-value of about 133°C [10]. The isotactic modification, made with a stereospecific heterogeneous Ziegler Natta catalyst is also a semicrystalline polymer (crystallinity about 50 %wt.) with a Tm-value of about 160°C and contains nearly always 2 %wt. - 5 %wt. of atactic material. 

A series of C6 up to C18 a-olefin fractions prepared by the so-called SHOP process were polymerised with a Ziegler-Natta catalyst system. The purity of these fractions was > 98 %wt. The (peak) molecular weights of the polymers proved to be > 200.000 NMR analysis showed atactic material and some stereoregularity. The results of the fusion/recrystallisation measurements (see 1.1.4) are listed in Table 1.6. 

The most likely cause of the crystallinity variations in polypropylene spherulites is the accumulation of rejected atactic and low molecular weight impurities. This view is supported by the observation that adding increasing amounts of atactic material to polypropylene purified by octane extraction, leads to changes in the crystallinity distribution (Figure U). 

We are now analysing the rejection behaviour of the atactic material along similar lines to those used for the uv absorbing additives to determine the mobility of the atactic fractions within the polymer. 

Some polymer surface studies that have been reported recently are the detection of the molecular orientation at polymer film surfaces. For instance, the spectrum of iostactic polystyrene is different from that of the atactic material [82]. The spectra of thin films of poly(methylmethacrylate) (PMMA) cast on An, Al, or Cu were also different especially the intensity of the C = 0 stretching band at 1710 cm" varied considerably [83]. Thus HREELS seems to be capable of identifying molecular long-range order in polymeric surfaces. 

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