Polypropylene resins syndiotactic

Syndiotactic polypropylene resins allow the production of small flexible containers with very good y-radiation resistance and a degree of transparency never achieved before with any polypropylene. Moreover, the final objects transmit a feeling by contact that has been qualified as soft and warm. Only some highly expensive specialty and engineered polymers can achieve the same combination of properties, but require a significantly longer cycle time. 

The syndiotactic polymer configuration is not obtained in pure form from polymerizations carried out above 20°C and, thus has not been a serious concern to most propylene polymerization catalyst designers. Eor most commercial appHcations of polypropylene, a resin with 96+% isotacticity is desired. Carbon-13 nmr can be used to estimate the isotactic fraction in a polypropylene sample. Another common analytical method is to dissolve the sample in boiling xylene and measure the amount of isotactic polymer that precipitates on cooling. 

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

There are three types of polypropylene amorphous (aPP), isotactic (PP), and syndiotactic (sPP). Performance of these resins depends on the tacticity content. PP was commercialized in 1957 by Hoechst. 

Much of the work on the conformation of polymers concerns itself with reactions of the lower olefins. By the use of certain catalytic systems, crystalline syndiotactic polypropylene may be formed exclusively, rather than as a co-product in the formation of the isotactic macromolecule. To accomplish this, rather specific reaction conditions are required. For example, a catalytic system of approximately five molar equivalents of aluminum dialkylmonochloride to one of vanadium triacetylaceonate at less than 0°C initiates the stereospecific polymerization of propylene to a syndiotactic resin. Other catalytic systems have also been described [108]. As discussed above, these polymers are not expected to be optically active. 

When the insertion of propylene molecules in the growing chain is such that all methyl branches are on the same hand, the regularity of the chain allows it to crystallize this is the isotactic polypropylene (iPP), synthesized by Natta [8, 9] and the most commercial type obtained by the Ziegler-Natta stereoregular polymerization process. When the monomer insertion is consistently in the opposite hand to previous monomer insertion, the polymer obtained is syndiotactic polypropylene (sPP), which achieves lower crystallinity and today has less commercial interest. The random stereo incorporation of monomer units results in an amorphous resin, atactic polypropylene (aPP). 

Syndiotactic polypropylene can be used as an impact modifier. Indeed, the addition of s-PP to i-PP can improve impact characteristics over pure i-PP [222-225]. When s-PP is blended with i-PP, the resulting resin has a processability better than that of i-PP and impact and transparency properties better than those of pure i-PP. As an impact modifier to a controlled rheology i-PP copolymer, s-PP does not crosslink or affect the peroxide efficiency of the copolymer while improving the Izod notched impact and maintaining the similar processability of the copolymer. 

Polypropylene, Polypropylene and its copolymers account for about 13 percent of U.S. resin sales. Its manufacture and properties have much in common with HDPE. Unlike polyethylene, however, isotactic, syndiotactic. Nomenclature ). Atactic polypropylene, and atactic configurations are possible as a consequence of the pendant methyl group (see above, under Polymer Structure and... 

The relatively open active sites of metallocene catalysts permit the copolymerization of nontraditional cyclic comonomers, snch as styrene and norbomene, with ethylene. Although such resins are not cormnercially available at present, they have the potential for exhibiting novel physical characteristics, possibly expanding the use of polyethylene into new markets. Metallocene technology has also been developed for the production of isotactic and syndiotactic polypropylene, copolymers of propylene with other olefins, and syndiotaetie polystyrene. 

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