Polymers – Lecture 5: Addition, Ionic and Coordination Polymerisation

Mechanisms, Catalytic Control, and Stereochemistry in Chain-Growth Systems

1 Introduction

Following our exploration of free-radical polymerisation (Lecture 4), we now examine two alternative chain-growth mechanisms that achieve a level of molecular and stereochemical control impossible with radicals:

Ionic polymerisation (both anionic and cationic)
Coordination polymerisation (metal-catalysed, including Ziegler–Natta and metallocene systems)

These mechanisms underpin many high-performance materials, from isotactic polypropylene to butyl rubber and living anionic block copolymers.
They rely on electrophilic or nucleophilic active centres, or on coordinatively activated monomers bound to transition metals.

2 From Radical to Ionic and Coordination Mechanisms

2.1 Key Differences

Feature	Radical Polymerisation	Ionic / Coordination Polymerisation
Active centre	Unpaired electron (radical)	Ion or metal complex
Sensitivity	Tolerant to impurities	Requires high purity, dry/inert conditions
Control	Moderate, broad MWD	High, narrow MWD; living possible
Tacticity	Usually, atactic	Can be isotactic/syndiotactic
Typical catalysts	Peroxides, azo initiators	Lewis acids, bases, transition-metal complexes

2.2 Energy Profile

Both are chain-growth systems, but ionic/coordination routes involve lower activation barriers for propagation and more stabilised active sites.
They are highly selective, producing polymers with defined architecture and stereochemistry.

3 Anionic Polymerisation

3.1 General Principles

Anionic polymerisation involves a nucleophilic initiator attacking an electron-deficient double bond.
The propagating species is a carbanion stabilised by adjacent groups such as CN, CO₂R, or phenyl.

Typical monomers:

Styrene
Butadiene
Acrylonitrile
Methyl methacrylate

Typical initiators:

Alkali metals (Na, K)
Organolithium compounds (n-BuLi)
Sodium naphthalenide

3.2 Mechanism

Initiation

Propagation

Termination
Often absent under inert conditions → living polymerisation.
Quenching (optional)
When desired, the reaction is stopped by proton donors (water, alcohol).

3.3 Characteristics

Living behaviour: no spontaneous termination or chain transfer.
Narrow molecular-weight distribution (PDI ≈ 1.05–1.2).
Enables block copolymer formation by sequential monomer addition.

3.4 Example: Living Polystyrene

In dry benzene, styrene polymerises with n-butyllithium:

Adding a second monomer (e.g. isoprene) yields a diblock copolymer (PS-b-PI).

Applications:
Thermoplastic elastomers such as Kraton™.

3.5 Solvent and Temperature Effects

Polar solvents (THF) stabilise ions → faster polymerisation.
Non-polar solvents (benzene) lead to aggregated ion pairs → slower, more controlled growth.
Low temperatures minimise side reactions such as back-biting.

3.6 Side Reactions

Proton abstraction from impurities → termination.
Anionic rearrangement (e.g. with MMA → head-to-tail irregularities).
Hence, rigorous, dry and oxygen-free conditions are essential.

4 Cationic Polymerisation

4.1 Overview

Cationic polymerisation is the electrophilic counterpart of anionic polymerisation.
It involves a carbocationic active centre and suits electron-rich monomers such as isobutylene, vinyl ethers, and styrene derivatives.

4.2 Typical Initiators

Protonic acids (H₂SO₄, HClO₄)
Lewis acids (BF₃, AlCl₃, TiCl₄) in the presence of co-initiators (water or alcohols)
Photoacid or redox systems (for UV-curing)

4.3 Mechanism

Initiation

Counter-anion (e.g. BF₄⁻) stabilises the cation.

Propagation

Termination
- Chain transfer to monomer or solvent
- Combination with counter-ion
- β-hydride elimination

4.4 Control and Limitations

Very sensitive to trace water and nucleophiles.
Requires low temperatures (often –80 °C to –100 °C for isobutylene).
Chain transfer reduces molecular-weight control, but recent advances (e.g. living cationic polymerisation using halides or special Lewis acids) provide better regulation.

4.5 Example: Polyisobutylene (PIB)

Initiated by BF₃ · ROH complex:

Produces soft, flexible, impermeable rubber.
Used in inner tubes, sealants, and adhesives.

5 Comparison: Anionic vs Cationic Polymerisation

Aspect	Anionic	Cationic
Active centre	Carbanion	Carbocation
Suitable monomers	Electron-deficient (styrene, acrylonitrile, MMA)	Electron-rich (isobutylene, vinyl ethers)
Typical initiators	Organolithium, Na/K metals	Lewis or Brønsted acids
Solvent	Non-polar or polar aprotic	Polar but low nucleophilicity
Temperature	Often ambient	Frequently sub-zero
Chain termination	Minimal (living)	Frequent (non-living)
Stereocontrol	Moderate	Limited
Industrial examples	SBR, block copolymers	PIB, poly(vinyl ether)

6 Living Ionic Polymerisation and Block Copolymers

A* = living polymer chain of monomer A (with an active anionic end, e.g. R – (A)_n^–M⁺
B = second monomer added after A is fully polymerised
A–B* = new living chain composed of an A block and a growing B block (still anionic and active)

Because the anionic end remains alive (not terminated or chain-transferred), it can immediately initiate polymerisation of a new monomer type when introduced.

Key Concept: “Living” Polymerisation

A polymerisation is living when:

There is no termination or chain transfer.
The reactive chain ends remain active indefinitely (as long as impurities like water or oxygen are excluded).
Polymer growth stops only when the monomer is exhausted, not because the chain dies.

In anionic systems (like with butyllithium initiators), the carbanion end persists and can be reactivated simply by adding more monomer.

Sequential Block Copolymer Synthesis

This property allows stepwise monomer addition:

Initiation:

→ Polymerisation of monomer A gives a living polymer with an active anionic end.

Sequential addition:
After A is consumed, add monomer B:

→ the same living end now propagates through monomer B.

Result:
A well-defined block copolymer:

Optionally, add more monomers (C, D, etc.) → triblock, multiblock, etc.

Advantages of Living Anionic Polymerisation

Feature	Effect
No termination/transfer	Predictable molecular weights (M_n = [M]₀/[I]₀ x M _monomer)
Narrow molecular-weight distribution	Typically D < 1.1
Control over architecture	Sequential addition → block, graft, star, comb polymers
Functional end-groups	Active ends can be terminated selectively with electrophiles

Example

Styrene–isoprene block copolymer:

Initiate polymerisation of styrene with n -BuLi → forms PS^–Li⁺.
Add isoprene → anion reinitiates, forming PS – PI^–Li⁺.
Quench with methanol → neutral PS – PI diblock.

This is the route used industrially for SIS thermoplastic elastomers.

Applications:

Thermoplastic elastomers: PS-b-PI-b-PS (styrene–isoprene–styrene).
Surfactant and dispersant systems: PEO-b-PPO (in detergents, biomedical use).

7 Coordination Polymerisation

7.1 Historical Context

In the 1950s, Karl Ziegler and Giulio Natta revolutionised polymer chemistry with transition-metal catalysts that polymerised ethene and propene at low pressure to yield highly regular (isotactic/syndiotactic) polymers.
They shared the 1963 Nobel Prize in Chemistry.

7.2 Basic Concept

A transition-metal complex coordinates to a monomer via its π-bond, activating it towards insertion into a growing metal–carbon bond.

Typical catalysts:

Ziegler–Natta: TiCl₄ + AlEt₃ (heterogeneous)
Metallocene (single-site): Cp₂ZrCl₂ + MAO (methylaluminoxane)

7.3 Mechanistic Steps

1. Activation / Formation of Active Site

Catalyst system: Transition metal halide (e.g. TiCl₄ or TiCl₃) + organoaluminium co-catalyst (e.g. triethylaluminium, AlEt₃).
What happens:
AlEt₃ reduces Ti(IV) → Ti(III) and transfers an ethyl group to titanium.
This generates an alkylated titanium species, the active site for polymerisation.
Result: A coordinatively unsaturated Ti–Et species capable of binding alkenes.

2. Monomer Coordination

The π bond of ethylene (or another α-olefin) coordinates to the empty orbital on titanium.
This forms a π-complex; the monomer is temporarily “held” in position near the metal.

3. Insertion (Chain Growth Step)

The coordinated monomer undergoes migratory insertion into the Ti–C bond.
The ethylene inserts between the metal and the existing alkyl chain.
A new, longer metal–alkyl bond forms, and the polymer chain length increases by one monomer unit.

This chain propagation repeats:

Each new monomer adds in a head-to-tail fashion.

4. Propagation and Stereocontrol

The geometry of the active site (on the solid TiCl₃ or TiCl₄/MgCl₂ support) dictates how monomers orient as they insert.
That’s how isotactic, syndiotactic, or atactic polymers are formed. The Ziegler–Natta catalyst site structure enforces the stereochemistry of insertion.

5. Termination

Typical termination routes:

β-Hydride elimination:

→ polymer chain releases as an olefin.

Chain transfer to monomer or hydrogen:
Hydrogen addition stops growth, forming a saturated polymer end and regenerating Ti–H.

These processes control molecular weight (chain length).

Summary of the Catalytic Cycle

Step	Reaction	Role
Activation	TiCl₄ + AlEt₃ → Ti-Et + AlEt₂Cl	Forms the active site
Coordination	Ti-Et + CH₂=CH₂ → Ti(Et)(η²-CH₂=CH₂)	Monomer binds
Insertion	Ti(Et)(η²-CH₂=CH₂) → Ti–CH₂–CH₂–Et	Chain growth
Propagation	Repeat insertion	Builds high MW polymer
Termination	β-H elimination / chain transfer	Ends chain growth

7.4 Stereoregularity Control

Chiral environments at the metal centre enforce selective monomer insertion orientation:

Isotactic: same configuration at each stereocentre.
Syndiotactic: alternating configuration.
Atactic: random (typical of radicals).

Natta demonstrated that TiCl₄/MgCl₂ catalysts can produce isotactic polypropylene, whereas certain metallocenes yield syndiotactic forms.

7.5 Catalyst Types

Catalyst Type	Example	Key Features
Heterogeneous Ziegler–Natta	TiCl₄/MgCl₂ + AlEt₃	Multiple sites → mixed tacticity
Homogeneous Metallocene	Cp₂ZrCl₂ + MAO	Single site → uniform chains
Post-metallocene	e.g. pyridyl-amido Ti or Fe complexes	Tunable activity, comonomer incorporation

8 Industrial Polyolefin Processes

8.1 Polyethene (PE)

HDPE: Produced via Ziegler–Natta or metallocene catalysts at low pressure. Linear, dense, crystalline.
LLDPE: Copolymer of ethene with α-olefins (1-butene, 1-hexene), giving flexibility.
UHMWPE: Extremely high molecular weight, used in prosthetics and high-wear parts.

8.2 Polypropylene (PP)

Ziegler–Natta catalysts → isotactic PP (Tₘ ≈ 170 °C).
Metallocene catalysts → syndiotactic PP (Tₘ ≈ 130 °C).
Atactic PP (from radical process) is amorphous and sticky, of little use.

Applications: fibres, automotive components, packaging, medical devices.

9 Other Coordination Systems

9.1 Phillips Catalyst (CrO₃/SiO₂)

Used in gas-phase polymerisation of ethene → HDPE.
Heterogeneous, non-Ziegler system; active sites involve Cr²⁺ species.

9.2 Kaminsky–Sinn (Metallocene/MAO)

High activity, narrow MWD, and capability to co-polymerise polar monomers.

9.3 Coordination of Dienes and Polar Monomers

Ti, Zr, or Ni catalysts can polymerise butadiene and isoprene with cis/trans control.
Late-transition-metal catalysts (Ni, Pd) allow incorporation of polar comonomers (acrylates, vinyl ethers).

10 Kinetic and Mechanistic Considerations

Rate of polymerisation:

The number of active sites remains approximately constant (heterogeneous systems).

Molecular weight depends on chain-transfer to monomer or hydrogen.
Adding small amounts of H₂ reduces chain length (useful in PE process control).
Stereocontrol arises from the geometry of monomer coordination and migratory insertion.

11 Characterisation of Stereoregular Polymers

Technique	Purpose	Observation
¹³C NMR	Determines tacticity via triad distributions	Distinguishes iso-, syn-, atactic PP
DSC	Measures crystallinity and melting point	Isotactic > syndiotactic > atactic
X-ray diffraction	Reveals crystal form (α, β, γ phases)	Confirms ordered chain packing
GPC (SEC)	Determines molecular-weight distribution	Single-site catalysts → narrow MWD

12 Environmental and Economic Aspects

Energy efficiency: Coordination polymerisation occurs at ambient pressure and temperature, reducing energy consumption relative to radical LDPE processes.
Catalyst recovery: Transition-metal residues are minimised through supported catalysts and post-treatment.
Recycling: High-purity, linear polymers (HDPE, PP) are easier to remelt and reuse.
Sustainability: Development of bio-based olefins and recyclable metallocene catalysts is a major research focus.

13 Applications of Ionic and Coordination Polymers

Polymer	Mechanism	Key Properties	Applications
Polystyrene (anionic)	Living anionic	Narrow MWD, block copolymers	TPEs, resins
Polyisoprene (anionic)	Controlled cis/trans ratio	Elastic	Natural rubber substitute
Polyisobutylene (cationic)	Low gas permeability	Sealing, damping	Inner tubes, adhesives
HDPE (coordination)	Linear, crystalline	Tough	Containers, pipes
PP (coordination)	Isotactic, semicrystalline	High strength	Fibres, mouldings
Block copolymers (living)	Sequential monomer addition	Microphase separation	Nanostructured materials

14 Modern Research Directions

Single-site catalysts for perfectly uniform chains.
Switchable polymerisation: catalysts toggling between ionic and coordination modes.
Recyclable catalysis using non-precious metals (Fe, Ni).
Green polymerisation media (ionic liquids, supercritical CO₂).
Precision synthesis of stereoblock and gradient polymers.

15 Summary and Comparison Table

Mechanism	Active Species	Control	Typical Polymers	Stereochemistry	Living?
Radical	Free radical	Moderate	PE (LDPE), PS, PVC	Atactic	No
Anionic	Carbanion	Excellent	PS, PI, PBD	Moderate	Yes
Cationic	Carbocation	Poor–moderate	PIB, poly(vinyl ether)	Limited	Partially
Coordination	Metal–carbon	Excellent	HDPE, PP, poly-1-butene	High	No (quasi-living variants)

16 Further Reading and Live Learning Links

These interactive sources include reaction animations, catalyst diagrams, and industrial case studies.

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