Olefin Polymerization Organometallic Catalysis 11/16/2018 Olefin Polymerization

Olefin Polymerization What is a polymer? [ -monomer- ]n A polymer is anything that is hard to get out of a Schlenk tube A polymer is anything that gives broad NMR spectra Plastics, rubbers, superabsorbent materials, starch, cellulose, peptides, DNA 11/16/2018 Olefin Polymerization

Olefin Polymerization Atactic polypentene - 1H NMR 11/16/2018 Olefin Polymerization

Olefin Polymerization Atactic polypentene - 13C NMR 11/16/2018 Olefin Polymerization

Polymers and oligomers Typical "polymer properties" appear at » 1000 monomer units * more for small apolar monomers (polyethene) * less for large polar monomers (polyester) "Oligomers" consist of 5-50 units. This is the region where separation of individual components is difficult. Oligomers are mostly used as "performance chemicals" (synthetic detergents, fuel additives) 11/16/2018 Olefin Polymerization

Olefin Polymerization Questions If you tear a plastic fiber, do you break individual polymer chains? And if you cut it? What is the strongest polymer? or 11/16/2018 Olefin Polymerization

Olefin Polymerization Physical properties are important Crystallinity Transparency Strength Stiffness Viscosity Tg "Melt flow" Paintability 11/16/2018 Olefin Polymerization

Olefin Polymerization Solid polymers Amorphous or Partially crystalline: small crystallites, with amorphous regions between them. No sharp melting point, but a melting range of up to »10°C. Also often a "glass transition temperature" (Tg) between a brittle glass-like state and a rubber-like state. 11/16/2018 Olefin Polymerization

Chemistry determines properties Monomer solubility, interaction between chains, surface reactivity Chain regularity crystallinity Crosslinking rubber-like properties Molecular weight and distribution viscosity, melt flow 11/16/2018 Olefin Polymerization

Chemical reactions in polymer synthesis Polymerization Þ chain structure Stereoselectivity Þ chain regularity Chemoselectivity Þ branching Initiation, termination, chain transfer Þ mol wt and distribution Chemical modification of polymers after their formation 11/16/2018 Olefin Polymerization

Molecular weight distribution Mi = mol wt of polymer i Ni = number of molecules of this polymer Number-average: Weight-average: Polydispersity: Depends on polymerization kinetics, but is often independent of molecular weight. 11/16/2018 Olefin Polymerization

Olefin polymerization Exothermic reaction, but slow at RT: "just add the right catalyst" Anionic polymerization Insertion polymerization 11/16/2018 Olefin Polymerization

Olefin Polymerization Cationic polymerization Radical polymerization Ring-opening metathesis polymerization 11/16/2018 Olefin Polymerization

Metal-centered olefin polymerization Basic mechanism: 2+2 addition is normally "forbidden". "Allowed" here because of asymmetry in M-C bond: Empty acceptor orbitals at M Polarity Md+-Cd- bond d-orbitals at M Þ easier to form small CMC angles Insertion also happens at main-group metals, but is much slower there 11/16/2018 Olefin Polymerization

Olefin Polymerization 11/16/2018 Olefin Polymerization

Requirements for an active catalyst M-C or M-H bond (can be formed in situ) Empty site or labile ligand (anion) Highly electrophilic metal center No easily accessible side reactions For stereoregular polymerization: fairly rigid metal environment 11/16/2018 Olefin Polymerization

Insertion is a 2-site mechanism Original Cossee mechanism Modified mechanism 11/16/2018 Olefin Polymerization

Olefin Polymerization The carbene mechanism (Green) Involves change in oxidation state of metal Þ unlikely for LnIII, Ti/Zr/HfIV, Ni/PdII Modified Green mechanism (a-agostic assistance) M-H (or M-CH) interaction could facilitate rotation of the C sp3 orbital. 11/16/2018 Olefin Polymerization

Olefin Polymerization 11/16/2018 Olefin Polymerization

Olefin Polymerization Agostic interactions occur frequently in electron-poor transition metal complexes. a- and b-agostic interactions are most the common types. Examples of both have been found in crystal structures. The interactions are usually weak (0-6 kcal/mol) and fluxional. b-agostic interactions are strongest. 11/16/2018 Olefin Polymerization

Olefin Polymerization Chain transfer Main chain transfer mechanism: b-elimination Balance between M-H and M-C bond strengths very important for chain lengths. General trends: 11/16/2018 Olefin Polymerization

Other chain transfer reactions Alkyl transfer to cocatalyst b-Me elimination 11/16/2018 Olefin Polymerization

Termination mechanisms Allyl formation M-C or M-H bond homolysis (reduction of metal) Reactions with impurities in monomer "Burning" 11/16/2018 Olefin Polymerization

Olefin Polymerization Fast chain transfer Isomerization primary alkyls more stable and more reactive equilibrium favours internal olefins Dimerization Insertion rates: ethene > a-olefin >> internal olefins 11/16/2018 Olefin Polymerization

Olefin Polymerization Slow chain transfer M-H (relatively) unstable Polymers up to 106-107 D possible (but often not desirable) If: there is only a single "site" kCT is independent of chain length kinit is not too small tpol >> tchain then Q = Mw/Mn » 2 Any deviation from the first 3 conditions increases Q. Determination of Mn by NMR (for up to 1000 units): detectable vinyl and vinylidene end groups Main MWD analysis method: GPC 11/16/2018 Olefin Polymerization

No chain transfer "living" polymerization If: there is only a single "site" kCT "zero" (on time scale of experiment) kinit is large relative to kprop then Q = Mw/Mn » 1 Any deviation increases Q. Living polymerization can occur if the M-H bond is very weak. 11/16/2018 Olefin Polymerization

Olefin Polymerization Block copolymers If kterm is "zero" at the laboratory time scale, one can make block copolymers, e.g. Not often used for polyolefins, except (Doi): Also used for SBS rubbers: 11/16/2018 Olefin Polymerization

Controlling the molecular weight Very high MW is not always desirable Þ add an MW "control agent" H2 saturated end groups (NMR) other olefin MW control agents increase kct and lower the MW, but do not necessarily affect Mw/Mn 11/16/2018 Olefin Polymerization

Olefin Polymerization MW distributions Schulz-Flory distribution (polymerization with chain transfer) characterized by g = kprop/(kprop+kct) mole fraction of n-mer = gn(1‑g) Poisson distribution (living polymerization) characterized by a = ratio monomer:"catalyst" mole fraction of n-mer = ane-a/n! 11/16/2018 Olefin Polymerization

Olefin Polymerization 11/16/2018 Olefin Polymerization

Typical polymerization conditions (Pre)catalyst on a support Active species generated in situ (e.g., by addition of "cocatalyst") Al alkyls or similar added as "scavengers" Very pure monomer used 70 - 150°C Gas phase (ethene, ethene/propene) Liquid olefin (propene, higher olefins) 11/16/2018 Olefin Polymerization

Classical Ethene Polymerization Catalysts Ziegler catalysts Role of Al alkyl alkylation reduction of M (via M-C or M-H bond homolysis?) scavenger weakly coordination anion? "Almost any metal can polymerize ethene" 11/16/2018 Olefin Polymerization

Commercial Ziegler catalysts are always heterogeneous High-surface TiCl3 formed in situ from TiCl4 and Al alkyl TiCl3 on support (e.g. MgCl2), prepared in a complicated process starting e.g. from TiCl4 and Mg(OAr)2. Nature of active site not very clear. Chain transfer is slow Þ high MW H2 or temperature used to control MW. Distribution of different sites on surface Þ broad MWD (Q = 5-30) Catalyst productivity > 106 g/g Ti/hr Þ catalyst residue removal not needed. 11/16/2018 Olefin Polymerization

Types of polyethylene different application areas High-density (HDPE) produced by Z-N or metallocene catalysis Low-density (LDPE) produced by high-temperature radical polymerization Linear low-density (LLDPE) produced by Z-N or metallocene catalysis with some a-olefin comonomer Long-chain-branched produced by Brookhart-type catalysts or by metallocene catalysis with long-chain a-olefin comonomer 11/16/2018 Olefin Polymerization

Other classical PE catalysts (1) VCl4/EtAlCl2 and related systems "Possibly the most active ethene polymerization catalysts" (Shell, 1972). But: V catalyzes PE autoxidation and most be removed Homogeneous? VIII? Halogenated compounds added to improve catalyst stability (reoxidation of VII to VIII?) Fairly narrow MWD (Q = 2-4) VIII is paramagnetic Þ this is a difficult system to study! 11/16/2018 Olefin Polymerization

Other classical PE catalysts (2) CrO3 or Cp2Cr on silica (Phillips) Cr oxides need to be "activated" with a reductant (H2, ethylene) Metallocenes are currently replacing Z-N catalysts for commercial production of specific types of PE. 11/16/2018 Olefin Polymerization

Olefin Polymerization Generalization In simple, hard-donor ligand environments, early first-row metals are the best catalysts. M-H vs M-C easier to form "naked" ions accessibility of different oxidation states cost 11/16/2018 Olefin Polymerization

Olefin Polymerization Polypropene Propene polymerization is more difficult than ethene polymerization: Insertion of a-olefins is slower than of ethene Termination by b-elimination is easier Generally, some degree of stereocontrol is necessary to make an interesting product But (isotactic) PP is attractive because it has a higher melting point than PE. 11/16/2018 Olefin Polymerization

Olefin Polymerization PP stereoregularity PP is not chiral!!! 11/16/2018 Olefin Polymerization

Commercial PP production Ziegler-Natta technology The process to make an iso-specific catalyst is complex. An example: Pre-treatment of support with an organic "donor" (ether or ester) Absorption of TiCl4 on surface Addition of Al alkyl mixed with a second "donor" (ester or di-ester) à Some surface sites are better for isospecificity than others à Aspecific sites can be converted into isospecific ones Fair syndiotacticity is also possible, but much more difficult. 11/16/2018 Olefin Polymerization

The active site of Z-N catalysts? There have been many proposals for the active site in Z-N catalysts and the reason for its isospecificity. These are probably all incorrect or very incomplete. Our basic understanding of the system is still poor, and this is the reason metallocenes have had such a dramatic impact recently. 11/16/2018 Olefin Polymerization

The active site of Z-N catalysts? There is consensus about the direct Ti environment on the surface; and it is probably TiIII. Insertion occurs primarily in a 1,2-fashion (determined from end-group analysis). Steric bulk near the Ti center must be responsible for the isospecificity (site control). The Cossee mechanism was originally proposed to explain the isospecificity. If the site has approximate C2 symmetry, the modified Cossee mechanism would also explain the results. 11/16/2018 Olefin Polymerization

Olefin Polymerization 11/16/2018 Olefin Polymerization

Olefin Polymerization Errors in Z-N PP Stereo-errors Regio-errors "1,3-insertion" 11/16/2018 Olefin Polymerization

Mechanism of 1,3-insertion (has been demonstrated by labelling) 11/16/2018 Olefin Polymerization

Olefin Polymerization Effect of addition of H2 Reduction of MW Increase in activity Increase in iso-specificity Explanation: "Dormant sites", formed by an insertion error, have lower propagation rate but still react rapidly with H2. The H2 effect allows a better determination of the number of active sites. From this, we can conclude that in the best Z-N catalysts, high isospecificity is obtained not just by blocking aspecific insertion, but by "optimizing" for isospecific insertion! 11/16/2018 Olefin Polymerization

Olefin Polymerization PP tacticity Analysis of errors by 13C NMR. Stereochemistry usually expressed in "linkages": NMR can be used to distinguish different linkages, e.g. mmrm vs mrrm "pentads". Formal view: site vs chain-end control Assume stereochemistry of next insertion is determined completely by: Geometry of metal site ("site control") Stereochemistry of last inserted unit ("chain-end control") 11/16/2018 Olefin Polymerization

Olefin Polymerization These extremes can be distinguished by the insertion errors. site control: chain-end control: Always observe a distribution of different linkages. Statistics can be used to analyze the results in terms of site control, chain-end control, mixed control, or mixture of different sites with different specificities. 11/16/2018 Olefin Polymerization

The Doi system for propene polymerization V(acac)3 + EtAlCl2 Mainly syndiotactic Living (Q = 1.0‑1.2) at low temperature (up to ‑40 °C) Activity in the order of kg/g V/hr Homogeneous Large ligand effects: ca 6% of V active ca 95% of V active 11/16/2018 Olefin Polymerization

Olefin Polymerization Doi system Possible explanation: active site relatively open: 2,1-insertion (at least for the syndiotactic blocks) chain-end control r linkage 11/16/2018 Olefin Polymerization

Metallocenes "the next generation" M = TiIV, ZrIV, HfIV X = weakly coordinating anion (MAO, borate, carborate) Kaminsky, Ewen, Marks 11/16/2018 Olefin Polymerization

Olefin Polymerization Cp2TiR+ polymerizes propene with slight and temperature-dependent isospecificity mainly 1,2-insertion chain-end control Stereopreference depends on difference in steric interaction with Cp of P and Me groups. 11/16/2018 Olefin Polymerization

The "trick" of metallocenes Discriminating between chain positions ("site control") Preferred chain position independent of previous insertion stereochemistry 11/16/2018 Olefin Polymerization

Olefin Polymerization One "real" catalyst 11/16/2018 Olefin Polymerization

Preferred alkyl chain orientation 11/16/2018 Olefin Polymerization

"Wrong" chain orientation 11/16/2018 Olefin Polymerization

Preferred olefin orientation 11/16/2018 Olefin Polymerization

"Wrong" olefin orientation 11/16/2018 Olefin Polymerization

Olefin Polymerization After every insertion, the chain moves to the other site at Zr. Chain preferences the same (i.e. related by 2-fold axis) Þ isospecific polymerization Chain preferences opposite (i.e. related by mirror plane) Þ syndiospecific polymerization One site with a strong preference, the other without preference Þ hemi-isotactic polymerization Double stereodifferentiation: the absolute configuration of the last inserted monomer affects the preference, so the tacticity of a unit after an insertion error is atypical. This complicates interpretation of NMR data. In an extreme case, one could (theoretically)make a new polymer: 11/16/2018 Olefin Polymerization

Olefin Polymerization Typical metallocenes see e.g. Brintzinger, Angew. Chem. IE 34(1995)1143 11/16/2018 Olefin Polymerization

Characteristics of metallocene polymerization Difficult to get MW as high as Z-N catalysts H2 has same effects as in Z-N catalysis (dormant sites) Narrow MWD (Q » 2) Rates can be extremely high (> 107 g/g Zr/hr) Chain transfer by b-H and b-Me elimination Errors same as in Z-N catalysis, plus: stereo-errors after insertion (demonstrated by labelling studies and by dependence of isospecificity on [monomer]) 11/16/2018 Olefin Polymerization

Mechanism of epimerization 11/16/2018 Olefin Polymerization

Olefin Polymerization Back-skip Inversion at the metal Does not affect isospecific polymerization but results in a stereo-error in syndiospecific polymerization Back-skip more difficult in highly bent systems (short bridges between the rings) Back-skip after each insertion will usually result in isospecific polymerization (cf Z-N systems) 11/16/2018 Olefin Polymerization

Non-metallocene ETM polymerisation catalysts (just a selection) Mostly non-stereoregular. Differences in rate of termination (MW) and incorporation of higher olefins. 11/16/2018 Olefin Polymerization

Olefin Polymerization General conclusion An M-C bond, an "empty site" and electrophilicity are enough for polymerization. Most unsaturated ETM compounds can in principle polymerize olefins. The ligand environment is needed for suppressing side reactions and for "tuning" MW, chemospecificity and stereospecificity 11/16/2018 Olefin Polymerization

Polymerisation at LTM centers Mostly low MW, but with "hard" donors MW can be higher. Isomerization ("chain running"). Lower sensitivity to functionalized olefins. 11/16/2018 Olefin Polymerization

Olefin Polymerization The SHOP process (1) Basic reaction: Main byproducts: Special properties of SHOP catalyst: isomerization rate low very little insertion of a-olefin 11/16/2018 Olefin Polymerization

Olefin Polymerization The SHOP process (2) Depending on market, desired products are: Hexene and Octene, as comonomers for polyethene and polypropene C10-C18 olefins for detergents (hydroformylation or sulphonation) Butene is "waste" Schulz-Flory distribution is broad Þ relatively large amount of undesired products 11/16/2018 Olefin Polymerization

Olefin Polymerization The SHOP process (3) Oligomerisation carried out in polar solvent (diol) in which products are insoluble. 11/16/2018 Olefin Polymerization

LTM oligomerization and polymerization Problem: easy chain transfer by b-elimination because M-H bond stable (relative to M-C bond). Low MW (dimers or oligomers instead of polymers) Isomerization But there are exceptions ! Why ? 11/16/2018 Olefin Polymerization

Chain transfer via b-elimination? According to theoretical studies, termination could never compete with propagation! 11/16/2018 Olefin Polymerization

Olefin Polymerization Explanation Chain transfer does not involve a free hydride Associative displacement: "chain transfer to monomer" Anion or solvent assistance: Monomer, anion and solvent can occur in chain transfer rate equations. 11/16/2018 Olefin Polymerization

The "famous” Brookhart system Enough space for insertion but not for associative displacement Þ high MW. Enough space for (reversible) b-elimination and olefin rotation Þ chain running 11/16/2018 Olefin Polymerization

Olefin Polymerization Chain running Product more linear than expected 11/16/2018 Olefin Polymerization

Olefin Polymerization 2,1- vs 1,2-insertion 1,2-insertion is the most common reaction 2,1-insertion can be favoured if: The metal is relatively open Insertion is in a M-H bond (as opposed to M-C) The olefin has a functional group in a-position 11/16/2018 Olefin Polymerization

LTM systems are more compatible with functionalized olefins Low incorporation of acrylate because M-acrylate coordination weaker than M-ethene coordination (why?). But after coordination, acrylate is more reactive. 11/16/2018 Olefin Polymerization

Acrylate inserts in a 2,1-fashion The 5-membered chelate ring is relatively stable. Copolymerisation is much slower than ethene polymerisation (factor 30-300). 11/16/2018 Olefin Polymerization

Characteristics of LTM polymerization catalysis Rates can be comparable to ETM catalysts Anion coordination is not a major problem Can be compatible with protic solvents and functionalized monomers MW tends to be (much) lower, unless associative displacement is blocked Chain running gives rise to unusual branching No convenient general "ligand skeleton" for easy tuning Hard ligands destabilize hydride Þ higher MW. 11/16/2018 Olefin Polymerization

Comparison of ETM and LTM catalysts 11/16/2018 Olefin Polymerization

Challenges in olefin polymerization Comonomer incorporation Stereocontrol Control over branching (long chain branching) Morphology (reactor fouling, space-time yield) "True gas-phase" processes New monomers New block copolymers 11/16/2018 Olefin Polymerization