Polyolefins propylene polymerization, catalyst

The vacant sixth coordination site of these Ti centres can take up an olefin molecule to form the reaction complex required for the initiation and subsequent growth of polyolefin chains. Due to their octahedral dichelate-type structure, these Ti(III) centres are chiral and thus able to steer each incoming molecule into a preferred enantiofacial orientation. The stereospecificity with which subsequent propylene units insert into the growing polymer chain is most likely based on a mechanism analogous to that determined for soluble polymerization catalysts (Section 7.4.3). 

Unlike molecules containing electron-rich heteroatoms, boron compounds do not poison Ziegler-Natta or metallocene polymerization catalysts. Borane-containing olefin comonomers are therefore well suited to produce olefin copolymers while retaining good catalyst activity. The resulting polymers are suitable for subsequent conversion into a variety of functional groups. In principle, two approaches are possible (1) hydroboration of the terminal double bond (formed by typical chain transfer processes) of a preformed polyolefin, and (2) direct copolymerization of propylene or a 1-alkene with an alkenyl borane (Scheme 11.4). 

Further support for this mechanism was provided by Ewen in the form of a catalyst which polymerizes propylene to hemiisotactic polypropylene. The metallocene shown in Scheme IV has two different coordination sites, one which is isospecific and one which is aspecific.51 When used for propylene polymerization, the alternation between iso- and aspecific sites results in a hemiisotactic polymer (Scheme IV). The polymer was readily characterized due to the pioneering work of Farina, who independently prepared this material previously. The rational synthesis of isotactic, syndiotactic, and hemiisotactic polyolefins represents a crowning achievement in the application of transition metal catalysts in stereocontrolled reactions. 

The hydrogenation of cis-1,4 copolymers of B and I would lead to polyolefins with composition and sequence distribution consisting of ethylene (E) blocks and alternating ethylene/propylene (E/P) blocks. These novel polyolefins are difficult or almost impossible to obtain directly by simple polymerization of E and P monomers using any existing polymerization catalysts. Since structural variations in these polyolefins, such as composition aind monomer sequence distribution, would significantly affect the polyolefin properties, the hydrogenated cis-1,4 B/I copolymers with uniformly random distribution of E and E/P imits may serve as model polymers to study structure-property relationships and be useful as polymers with unique properties. 

Sugano T (1999) Novel metallocene catalysts for propylene polymerization. In Proceedings Schotland conference on specialty polyolefins (SPO 99), October 12-13, 1999, Houston, Texas. Schotland Business Research, p 33... 

Electron donors play an important role in modem catalysts for propylene polymerization. Recent breakthroughs in polyolefin catalyst and process technology have largely come about through an increase in empirical understanding and use of Lewis bases in a right way. However, the functional mechanisms of the reactions are poorly known. 

Transition metal catalysis plays a key role in the polyolefin industry. The discovery by Ziegler and Natta of the coordination polymerization of ethylene, propylene, and other non-polar a-olefins using titanium-based catalysts, revolutionized the industry. These catalysts, along with titanium- and zirconium-based metallocene systems and aluminum cocatalysts, are still the workhorse in the manufacture of commodity polyolefin materials such as polyethylene and polypropylene [3-6],... 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

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