State of the art of cyclic olefin polymers
- PDF / 439,495 Bytes
- 7 Pages / 585 x 783 pts Page_size
- 114 Downloads / 178 Views
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.1–5 The 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.2 The two alternative cycloolefin polymerizations are illustrated in Scheme 1. 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. The ring strain release is the main driving force of ROMP reaction cycles, thus strained monomers such as norbornene are readily polymerized.6 After 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 β-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 Kaminsky12 and to great interest in these polymers. Cycloolefin homopolymers synthesized by metallocene polymerization are not processable because of their low solubility in organic solvents and their high melting points.8 By copolymerization of cycloolefins, namely norbornene with ethene, amorphous cycloolefin copolymers (COC) (Topas) are produced with excellent transparency and high refractive index, making them suitable for optical applications. Soluble saturated cycloaliphatic homopolymers can now be obtained by late transition metal catalysts (palladium, nickel, and cobalt) in high-yield. Polymers with cycloaliphatic repeating units display good thermomechanical properties, high optical clarity, and low dielectric constants and are suited for microelectronic and optical applications. A brief review of historical aspects and of the state of the art of cycloolefin polymerization by ROMP and via addition of cycloolefi
Data Loading...
