p olyolefins: catalysis and dedicated analysis

Polyolefins: Catalysis and dedicated analysis

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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)

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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.

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…

Polyolefins: Catalysis and dedicated analysisIntroduction - overview of polymerization catalysis

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.

Organometallics – basics

18 VE

16 VE

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.



Some reagents have different functions (Lewis/Brønsted acids/bases or electrophiles/nucleophiles?

Some hydrocarbons can be acidic. Which of the following are Brønsted acids?

Going from sp sp➞ 2 sp➞ 3: the e- is more distant from the positively charged nucleus which makes it less stable.

Lewis acid Lewis base Brønsted acid Brønsted base Brønsted base

Nucleophiles electrophiles, Brønsted and Lewis acids and bases.


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).



Relative bond strength.

Ti-Np = 185Zr-Np = 221Hf-Np = 240

Ti-Bz = 217Zr-Bz = 263

Ti-NEt2 = 307Zr-NEt2 = 337Hf-NEt2 = 364

Ti-OiPr = 447Zr-OiPr = 517Hf-OiPr = 535

M = Ti, Zr, Hf;

R =

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.

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

Metal

Ancillary ligand

d+d-

X

Lewis base

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Organometallics – the requirements for catalysisIn 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:

Coordination polymerization is generally a chain growth process.

─ = step growth polymerization─ = chain growth polymerization

kinitiation

n

kpropagation ktermination or

kchain transfer +

The word chain is used in a statistical sense and has no relationship with the actual growing polymer chain.

n

kpropagation

Chain growth polymerization

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…

…

…

…


Polycondensation is a step growth process.

Step growth polymerization

During step growth reactions all monomers are reactive at the same time.

High conversion is required in order to obtain high molecular weight polymers.

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-Overview of polymerization catalysis

Different coordination polymerization mechanisms:ROP, ROMP, (meth)acrylate polymerization, olefin polymerization.

Different catalysts:Metal-based catalysts, organic catalysts and enzymes.

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Ring opening polymerization is a versatile process to polymerize a wide range of cyclic monomers. For example:

Why would such polyestersbe formed?

Enthalpy or entropy driven?

Ring Opening Polymerization (ROP)

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Not only for cyclic esters, also for example for cyclic ethers and the combination of different cyclic molecules can be ring opened. For example:

Ring Opening Polymerization (ROP)

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ROP of cyclic esters.

What are the requirements of the catalyst?




Vacant coordination site to bind monomer

Metal

Ancillary ligand

d+ d-

OMe

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Catalysts for ROP of cyclic esters.

Examples of catalysts for ring opening polymerization of cyclic esters.

Cocat: [Ph2PNPPh2]+Cl-, NEt4+Br-,

M = Al, Cr, Mn, Co

M = Mg, ZnM = Y, LaM = Mg, Ca, Zn

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Ring Opening Polymerization – mechanismROP of e-caprolactone.

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xs

H+

Ring Opening Polymerization – mechanismROP of e-caprolactone.

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

Polyhydroxybutyrates

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DL-lactide L-lactide D-lactide

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

Polylactide and poly(lactide-co-glycolide)

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Ring Opening Polymerization – mechanism

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Oxirane-based (co-)polymers

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Two similar but different synthetic polymers.

Oxirane – carbon monoxide copolymers

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100%0

Mw

monomer consumption

Step growth versus chain growth

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Inversion of configuration

Oxirane – carbon monoxide copolymers

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Oxirane – carbon monoxide copolymersInversion of configuration

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Oxirane – anhydride copolymers

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Oxirane-carbon dioxide copolymers

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Oxirane-carbon dioxide copolymers

Bimetallic mechanism.

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ROP – cocatalyst assisted

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Bacteria (Alcaligenes latus).Up to 90% polymer

poly(R-3-hydroxybutyrate) - PHB

lipase Pseudomonas cepaciapoly(R-2-hydroxypropionate) - PLA

Enzymatic polymerization

Monomer activation:

+O

O

OHLipase OC(CH2)5OHLipase

O

(EAM)

Initiation:

+ H2O HOC(CH2)5OH

O

+OC(CH2)5OHLipase

O

(EAM)

OHLipase

Propagation:

OHHO+ LipaseHO +OC(CH2)5OHLipase

O

(EAM)

C(CH2)5O

O

Hn n+1

C(CH2)5O

O

H

Enzymatic polymerization of e-caprolactone.

Enzymatic ring opening polymerization

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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.

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+

Nucleophilic catalyst

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+

Nucleophilic catalyst

ROH

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Acyclic Diene Metathesis (ADMET)Ring Opening Metathesis Polymerization (ROMP)

Olefin metathesis polymerization

Metal

Ancillary ligand

d+ d-

C(H)R1

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Schrock type Grubbs type


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ADMET – Cross metathesis polymerization

Polyolefins by step growth polymerization (polycondensation).


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Ring opening metathesis polymerization


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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.

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?

ROMP – an example

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Coordination polymerization – acrylates

Metal

Ancillary ligand

d+

d-

Coordination intermediate Resting state

➘

Chirality*

Metal mediated Michael addition

Migratory reaction

MMA is prochiral which leads to tacticity

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Coordination polymerization – olefins

Migratory insertion

propylene is prochiral which leads to tacticityMetal

Ancillary ligand

d+ d-

CH3

➘

Chirality

*

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Coordination polymerization – olefins - MMA

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Metal

Ancillary ligand

d+ d-

Metal

Ancillary ligand

d+ d-

CH3

Coordination polymerization – catalysts

Metal

Ancillary ligand

d+ d-

C(H)R1

Metal

Ancillary ligand

d+ d-

OMe

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Organometallics – the requirements for catalysisIn 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.


Vacant coordination site



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:

p olyolefins: catalysis and dedicated analysis

Documents

electron rule

electron pair

electronpoor substrate

electronrich substrate

polymerization catalysis

polyolefin endproduct

end product

low electron affinity