Cocatalysts Heterogeneous Ziegler-Natta type catalysts are generally activated with aluminum alkyls or aluminum alkyl-halides (AlEt3, Et2AlCl, …) Function: reduction + alkylation Heterogeneous "chromium-on-silica" Phillips catalysts can be activated with CO or are self-activating upon treatment with ethylene Function: reduction Homogeneous catalysts are generally activated with aluminoxanes or the combination of an alkylating agent plus a cation generating agent Function: alkylation, cation generation

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.

Cocatalysts – proposed structures of MAO

Cocatalysts – methylaluminoxane Four tasks have been identified (currently accepted scheme): 1. Scavenger for oxygen and moisture and other impurities in the reactor 2. Introduced methyl groups on the transition metal 3. Methylated metallocene is not a sufficiently good electrophile to coordinate olefins. MAO takes away a chloride or methyl anion to give a positively charged – more electrophilic complex 4. Three dimensional structure delocalizes or diffuses the anionic charge that was previously held tightly by the chloride.

Cocatalysts – methylaluminoxane MAO is likely a number of cage species Despite extensive research, the exact composition and structure of MAO are still not entirely clear or well understood The MAO structure is difficult to elucidate because of the multiple equilibria present in MAO solutions Its nature and behavior critically depends on preparation Needs to be used in large excess (100 – 10,000 equiv) MAO is unstable and tends to precipitate from solution over time and tends to form gels - considerably limits its utility. Residual trimethylaluminum in MAO solutions appears to participate in equilibria that interconvert various MAO oligomers – this is a well-known problem with this materials Residual trimethylaluminum in MAO can deactivate the catalyst and sometimes needs to be removed.

Cocatalysts – methylaluminoxane Residual trimethylaluminum in MAO can deactivate the catalyst and sometimes needs to be removed. Problem Way out

Cocatalysts – boranes & borates Alkyl/Hydride Abstraction by Neutral Strong Lewis Acids. Reaction of borane (B(C6F5)3 to remove a Me group. Cation-anion ion pairing stabilizes highly electron-deficient metal centers Sufficiently labile to allow an a-olefin to displace the anion  

Cocatalysts – boranes & borates With a sub-stoichiometric amount of borane, the electron deficient cation forms a Lewis base adduct with the neutral dimethyl species. + ½ B(C6F5)3 [MeB(C6F5)3]-

Cocatalysts – boranes & borates Instead of prior alkylation, the chloride precursors can be treated simultaneously with TiBA (Al[CH2C(H)Me2]3) and the borane or (trityl or ammonium) borate.

Cocatalysts The cost of the cocatalyst is frequently more than that of the precatalyst, especially for group 4 metal-catalyzed olefin polymerization - can represent 1/2 to 1/3 of the total cost Often a large excess of cocatalyst relative to the amount of precatalyst is required Presents compelling reasons to discover more efficient, higher performance and lower cost cocatalysts - understand their role in the polymerization processes Activators affect the rate of polymerization, the polymer molecular weight, thermal stability of the catalyst system, stereochemistry of polymer

Polyolefins: Catalysis and dedicated analysis Catalyst structure versus polymer microstructure Homopolymers Random copolymers Stereoregular polymers Multiblock copolymers

Tacticity of polypropylene Stereoselective propylene polymerization Tacticity of polypropylene atactic PP isotactic PP block isotactic PP hemisotactic PP syndiotactic PP

Site-control – lock & key model Site Polypropylene non-selective regio- and stereoirregular regioselective stereoselective (atactic) regioselective = regioselective stereoselective regioselective stereoselective (isotactic)

Enantiomorphic site-control Enantiomorphic site control – the key in lock model: two locks, one key =

Enantiomorphic site-control C2 symmetric Cs symmetric +

Chain-end control Chain-end control si-chain The difference in energy between the two configurations is small (∼2 kcal⋅mol-1). Configuration A affords an isotactic sequence, whilst configuration B results in a syndiotactic one. si-propylene si-chain re-propylene

Tacticity of polypropylene – characterization Regio- and stereo errors Tacticity of polypropylene – characterization primary alkyl 1,2-insertion stereo-error primary alkyl 2,1-insertion secondary alkyl D isomerization (1,3 insertion) 1,2-insertion

Effect of miss-insertions Site control (isotactic) Chain-end control (isotactic) … m r … m r defective triads: mr, rr (2:1 ratio) defective pentads: mmmr, mmrr, mrrm (2:2:1 ratio) defective triad: mr defective pentads: mmmr, mmrm (1:1 ratio)

m (meso) linkage r (racemic) linkage Tacticity of polypropylene NMR proved to be a powerful tool to study the tacticity of polypropylenes Stereochemistry usually expressed in "linkages": m (meso) linkage r (racemic) linkage NMR can be used to distinguish different linkages, e.g. mmrm vs mrrm "pentads"

Determination of tacticity of polypropylene by 1H NMR Determination of tacticity by NMR Determination of tacticity of polypropylene by 1H NMR isotactic … m triads: mm pentads: mmmm syndiotactic Check Busico's artikelen voor goeie NMR spectra (zie COP course sheets en Oostende sheets) Zie Razavi sheets voor sPP … r triads: rr pentads: rrrr

Determination of tacticity of polypropylene by 13C NMR Determination of tacticity by NMR Determination of tacticity of polypropylene by 13C NMR isotactic … m triads: mm pentads: mmmm syndiotactic Check Busico's artikelen voor goeie NMR spectra (zie COP course sheets en Oostende sheets) Zie Razavi sheets voor sPP … r triads: rr pentads: rrrr

Determination of tacticity by NMR Determination of tacticity of polypropylene by 13C NMR - single misinsertions isotactic … m r defective triads: mr, rr (2:1 ratio) defective pentads: mmmr, mmrr, mrrm (2:2:1 ratio) syndiotactic Check Busico's artikelen voor goeie NMR spectra (zie COP course sheets en Oostende sheets) Zie Razavi sheets voor sPP … r m defective triads: rm, mm (2:1 ratio) defective pentads: rmmr, mmrr, rrrm (1:2:2 ratio)

Effect of miss-insertions Site control (isotactic) Chain-end control (isotactic) … m r … m r defective triads: mr, rr (2:1 ratio) defective pentads: mmmr, mmrr, mrrm (2:2:1 ratio) defective triad: mr defective pentads: mmmr, mmrm (1:1 ratio)

13C NMR i-PP site control & chain-end control Determination of tacticity by NMR 13C NMR i-PP site control & chain-end control site control … m r defective triads: mr, rr (2:1 ratio) defective pentads: mmmr, mmrr, mrrm (2:2:1 ratio) chain-end control … m r defective triad: mr defective pentads: mmmr, mmrm (1:1 ratio)

Determination of tacticity by NMR Distribution of chain defects in syndiotactic PP Site epimerization for Cs symmetric systems Ea … r m defective triads: rm defective pentads: rrrm, rrmr

End-groups provide relevant information about the termination process. End-group analysis End-groups provide relevant information about the termination process.

Polypropylene and higher α-olefin polymers Polymerization of α-olefins is more complex than ethylene polymerization: Polymerization of α-olefins is generally much slower than of ethylene As a consequent, β-H transfer is relative fast resulting in lower Mw Stereo (and regio) control is crucial to obtain useful products, which requires stereospecific catalysts that are more difficult to synthesize But the high melting point of products such as i-PP (Tm = 165oC) and i-poly-4-methyl-1-pentene (Tm = 256oC) makes them very attractive

Different types of polyolefins HDPE LDPE LLDPE i-PP, i-P4M1P s-PP, s-PS stereoblock i-PP OBC 6BM56, (08-09-2009)

Different types of polyolefins – HDPE How to synthesize different types of polyolefins LDPE: As an alternative to the high pressure, high temperature radical process, late transition metal catalysts can be used to produce LDPE under mild conditions. The branching originates from the fact that for late transition metals the M-H bond is stronger than the M-C bond. Consequently, M-R(C2H4) is the resting state and β-hydride transfer is a facile/rapid process leading to "chain walking" and the formation of branched and low molecular weight products. The branching can be tuned by altering ethylene pressure. The molecular weight can be improved by increasing the steric bulk of the ancillary ligand system.

Chain walking mechanism

How to slow down the B-H transfer process? Pd

Advantage of late transition metal catalysts Palladium and nickel catalysts are stable towards water and functionalities, which allows: Building-in of functionalities such as acrylates and alcohol groups Polymerization in water or emulsion (making organic solvents superfluous)

Advantage of late transition metal catalysts Linear polymers Tolerant to polar groups Highly branched polymers Tolerant to polar groups Oligomers

Short and long chain branching How to synthesize different types of polyolefins LLDPE: Most early transition metal catalysts incorporate α-olefins well. The more open the structure, the better the α-olefins incorporation. The open structure also allows re-insertion of released polymer chains, thus affording long chain branching. Late transition metal catalyst show a much lower tendency to incorporate α-olefins.

Long chain branching The open structure of the catalyst allows it to re-insert a previously released polymer chain. re-insertion termination

Effect of branching density in polyethylenes

Effect of branching density in polyethylenes Thermoplastic elastomers Plastomers VLDPE Rubbers Impact PP HDPE LLDPE i-PP Sheet van Muelhapt copieren met lamellar and fringed micellar crystals 10 20 30 40 50 60 70 80 90 100 % 1-alkene comonomer

Stereoregular poly-α-olefins How to synthesize different types of polyolefins i-PP, s-PP, i-PB-1, i-P4M1P, … Most early transition metal catalysts incorporate α-olefins well. Dependent on the structure of their ligand structure irregular or stereoregular polymers are obtained

Stereoblock copolymers Switching catalyst symmetry during polymerization. isotactic PP atactic PP

Stereoblock copolymers Switching catalyst symmetry during polymerization. isotactic PP isotactic PP

Stereoblock copolymers Rapid and reversible alkyl degenerative transfer between active and dormant sites [Ph3C]+[B(C6F5)4]- - Ph3CCH3 C1

Stereoblock copolymers Rapid and reversible alkyl degenerative transfer between active and dormant sites

Stereoblock copolymers Rapid and reversible alkyl degenerative transfer between active and dormant sites