Monumental Discovery and Progress
More than 50 years have passed since Ziegler and Natta shared the Nobel Prize for the discovery of olefin polymerization catalysts. There was a plenty of research about the effects of various catalyst components (i.e., metal precursors (Mt), activators, electron donors, supports, and polymerization conditions) on the catalysts productivity and polymer properties. Some European labs which are involved now in these researches areas are presented by Bashir, 2020.
The field of metal-catalyzed polymerization has matured, mainly due to the development of several high-performance catalysts. Olefin polymerization catalysts and processes, and technological and commercial impacts on polyethylene (PE) and polypropylene (PP) in 2015-2025 [TCGR, 2017] and progress in technology for polyolefins production are presented elsewhere [TCGR, 2020].
This progress, however, has often occurred as a result of trial and error discovery [Chen, 2018]. Why?
“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 et al., 2011]
Insertion Mechanism and Open Questions
Despite intensive research, the generally accepted insertion mechanism (IM), proposed by G. Natta and elaborated by E. Arlman and P. Cossee (Fig. 1) has generated many open questions:
- What are true mechanisms of initiation, chain propagation and termination?
- What is the oxidation state and structure of active center?
- 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 the origin of polymer structure (molecular mass, MWD, stereoregularity)?
In the previous six decades almost thousand researchers, in hundreds laboratories worldwide, have performed perhaps million experiments using very sophisticated equipment and research methods in order to find answers on these questions. However, no definite answers have been found:
- “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, 1979]
- In spite of the improvements in catalyst technology, “there is still much to be learned about the elementary steps” in olefin polymerization [Jüngling et al, 1995].
- Despite intense research activity, “no definite, unequivocal polymerization mechanism has yet been defined” to describe the behavior of metallocene and Ziegler-Natta catalysts [Hamielec and Soares, 1996].
- 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 and Köppl 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 et al., 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 et al., 2016].
The Greek hero Odysseus, after the fall of Troy, was wandering to return to his homeland Ithaca. It is a well known proverbial myth of extremely long wandering. However, it lasted only ten years. More than sixty years ago, however, thousands of scientists have entered and wandered in the labyrinth of olefin polymerization by transition metals and did not find an exit, i.e. did not explain the polymerization. Evidently, they chose the wrong path. Some initial and fundamental presumptions are wrong and have to be re-examined! What should be done? Return to the entrance of the labyrinth, i.e. return to the very beginning to choose some another research path.
The support has a very important role to gather both components at the same location, thus enabling both reactions (oxidation-reduction of Mt and polymerization of monomer) are merged and performed simultaneously, in a common process. Support is the catalyst for both processes.Dragoslav et al., (2006)
Promising Research Path – Surface Science
“Process optimization based on empirical data for heterogeneous olefin polymerization without a fundamental understanding of the molecular processes for the polymer formation has reached the limit. Further improvements of the olefin polymerization system will require catalyst design and investigation of the molecular mechanism of the polymerization”. “The surface science results reported here prove the potential of such catalyst design”. “A combination of various surface science techniques is used for characterization of the surface composition, structure, and oxidation state of these model catalysts. X-ray photoelectron spectroscopy (XPS) can identify the oxidation states of Ti ions at the active sites. Temperature-programmed desorption (TPD) can distinguish the binding strength of probe molecules to the active sites that produce polypropylene with different tacticities.
These findings are of great benefit to polymerization science and technology”. It was found that in a TiClx monolayer on top of MgCl2 multilayer the oxidation states of the titanium species have distributions of 4+, 3+ and 2+. The result “rules out the bimetallic mechanism in which the Al-containing species bonded to the Ti active site is claimed to be responsible for the stereochemistry control”. “The polymerization reactivity is not correlated with the Ti3+ concentration”. The “polymerization rates correspond to nominal turnover frequencies of 3 – 6*1014 C3H6 molecules per cm2 catalyst per second and 1 – 1.8*1016 C2H4 molecules per cm2 catalyst per second, respectively”, i.e. correspond to the rate of monomer adsorption on the catalyst support [Kim and Somorjai, 2006].
The Support is Crucial – Charge Percolation Mechanism (CPM)
Despite the importance of Mt support has been recognized at the very beginning, it is usually omitted in IM schemes (Fig. 1).
Hence, some initial and fundamental presumptions have been re-examined in order to clarify the role of support as well as the roles of Mt and monomers [Stoiljković et al., 2007, Stoiljkovich, 2017]. Mt ions exist simultaneously in several oxidation states (Mt+2, Mt+3, Mt+4 etc.) producing an irregular charge distribution over the support surface. The tendency to equalize the oxidation states by charge transfer from Mt+2 to Mt+4 and both of them to transform the stable inactive Mt+3 cannot be performed, however, since Mt+2 and Mt+4 ions are immobilized and highly separated on the support.
But, monomer molecules are gradually adsorbed on the support producing a cluster (Fig. 2, left) which terminal monomer molecules make pi-complexes with Mt+2 and Mt+4, like “plug in socket”. Thus adsorbed monomer molecules gradually make a bridge of pi-electron cloud between Mt+2 to Mt+4. Once a bridge is completed (percolation moment), charge transfer occurs. The Mt+2 and Mt+4 equalize their oxidation states simultaneously with the polymerization of the monomer. The polymer chain is detached from the support (Fig. 2, right), freeing the surface for subsequent monomer adsorption. The whole process is repeated by the oxidation-reduction of other Mt+2 and Mt+4 tandems.
Mt+2 and Mt+4 tandem is deactivated to 2Mt+3. Polymerization is not possible without this kind of deactivation. This kind of deactivation is not possible without the polymerization of monomer. Monomer polymerization and deactivation of Mt+2 and Mt+4 are mutually interdependent acts; one is not possible without the other. The both components (Mt and monomer) are chemically changed. The both are reactants. Neither of them is the catalyst.
The support has a very important role to gather both components at the same location, thus enabling both reactions (oxidation-reduction of Mt and polymerization of monomer) are merged and performed simultaneously, in a common process.
Support is the catalyst for both processes.
The initial steps of CPM path are identical to those of the IM path (Fig. 3). Only the final monomer insertion step is excluded and replaced by the monomer bridge formation and the charge percolation steps, which are those “critical pieces” asked by Boor even in 1979. Thus, the most of empirical findings, theoretical considerations, and statements made by researchers in previous decades are of great importance for CPM, too.