Polyolefins: Catalysis and dedicated analysis 6BM56, (31-08-2009)

Scope & objectives Providing a comprehensive fundamental introduction into the chemical aspects of catalytic olefin polymerization. Get insight into several aspects of the macromolecular science of polyolefins by following the course of a polyolefin molecule from its origin at the catalyst site to its application in the end product. Generate the ability to rationally design a catalytic system, a production process and a processing methodology to produce a polyolefin end-product with certain predefined properties. 6BM56, (31-08-2009)

Scope & objectives Providing a comprehensive fundamental introduction into the chemical aspects of catalytic olefin polymerization. Get insight into several aspects of the macromolecular science of polyolefins by following the course of a polyolefin molecule from its origin at the catalyst site to its application in the end product. Generate the ability to rationally design a catalytic system, a production process and a processing methodology to produce a polyolefin end-product with certain predefined properties.

Polyolefins: Catalysis and dedicated analysis Introduction - overview of polymerization catalysis Generally, catalysts are (transition) metal coordination complexes. Generally, the complexes contain a reactive group that forms the initiating group of the growing chain. This group is often a halide or alkoxide (ROP), an alkylidene (ROMP), enolate (acrylate polymerization) or an alkyl or hydride (olefin polymerization). But what are the characteristics of a good catalyst? Before that, some basics…

Organometallics – basics 18 electron rule. The valence shell of transition metals can accommodate 18 electrons (1 S + 3 p + 5 d orbitals). Filling the valence shell will result in a noble gas configuration. Steric reasons often prevent the metal to fulfill the 18 electron rule. Transition metal complexes with less than 18 valence electrons show enhanced reactivity. 18 VE 16 VE

Organometallics – basics A nucleophile is a reagent that is “nucleus-loving”. It has an electron-rich side and can form a bond by donating an electron pair to an electron-poor substrate, the electrophile. An electrophile is a reagent that is “electron-loving”. It has an electron-poor side and can form a bond by accepting an electron pair from an electron-rich substrate, the nucleophile. Closely related to nucleophiles and electrophiles are acids and bases.

Organometallics – basics Some hydrocarbons can be acidic. Which of the following are Brønsted acids? Going from sp ➞ sp2 ➞ sp3: the e- is more distant from the positively charged nucleus which makes it less stable. Some reagents have different functions (Lewis/Brønsted acids/bases or electrophiles/nucleophiles? Lewis acid Lewis base Brønsted acid Brønsted base Brønsted base

Organometallics – basics Nucleophiles electrophiles, Brønsted and Lewis acids and bases.

Organometallics – basics Hardness and softness. The difference in the ionization energy of a neutral atom to its ion is a measure for the so-called hardness of an element (the hardest atoms are those with high ionization energies and low electron affinity). Simple rule of thumb: the hardness of an atom is related to the charge of the atom divided by the ionic radius. For example, AlF3 is a very hard molecule, while SnI2 is a soft molecule. Polarizability. The harder the atom/molecule, the less polarizable it will be. Clearly, the more diffuse orbitals of heavier elements in a group are more easily to polarize than the orbitals of the lighter elements, and hence the heavier elements are softer (e.g. fluorine is hard, iodine is soft).

Organometallics – basics Relative bond strength. M = Ti, Zr, Hf; R = Ti-Np = 185 Zr-Np = 221 Hf-Np = 240 Ti-Bz = 217 Zr-Bz = 263 Ti-NEt2 = 307 Zr-NEt2 = 337 Hf-NEt2 = 364 Ti-OiPr = 447 Zr-OiPr = 517 Hf-OiPr = 535

Mechanism – termination For late transition metals, the DE(M-H – MC) is significantly larger than for early transition metals. Consequently, late transition metals undergo b-H elimination more rapidly.

Organometallics – basics Summarizing. Transition metal complexes with less than 18 VE are reactive the metal in such complexes – are Lewis acids/electrophiles the metal in such complexes – form polar bonds the metal in such complexes – tend to bind Lewis bases d- d+ X Metal Lewis base Ancillary ligand

Organometallics – the requirements for catalysis In catalysis, we want to activate a substrate so it can react with another reagent already bonded to the catalyst site. In catalytic coordination polymerization, the monomer coordinates to the metal site and is thereby activated, which allows it to react with the growing chain. Electrophilic metal center (can be cationic) Vacant coordination site Polarized metal-polymer bond Robust and tunable ancillary ligand system Sometimes a cocatalyst is required No easily accessible side reactions Low costs Polymer Monomer Metal Ancillary ligand d+ d- Cocat. Characteristics of a metal-based catalyst:

Chain growth polymerization Coordination polymerization is generally a chain growth process. n kpropagation n kinitiation kpropagation ktermination or kchain transfer + The word chain is used in a statistical sense and has no relationship with the actual growing polymer chain. ─ = step growth polymerization ─ = chain growth polymerization

Step growth polymerization Polycondensation is a step growth process. … During step growth reactions all monomers are reactive at the same time. High conversion is required in order to obtain high molecular weight polymers. ─ = step growth polymerization ─ = chain growth polymerization

-Overview of polymerization catalysis Different coordination polymerization mechanisms: ROP, ROMP, (meth)acrylate polymerization, olefin polymerization. Different catalysts: Metal-based catalysts, organic catalysts and enzymes.

Ring Opening Polymerization (ROP) Ring opening polymerization is a versatile process to polymerize a wide range of cyclic monomers. For example: Why would such polyesters be formed? Enthalpy or entropy driven?

Ring Opening Polymerization (ROP) Not only for cyclic esters, also for example for cyclic ethers and the combination of different cyclic molecules can be ring opened. For example:

ROP of cyclic esters. What are the requirements of the catalyst? d- d+ OMe Metal Ancillary ligand Robust and tunable ancillary ligand system Electrophilic metal center (can be cationic) Polarized metal-polymer bond Vacant coordination site to bind monomer

Catalysts for ROP of cyclic esters. Examples of catalysts for ring opening polymerization of cyclic esters. M = Al, Cr, Mn, Co Cocat: [Ph2PNPPh2]+Cl-, NEt4+Br-, M = Mg, Ca, Zn M = Y, La M = Mg, Zn

Ring Opening Polymerization – mechanism ROP of e-caprolactone.

Ring Opening Polymerization – mechanism ROP of e-caprolactone. xs H+

Polyhydroxybutyrates Catalyst requirements: Lewis acidic metal. Free coordination site. Sometimes a cocatalyst is required.

Polylactide and poly(lactide-co-glycolide) Chirality plays an important role in polylactide and poly(lactide-co-glycolide). DL-lactide L-lactide D-lactide

Ring Opening Polymerization – mechanism

Oxirane-based (co-)polymers

Oxirane – carbon monoxide copolymers Two similar but different synthetic polymers.

Step growth versus chain growth ─ = step growth polymerization ─ = chain growth polymerization 1 % M w m o n er c s u p t i on

Inversion of configuration Oxirane – carbon monoxide copolymers Inversion of configuration

Inversion of configuration Oxirane – carbon monoxide copolymers Inversion of configuration

Oxirane – anhydride copolymers

Oxirane-carbon dioxide copolymers

Oxirane-carbon dioxide copolymers Bimetallic mechanism.

ROP – cocatalyst assisted

Enzymatic polymerization poly(R-3-hydroxybutyrate) - PHB Bacteria (Alcaligenes latus). Up to 90% polymer poly(R-2-hydroxypropionate) - PLA lipase Pseudomonas cepacia

Enzymatic ring opening polymerization Enzymatic polymerization of e-caprolactone. Monomer activation: Initiation: Propagation:

ROP polymerization using organic catalysts N-heterocyclic carbenes are effective nucleophilic catalysts for the ROP of cyclic ethers and esters. Chain transfer agents such as alcohols can be added to control the molecular weight and produce end-functionalized polymers.

ROP polymerization using organic catalysts Nucleophilic catalyst +

ROP polymerization using organic catalysts Nucleophilic catalyst ROH +

Olefin metathesis polymerization Acyclic Diene Metathesis (ADMET) Ring Opening Metathesis Polymerization (ROMP) d- d+ C(H)R1 Metal Ancillary ligand

Olefin metathesis polymerization Schrock type Grubbs type

ADMET – Cross metathesis polymerization Olefin metathesis polymerization ADMET – Cross metathesis polymerization Polyolefins by step growth polymerization (polycondensation).

Ring opening metathesis polymerization Olefin metathesis polymerization Ring opening metathesis polymerization

ROMP – an example 3 homopolymerizes norbornene but does not homopolymerize cyclooctene. However, 3 does copolymerize norbornene and cyclooctene. Why does 3 copolymerize these monomers and what is the structure of the copolymer? A is sterically less hindered than D. A➝B➝C➝D will only occur when severe ring strain is released since finally a sterically more hindered species is formed. D➝E➝F➝A will occur also for non-strained cyclic olefins since finally a sterically less hindered species is formed.

Coordination polymerization – acrylates Metal mediated Michael addition Migratory reaction MMA is prochiral which leads to tacticity Metal Ancillary ligand * ➘ Chirality Coordination intermediate Resting state

Coordination polymerization – olefins CH3 Migratory insertion propylene is prochiral which leads to tacticity Metal Ancillary ligand Chirality ➘ *

Coordination polymerization – olefins - MMA

Coordination polymerization – catalysts Metal Ancillary ligand d+ d- OMe d- d+ C(H)R1 Metal Ancillary ligand d- d- d+ d+ CH3 Metal Metal Ancillary ligand Ancillary ligand

Organometallics – the requirements for catalysis In catalysis, we want to activate a substrate so it can react with another reagent already bonded to the catalyst site. In catalytic coordination polymerization, the monomer coordinates to the metal site and is thereby activated, which allows it to react with the growing chain. Electrophilic metal center (can be cationic) Vacant coordination site Polarized metal-polymer bond Robust and tunable ancillary ligand system Sometimes a cocatalyst is required No easily accessible side reactions Low costs Polymer Monomer Metal Ancillary ligand d+ d- Cocat. Characteristics of a metal-based catalyst: