Isotactic materials

An important group of polymers used as moulding resins and in extruded forms (e.g. film). Can be electroplated. Useful polymerization is by Ziegler catalysis and gives an isotactic material. U.S. production 1983 1 -7 megatonnes. 

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... 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Syndiotactic materials are generally softer, tougher and more transparent than isotactic materials but exhibit similar melting points Table II. 6). 

In appearance and on handling the material is somewhat intermediate between a wax and a rubber. It is also semi-tacky. Like isotactic polypropylene it is attacked by oxygen but unlike the isotactic material it swells extensively in aliphatic and aromatic hydrocarbons at room temperature. It is also compatible with mineral fillers, bitumens and many resins. 

The commercial poly-(4-methypent-1-ene) (P4MP1) is an essentially isotactic material which shows 65% crystallinity when annealed but under more normal conditions about 40%. For reasons given later the material is believed to be a copolymer. In the crystalline state P4MP1 molecules take up a helical disposition and in order to accommodate the side chains require seven monomer units per two turns of the helix (c.f. three monomers per turn with polypropylene and polybut-I-ene). Because of the space required for this arrangement the density of the crystalline zone is slightly less than that of the amorphous zone at room temperature. 

A related enantiomerically pure zinc amide initiator, (340), has also been described.966 This complex catalyzes the alternating copolymerization of CHO and C02 to yield isotactic material (RR SS = 86 14). Similar enantiomeric excesses have been achieved using a mixture of Et2Zn and the chiral amino alcohol (341).967 Molecular weight distributions are much broader than using catalyst (340), but this protocol is still a convenient way to prepare optically pure diols (Scheme 23). 

An almost isotactic material has been obtained in CH2CI2 with the palladium catalyst [Pd(Me)(NCMe)((R,S)-BINAPHOS)]BAr4 (Ar = 3,5-bis(trifluoromethyl)phe-nyl) containing the unsymmetrical chiral bidentate phosphine-phosphite (i, S)-BI-NAPHOS = [(R)-2-(diphenylphosphino)-(S)-l,l -binaphthalene-2,2 -diyl phosphite]... 

Highly crystalline isotactic polyolefins are not soluble in organic solvents at room temperature. However, most amorphous polyolefins and oligomers of cr-olefins are easily soluble in saturated and aromatic hydrocarbons at ambient temperature. This difference in solubility can be used to separate amorphous atactic components of polyolefins from crystalline isotactic material in crude polyolefins mixtures. 

The mechanical 8 relaxation is slightly affected by tacticity. For an isotactic material the peak is narrower and not as pronounced as for an atactic sample [8,17]. 

Polymer Stereochemistry and Optical Activity.—Although stereoregular products from carbocation propagations are not common, there are some notable examples, perhaps the most important being the isotactic materials from alkyl vinyl ethers. Recently, novel catalysts based on phosphoryl and thionyl chlorides with vanadium pentoxide have been added to those initiator systems capable of producing stereospecific reactions. 

NB The data compares a syndiotactic material with a melt index of 30 and an isotactic material with an index of 0.3. This should however not invalidate the large differences seen in such propenies as modulus, impact strength and opacity.)... 

The elfects of solvent and temperature arc manifest in the polymerization of methyl methacrylate with w-butyl hthium at 243 K in a series of mixed solvents prepared from tolume and dimethoxyethane (DME). The NMR spectra of the products indicate the compositions in Table 6.2 and reveal that a predominantly isotactic material is produced in a low-polarity medium, but that this becomes highly syndiotactic as the solvating powCT of the medium increases. 

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. 

For instance, the Tg of PP depends strongly on its tacticity. Atactic PP has a Tg—19 C and is used in bitumen and hot-melt adhesive formulations, while for isotactic PP, it is approximately -8°C. The latter is a tough, semicrystalline plastic used in packaging films. The Tg of syndiotactic form of PP can be as high as ISO C Sophisticated catalyst systems even aUow synthesis of polymers that have ordered blocks of atactic materials and isotactic materials on the polymer chains. 

Unlike polyethylene, which crystallizes in the planar zigzag form, isotactic polypropylene crgtaUizes in a helical form because of the presence of the methyl groups on the chain. Commercial polymers are about 90 to 95 percent isotactic. The amount of isotac-ticity present in the chain will influence the properties. As the amount of isotactic material (often quantified by an isotactic index) increases, the amount of crystallinity will also increase, resulting in increased modulus, softening poinL and hardness. 

The stereochemistry from chain-end control and catalyst-control are not necessarily different. More details on the stereochemistry of alkene pol3mnerization are provided in Chapter 22. In brief, however, a symmetric catalyst like the ones used for the copolymerization of carbon monoxide with styrene will generate isotactic polymer, but polymerizations with stereochemistry dictated by chain-end control can form either syndiotactic or isotactic material. Catalysts displaying other symmetries have been designed for other types of polymerizations to generate polymer architectures beyond isotactic. 

Employing a dilatometric technique, Booth and coworkers determined the melt behavior of fractionated PPO as a function of crystallization temperature and by this technique found the thermodynamic melting point for PPO to be near 82° (Fig. 31), surprisingly 100° C lower than that of POM. These authors also detected three dilatometric transitions for partially isotactic material. The first below 60° was ascribed to the melting of lamellar crystals of limited thickness arising from the isotactic sequence. The other two transitions which depended on Tf- were attributed to the melting of lamellae with thicknesses determined by primary nudeation. 

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