The fundamentals of catalytic olefin polymerization Basics of initiation, propagation and termination. Olefin polymerization versus olefin oligomerization. Chain transfer agents.

π σ 2 σ Why would polyolefins form? σ + π ΔH∅ = -21.2 kcal/mol Energy-rich monomer ΔH∅ = -21.2 kcal/mol

Ways to prepare polyolefins Free radical polymerization Cationic/anionic polymerization Ring opening metathesis polymerization Ziegler-Natta type coordination polymerization LDPE a-PS

Free radical polymerization Initiation and propagation Initiation propagation 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

Free radical polymerization termination by coupling 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

Free radical polymerization termination by H-transfer 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

Free radical polymerization termination by radical transfer (gives branches) 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

Ways to prepare polyolefins Free radical polymerization Cationic/anionic polymerization Ring opening metathesis polymerization Ziegler-Natta type coordination polymerization

Cationic/anionic polymerization Cationic polymerization Anionic polymerization Disadvantages: Little control. Very low temperatures required.

Ways to prepare polyolefins Free radical polymerization Cationic/anionic polymerization Olefin metathesis polymerization Ziegler-Natta type coordination polymerization

Ring opening metathesis polymerization

Ways to prepare polyolefins Free radical polymerization Cationic/anionic polymerization Ring opening metathesis polymerization Ziegler-Natta type coordination polymerization a-PP, i-PP, s-PP

Ziegler-Natta coordination polymerization How to activate an olefin: First the olefin has to coordinate to the metal. High valent, electron poor transition metal center that interacts with the  orbital of the olefin. Low valent, electron rich transition metal center that interacts with the * orbital of the olefin. xz yz xy x2-y2 Z2

Ziegler-Natta polymerization – catalyst High valent catalyst systems - general requirements Cocat. Electrophilic metal center (can be cationic) Vacant coordination site Polarized metal-polymer bond Robust ancillary ligand system Sometimes a cocatalyst is required d- d+ CH3 Metal Ancillary ligand

Ziegler-Natta polymerization – catalyst - Molecular weight - Tacticity Comonomer content Block structure Cross linking Polar function groups Cocatalyst/counterion - Coordinative tendency - Electronics - Stereodirecting characteristics Cocat. d- d+ CH2 Metal Ancillary ligand Monomer - Electronics - Sterics - Stereodirecting characteristics - Electronics - Sterics - Polar functional groups

Ziegler-Natta polymerization – catalyst

Ziegler-Natta polymerization – catalyst

Ziegler-Natta polymerization – mechanism General reaction mechanism Initiation Activation of the catalyst precursor Propagation Chain growth Termination Chain transfer, catalyst decomposition

Mechanism – poisoning Problem: sensitivity of the catalysts to oxygen, moisture and other heteroatom containing impurities. For most catalysts, the solvent and feed should be extremely pure… …Why is that? Ziegler's original reactor.

Mechanism – poisoning Problem: sensitivity of the catalysts to oxygen, moisture and other heteroatom containing impurities.

Mechanism – initiation Generation of an electrophilic metal site that contains a metal-carbon or metal-hydrogen bond. Catalyst precursors are generally metal halide species. In most cases the alkylation is done with aluminum alkyls. Ziegler-Natta precatalyst (MgCl2 / TiCl4 / ester) is treated with AlEt2Cl for activation. Homogeneous catalyst precursors are generally treated with methylalumoxane.

Mechanism – initiation If the competing electrophile is Lewis acidic enough, it can even abstract a chloride ion. Strong Lewis acid competes for the electrons of the chloride ion. Alkyl - chloride ion exchange finally affords the active catalyst consisting of a cationic zirconium alkyl species.

Mechanism – initiation

Mechanism – initiation

Mechanism – initiation C* < [Zr] TOF > 103 s-1

Modified Cossee-Arlman mechanism – migratory insertion Mechanism – propagation Propagation Modified Cossee-Arlman mechanism – migratory insertion Important for stereospecific polymerization

Mechanism – propagation

Green-Rooney carbene mechanism Mechanism – propagation Propagation Green-Rooney carbene mechanism This mechanism requires a base to scavenge the proton.

Green-Rooney carbene mechanism Mechanism – propagation Propagation Green-Rooney carbene mechanism This mechanism involves a change in the oxidation state of the metal, which is unlikely for LnIII, TiIV/ZrIV/HfIV, NiII/PdII… …but it did result in another modification of the Cossee-Arlman mechanism.

Modified Cossee-Arlman mechanism – agostic interaction Mechanism – propagation Propagation Modified Cossee-Arlman mechanism – agostic interaction α α β The agostic interaction increases the interaction of the C sp3 with the olefin. α

The extreme of agostic interaction is hydrogen transfer to the metal Mechanism – termination Termination β-hydrogen elimination The extreme of agostic interaction is hydrogen transfer to the metal Depends on relative bond strength of M-C and M-H whether or not this occurs

Generally most accepted termination mechanism Mechanism – termination Termination β-hydrogen transfer to monomer Generally most accepted termination mechanism

Mechanism – Choice of metal Early transition metals Fast insertion Slow b-H elimination kins >> kb-H EM-C ≈ EM-H Highly oxophilic Low tolerance to polar groups Late transition metals Slow insertion Fast b-H elimination kins << kb-H EM-C < EM-H Poorly oxophilic High tolerance to polar groups

Mechanism – termination β-alkyl elimination/transfer to monomer

Mechanism – termination Propagation versus termination Termination requires more room than propagation

Mechanism – chain transfer processes Besides spontaneous chain transfer processes, chain transfer can be induced by adding chain transfer agents (CTA’s). Chain transfer agents are used to: Control the polymer molecular weight Control polydispersity Introduce functional groups Reactivate dormant sites

Chain transfer – hydrogenolysis Dihydrogen is the most commonly used chain transfer agent (CTA) to control the molecular weight. d+ d- d+ d+ d- d- d- d+ Dihydrogen is a weak Lewis base (like olefins) that can easily be polarized formally producing an acidic proton that can protonate off the polymer chain.

H-X is already polarized, which facilitates the reaction. Chain transfer – CTA’s Substrates of the type H-X where X is more electropositive than H can also be used as chain transfer agents. d+ d- d+ d+ d- d- d- d+ d- d+ X = H, BR2, SiR3, … H-X is already polarized, which facilitates the reaction. HX

This process only works when M-X is not too strong. Chain transfer – CTA’s Substrates of the type H-X where X is more electronegative than H can also be used as chain transfer agent. d+ d- d+ d+ d- d- d- d+ d- d+ This process only works when M-X is not too strong.

Main group metal alkyls can also function as chain transfer agents. Chain transfer – CTA’s Main group metal alkyls can also function as chain transfer agents. d+ d- d+ d+ d+ d- d- d- d- d+ d- d+

Dormant sites and ways-out Reactivating dormant sites Dormant sites and ways-out primary alkyl ZnEt2 Chain transfer to zinc (fast) dormant site 1,2-insertion H2 primary alkyl 2,1-insertion secondary alkyl hydrogenolysis (very fast) D Isomerization (slow) 1,2-insertion (very slow) ethylene insertion (fast)

Reactivating dormant sites Enforced β-H elimination Dormant site

Chain transfer – CTA’s Chain transfer to main group metal alkyl CTA's is a relative new way to: Control the molecular weight PDI Produce end-functionalized polyolefins Multi-block copolymers. For this mechanism to be effective, a living catalyst is required.

Chain transfer – multi-block copolymers What else can be done with chain transfer agents? Could it transfer chains between different catalysts?

Chain transfer – multi-block copolymers CTA Both catalysts (M1 and M2) should be living catalysts. Both catalysts (M1 and M2) should have a good response for the CTA. Chain transfer should be slower than insertion but not too much slower.

Chain transfer – multi-block copolymers Ethylene / 1-alkene copolymers - Shuttle chemistry

Chain transfer – multi-block copolymers Ethylene / 1-alkene copolymers - Shuttle chemistry High Tm High modulus Low solubility High clarity Low PDI A copolymer is obtained with the soft-characteristics of an amorphous random copolymer and the hard-characteristics of a crystalline homopolymer.

Summary How does the reaction of the active catalyst with the olefin take place? The electron rich olefin is attracted to the electron poor metal center The olefin binds to the empty metal orbital The polar M-C bond introduces an induced dipole on the olefin The opposite charges attract each other which leads to -bond formation Metal-alkyl with -agostic interaction as resting state The alkyl group rotates away making space for the new olefin to approach

Summary How does termination takes place? -H transfer/elimination (spontaneous) Hydrogenation by H2 (chain transfer agent)

Polyolefins: Catalysis and dedicated analysis Market Homogeneous olefin polymerization catalysts Metallocenes Post-metallocenes Middle and late transition metal catalysts Cocatalysts

Polyolefins in the global thermoplastic resins market Coordination polymerization – market Polyolefins in the global thermoplastic resins market

Polyolefins in the global thermoplastic resins market Coordination polymerization – market Polyolefins in the global thermoplastic resins market

Coordination polymerization – market 53

Quite some plastic per year! Coordination polymerization – market Quite some plastic per year! 4 mm 24 km

Coordination polymerization – market

Coordination polymerization – market

Coordination polymerization – market

Coordination polymerization – catalyst types Ziegler-Natta heterogeneous Phillips heterogeneous Single-site homogeneous

Applied for i-PP, HDPE, LLDPE Heterogeneous Ziegler-Natta catalysts Applied for i-PP, HDPE, LLDPE Mostly titanium based (TiCl4, Ti(OBu)4) Supported on TiCl3 (1st gen.), MgCl2 and/or SiO2 Co-catalysts: Al(CH2CH3)3, Al(CH2CH3)2Cl, …. Non-uniform active species, leading to Broad molecular weight distribution Heterogeneous melting behavior Reactor blend

Heterogeneous Ziegler-Natta catalysts MgCl2

Heterogeneous Ziegler-Natta catalysts MgCl2 + TiCl4

Heterogeneous Ziegler-Natta catalysts MgCl2 + TiCl4 These chlorides are displaced by one alkyl and the metal is being reduced, which forms the active site.

Heterogeneous Ziegler-Natta catalysts MgCl2 + TiCl4 C2

Applied for mainly HDPE. Heterogeneous Phillips catalysts Applied for mainly HDPE. Chromium based e.g. CrO3/SiO2 High temperature activation required Co-catalyst: none, B(CH2CH3)3, AlR3 Non-uniform active species, leading to Broad molecular weight distribution (typical >10) Heterogeneous melting behavior Reactor blend

Heterogeneous Phillips catalysts Over 30 % of the HDPE is currently synthesized using the Phillips catalyst. No hydrogen response, Mw can only be altered by temperature.

Applied for i-PP, s-PP, HDPE, LLDPE, VLDPE, …. Homogeneous single-site catalysts Applied for i-PP, s-PP, HDPE, LLDPE, VLDPE, …. Group 4 metal (Ti, Zr, Hf) based Co-catalysts: MAO, boranes, borates, …. Uniform active species yielding narrow molecular weight distribution In general improved product quality In principle easier product/catalyst tailoring Depending on the process the catalysts are supported on SiO2 or MgCl2

Catalyst costs Quick and dirty calculation 1 ton of polymer €1.000,- Ziegler-Natta Phillips Single-site Catalyst price €50-€400/kg €20-€80/kg €100-€750/kg Catalyst activity 20-50 kgpp/gcat 2-10 kgPE/gcat 2-20 kgpp/gcat Catalyst costs 1-20 €/T 2-40 €/T 5- €/T 375 Quick and dirty calculation 1 ton of polymer €1.000,- 1 ton of monomer €650,- Business costs* €250,- Profit €100,- * of which €20 for cat costs. For a producer of 1000 kton polymer/year, decreasing catalyst costs by €1/T can save €1.000.000/year.

Homogeneous single-site catalysts Advantages of well-defined single-site catalysts. Tunability of the catalyst performance through designed ligand modification Enhanced stereo control Uniform, random and tunable comonomer incorporation

Titanocene dichloride (16 VE) Early metallocene catalysis Ferrocene (18 VE) Titanocene dichloride (16 VE) Breslow, 1955 + AlEt2Cl HDPE very low yield Limited activity Propene; only dimerization Active species unclear

Early metallocene catalysis Catalyst precursor Active species Inactive in ethylene polymerization unless a small amount of water is added. Careful hydrolysis of AlMe3 yields methylaluminoxane (MAO) with unprecedented cocatalyst abilities. polyethylene + [MeAlO]n atactic polypropylene high activity

Metallocene catalysts – C2v symmetric Dh symmetric – no stereo control

Metallocene catalysts – C2v symmetric No preference for the position of the polymer chain. atactic PP

Metallocene catalysts – C2 symmetric C2-symmetric – stereo control Ziegler-Natta catalyst

Metallocene catalysts – C2 symmetric Growing chain preferentially points away from the steric bulk of the ligand system. Growing chain is responsible for "indirect stereo control". Without growing chain, there is no stereo control. isotactic PP

Metallocene catalysts – C2 symmetric isospecific isospecific isospecific

Metallocene catalysts – Cs symmetric Cs-symmetric – stereo control

Metallocene catalysts – Cs symmetric Growing chain preferentially points away from the steric bulk of the ligand system. Growing chain is responsible for "indirect stereo control". Without growing chain, there is no stereo control. syndiotactic PP

Metallocene catalysts – Cs symmetric syndiospecific syndiospecific syndiospecific

Metallocene catalysts Different ligand symmetries and their effect on tacticity. By designing the catalyst's ligand system, various including some unprecedented types of polypropylene are available. C2v (atactic) C2 (isotactic) Cs (syndiotactic) What if we remove the symmetry? C1 (???)

Metallocene catalysts – C1 symmetric The effective steric hindrance of the methyl group is comparable to that of the fused aryl group. hemi-isotactic PP

Metallocene catalysts – C1 symmetric aspecific isospecific aspecific

Metallocene catalysts – C1 symmetric One side is sterically too crowded for the polymer chain. As a result, directly after insertion of a propylene molecule the polymer chain skips back to the less crowded side. isotactic PP isotactic PP

Metallocene catalysts – C1 symmetric insertion isospecific back skipping

Different ligand structures lead to different types of polypropylene. Development of metallocene catalysts Different ligand structures lead to different types of polypropylene. stereoblock i-PP (30% mmmm) i-PP (83% mmmm) i-PP (92% mmmm) i-PP (93% mmmm) High Mw > 0oC a-PP -45oC i-PP (83% m) i-PP (90% mmmm) i-PP (89% mmmm) High Mw i-PP (99% mmmm) High Mw s-PP (80% rrrr) s-PP (90% rrrr) hemi-i-PP i-PP (95% mmmm) UHMW a-PP s-PP (90% rrrr)

Synthesis of metallocene catalysts Ligand synthesis – the carbon skeleton is the most difficult part Metal halide complexes – there is no standard synthetic route for metallocenes – problem with formation of meso species. Metal alkyl complexes – relatively unstable – several improved routes are available Well-defined metal alkyl cations – unstable – boranes or borates are the standard cation generators

Synthesis of metallocene catalysts Ligand synthesis is often a multi-step process giving low overall yields. major – rac minor – meso A problem with C2 symmetric catalysts is that is Cs product is also formed. Whereas the C2 symmetric catalysts yields i-PP, the Cs symmetric one affords a-PP.

Characteristics of metallocene catalysts Single-site catalyst afford low PDI Homogeneous comonomer incorporation Polymerization rates can be extremely high (> 107 g·g(Zr)-1·h-1) Difficult to get high molecular weight polypropylene Metallocenes are somewhat less sensitive to H2 for Mw control

Early transition metal post-metallocenes more electron deficient – more 'open' Activated with MAO gave an excellent copolymerization catalyst. Copolymerizes α-olefins (short chain branching) and re-inserts macromonomers (long chain branching). Living 1-hexene polymerization.

Development of metallocene catalysts

Characteristics of post-metallocene catalysts Easy to synthesize – allows parallel synthesis and high throughput screening No isomer separation needed Robust – can be used at high temperature Open ligand system comonomer incorporation Generally less suitable for the synthesis of highly isotactic PP, exception forms the new generation of octahedral complexes

Middle and late transition metal catalysts Keim, Starzewski, Ittel oligomers, HDPE Brookhart catalyst, HDPE Brookhart catalyst, HDPE Grubbs catalyst, moderately branched PE Brookhart catalyst, highly branched PE Brookhart Gibson catalyst, α-olefins → HDPE

Early versus late transition metal catalysts Very active catalysts (easily 107 – 108 gpol∙molcat-1∙h-1∙bar-1) Very reactive catalyst - difficult to investigate [M]-P (with agostic interaction) is the resting state Coordination is rate limiting? LTM resting state ETM resting state Much slower catalyst (easily 102 – 103 gpol∙molcat-1∙h-1∙bar-1) Easy to study by NMR [M](C2H4)-P is the resting state Insertion is rate limiting

Early versus late transition metal catalysts Polymerization rate is dependent on [C2H4] Linear polyethylene Also excellent for a-olefins Poorly tolerant to polar groups Polymerization rate is zero-order with respect to [C2H4] Branched polyethylene Poor activity for a-olefins Tolerant to polar groups

Processes and catalysts

Processes and catalysts

Processes and catalysts

Processes and catalysts

Processes and catalysts Reactor type CSTR Loop Fluidized bed Operation Solution Slurry Gas conditions High Temp Medium Temp Short residence time Longer residence time Long residence time Low conversion High conversion Medium-high conversion Polymer specs preferably amorphous or low melting Preferably highly crystalline Medium to highly crystalline Catalyst specs Thermally robust Good morphology Excellent morphology High Mw capability Mechanically resistant Good comonomer Stable kinetic profile incorporation Good H2 response

Immobilization of homogeneous catalysts Advantages of immobilization: Can be applied in existing processes Polymer particle morphology control by replication High bulk density No reactor fouling Less cocatalyst required Homogeneous solution process Heterogeneous slurry process

Immobilization of homogeneous catalysts Advantages of immobilization: Can be applied in existing processes Polymer particle morphology control by replication High bulk density No reactor fouling Less cocatalyst required

Immobilization techniques Amorphous silica is most commonly used support. High (and tunable) degree of surface functionalities (silanols) to anchor catalyst High surface area and pore volume Fragments evenly Grafting Electrostatic interaction with grafted cocatalyst Tethering Advantage: Advantage: Advantage: easy to prepare - easy to prepare - well-defined - well-defined single sites single sites - no leaching Disadvantage: Disadvantage: Disadvantage: leaching - leaching (?) - difficult to prepare low activity - varying activity ill-defined

Importance of catalyst immobilization When do we need catalyst immobilization ? When we want to use a homogeneous catalyst in a gas-phase or slurry/suspension process. When can we use homogeneous catalysts ? When the polymer is soluble in the polymerization medium: amorphous (non-crystalline) elastomers (EPDM) or plastomers containing high comonomer content (VLDPE) When the polymer is crystalline but the polymerization temperature is higher than the melting point of the polymer produced (solution process for LLDPE: 150 – 250 oC)