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Ziegler-Natta catalysis: 50 years after the Nobel Prize
Introduction
The Ziegler-Natta discovery of catalysts for olefin polymerization and stereospecific α-olefin polymerization has led to the search for new transition metal catalysts and co-catalysts for other olefins, among them, the most interesting ones are the cyclic olefins (cycloolefins). Truett, Natta, and Sartori discovered almost at the same time that a cycloolefin such as norbornene could be polymerized by using heterogeneous systems based on Ti, W, or Mo halides and strong Lewis acidic co-catalysts.1-5The resulting polymer was unsaturated with 1,3-dimethylenecyclopentane repeating units, suggesting that polymerization occurred via ring-opening. Conventional heterogeneous Ziegler-Natta catalysts generally yield cycloolefin polymers containing both addition and ring-opening metathesis polymerized (ROMP) units.2The two alternative cycloolefin polymerizations are illustrated in Scheme 1 .
Scheme 1.
Addition polymerization and ring-opening olefin metathesis polymerization (ROMP) of cyclic olefins.
Cycloolefins such as cyclopentene, cyclooctene, and norbornene (common name of bicyclo(2.2.1)hept-2-ene) can be polymerized via ROMP and via addition ( Scheme 2 ). There is tremendous interest in their homo- and copolymers because of the easy availability of the monomers and interesting polymer properties.
Scheme 2.
Examples of cyclic olefins and functionalized norbornenes that can be homo- or copolymerized by transition metal catalysts. Note: t-Bu, tert-butyl; Ac, acetyl; n-Bu, normal butyl; Me, methyl.
The ring strain release is the main driving force of ROMP reaction cycles, thus strained monomers such as norbornene are readily polymerized.6After the initial studies, synthesis of unsaturated ROMP polymers was reported from homogeneous catalysts, with reproducible activities, creating prospects for industrial applications. This led to in-depth investigations and evidence for the metal carbene mechanism.7
In addition, with polymerization, both the ring strain of the cycloolefin and the non-planarity of the reacting double bond influence the reactivity, while the possibility of undergoing [beta]-hydrogen elimination, which leads to isomerization and chain termination, has consequences on polymer structure and molar mass.8-11
The discovery of homogeneous metallocene/methylaluminoxane (MAO) catalysts (see the Introductory and Busico articles in this issue) led to the first report on additional cycloolefin polymerization without ROMP by Kaminsky12and to great interest in these polymers. Cycloolefin homopolymers synthesized by metallocene polymerization are not processable...