REQUIEM FOR INSERTION MECHANISM OF OLEFINS POLYMERIZATION BY TRANSITION METALS Dragoslav Stoiljković, University of Novi Sad, Faculty of Technology, Serbia,
[email protected] Liljana Korugic-Karasz, Amherst, USA, April 11th, 2017 Abstract: Despite six decades of intense research, the generally accepted insertion mechanism (IM) has generated many open questions. It is shown in this paper why IM is incorrect and nonsense. We reexamine the fundamental presumption of this polymerization, evaluate the roles of each basic component and propose Charge percolation mechanism (CPM).
1. The insertion mechanism dogma Very soon after Karl Ziegler and Giulio Natta in 1950-ties discovered that olefins could be polymerized by transition metals (Mt), Natta proposed [1] that monomer molecule is inserted between metal atom and organo-metallic compound (fig. 1), and proposed detailed monometallic and bimetallic mechanisms. Other investigators postulated a similar mechanism, although there was disagreement concerning the nature of active centers [2].
Figure 1. Natta proposed insertion mechanism of the polymerization of vinyl monomers with formation of isotactic polymers (Fig. 2 in [1]).
P. Cossee and E. Arlman in 1964 elaborated this idea and explained the nature of active center [2]. This interpretation was generally accepted and denoted as "Cossee-Arlman mechanism". In subsequent decades it was applied to olefins polymerization by: 1. Classical Ziegler-Natta complexes, i.e. TiCl3/AlEt2Cl; 2. CW complexes, i.e. TiCl4/AlEt3 supported on MgCl2; 3. Phillips catalysts, i.e. CrxOy supported on SiO2/Al2O3, discovered in early 1950-ties by Paul Hogan and Robert Banks [3]; 4. Metallocenes based on Zr, Ti and Hf with methylaluminoxane (MAO), discovered in 1980 by Walter Kaminsky [4]. Since 1964 till now, many extensions, modifications and improvements of Cossee-Arlman mechanism have appeared as well as some alternative mechanisms, too. (Their presentation is out of scope of this manuscript.) The common feature and the main dogma of all these mechanisms is the monomer molecule insertion between Mt atom and a carbon atom of organometallic compound.
2. Six decades of insertion mechanism open questions and controversies Insertion mechanism (IM), however, has generated many unsolved questions in the last six decades: • What are true mechanisms of initiation, chain propagation and termination? • How are the active centers activated? • What is oxidation state and structure of active center? • Are the active centers uniform or non-uniform, i.e. are there one or several types of active centers? • How and why active centers are deactivated? • Does alkylaluminium participate in the active centers? • How can one correlate the kinetic equations to the IM models? • What is the exact role of support? • Why full polymerization activity is observed only if the ratio support/Mt is extremely high? • What are the roles of electron donors, hydrogen, MAO and other components? • What is origin of resulting polymer structure (molecular mass, MWD, stereoregularity)? In the previous six decades almost of thousand researchers, in hundreds laboratories worldwide, performed perhaps million experiments using very sophisticated equipment and methods in order to find answers on these questions. However, no definite, unequivocal answers on above questions have been defined. (See examples of open question listed below.) (The Greek hero Odysseus, after the fall of Troy, wandered to come back to his homeland Ithaca. This is a well known proverbial myth of extremely long wandering. It lasted, however, only ten years.) The thousand scientists, however, wandered more than sixty years and still did not arrive to the explanation of olefin polymerization by transition metals. Evidently, some initial and fundamental presumptions are wrong and have to be re-examined! Examples: - "Each worker has examined some aspect of the problem and has given his view of what is happening. The findings are similar pieces of a puzzle which have to be coupled to form the whole picture; only here, some critical pieces are still missing". [Boor J., "Ziegler-Natta Catalysts and Polymerizations", Academic Press, New York, 1979] - In spite of the improvements in catalyst technology, "there is still much to be learned about the elementary steps" in olefin polymerization. [Jügling S., Mülhaupt R., Stehling U., Brintzinger H., Fischer D., Langerhauser F., J. Polym. Sci., Part A: Polym. Chem., 33,1305(1995)] - It can be concluded that despite intense research activity, no definite, unequivocal polymerization mechanism has yet been defined to describe the behaviour of metallocene and Ziegler-Natta catalysts. [Hamielec A. E., Soares J. B. P., Prog. Polym. Sci., 21,651(1996)] - The effects of the nature of metallocene complexes of group IV metals on their performance in olefin polymerization are not clear. A research group, which synthesized over 650 metallocene and half-sandwich catalyst precursors to test their catalytic potential and to study the influences of various catalysts parameters on the properties of the resulting polymers, after a plenty of work concluded: "What did we learn? We have to confess that we still do not understand all details in sequence to be able to predict the exact properties of designed metallocene catalysts. Too many parameters are involved that determine the kinetics of the polymerization. Tiny changes at the metallocene complex can have a drastic effect on the activity of the catalyst and the properties of the polymers. Even molecular modeling cannot answer all the questions: it only can confirm trends." [Alt H. D., Köppl A., Chem. Rev., 100,1205(2000)] - "Yet, despite more than 50 years have passed, the mechanism of polymerization from Ti activation to its deactivation included, is still not fully understood." [Bahri-Laleh N., Correa A., Mehdipour-Ataei S., Arabi H., Haghighi M. N., Zohuri G., Cavallo L., Macromolecules, 44, 778(2011)] - "While the catalyst was commercialized more than 50 years ago, the exact structure of the site that is active for polymerization is still being debated, although the ingredients in the simplest form of the catalyst are only a silica support and chromium ions." [Potter K. C., Beckerle C. W., Jentoft F. C., Schwerdtfeger E., McDaniel M. P., J. Catalysis, 344,657–668(2016)] 2
3. An examination of fundamental presumptions [5] There are three basic participants in the polymerization: monomer, transition metal and support. The crucial question is: In what processes does each participant enter and why? The general answer is: Each participant has some problems that are solved by relevant processes and with the assistance of the other two participants. The mutual cooperation of all three participants is necessary to solve their individual problems. The next logical question is: What are individual problems of each participant and how they are solved by relevant processes? 3.1. Problems of monomers Polymerization means the breaking of double bonds (C=C) in the monomer molecules and creating single bonds (CC)n in them and between them. This possible if the change of free energy is negative value, i.e. ΔGpol=ΔHpol – TΔSpol
