chemistry of polymer

B BA A

Chain growth polymerization

I + M I[M]n

Step Growth polymerization

+ A-A-B-B A-AB-BA-AB-BA-A etc

ie individual monomers extend the chain

ie first monomers then oligomers extend the chain

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Polymer Chemistry

Useful books

I. Cowie Chemistry and physics of modern materials

G. Odian Principles of Polymerization


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Step growth polymerization

Chain growth polymerization

ARA + BR’B A-(RABR’) -Bn

conversion

Mol. Wgt.

conversion

Mol. Wgt.


Chain growth and Polymerizability

ANIONIC, CATIONIC, RADICAL plus insertion and GTP

Anionic polymerization

Propagation and initiation

I- + =XX

-

Two aspects 1) Reactivity of the alkene to the anion (a nucleophile)2) Stability of the anion

For an alkene this reaction is essentially Michael addition


Common monomers

O

O

ON

1) Monomers that can stabilize negative charge

2) Usually vinyl but cyclics too

3) Note alpha-substituants are too hindered

O

O


The propagating centres

OO

OO

_

O-O

OO

ON

ON

_

O-N

ON

__

_ _

So most anionic polymerized VINYL monomers are Michael-type acceptors


-

X

Propagation involves perturbations of the π*-alkene LUMO by the n-bonding HOMO of the nucleophile

Nu

HOMOLUMO π*

Important conclusions are

1) Propagation and initiation only occur if HOMO and LUMO are close enough

2) Propagation of a DIFFERENT comonomer occurs only if the HOMO can perturb the π* LUMO of the comonomer

E1

Reactivity of the alkene to the anion

Reactivity of the anion to the alkene is best dealt with using Frontier orbitals-It is an aspect that occurs early in the reaction-energy profile before the TS


2) Stability of the anion

X

Eg new homo too stable, too low in energy

HOMO

LUMO π*

Too stable


2) Stability of the anion

X

or new homo too instable, too high in energy

HOMO

LUMO π*

In stable does not perturb LUMO


The consequences

require strong nucleophiles

eg nBuLi

Not all nucleophiles are initiators and not all monomers can be copolymerized

Weak electron withdrawing groups decrease energy of the anion HOMO less than CN and MeCOO in -MeCA

CN

OO

Initiated by weak nucleophileseg water


CN

OO

_+ no propagation

CN

OO+

_ will propagate but side reactions a problem

OO

_+ no propagation

OO+

_ will propagate but side reactions a problem


Side reactions and problems

XX H2O XX

H

O2

XX

OO

O

OO

OO

KILLS INITIATOR AND USES UP MONOMER


O

O

O

O

O

O

Does not occur to a significant degree

Steric hindurance is important In Micheal additions

So us bulky initiators eg


Cationic polymerization

Propagation and initiation

I+ + =X

X

+

Two aspects 1) Reactivity of the alkene to the cation2) Stability of the cation

X is electron donatingO

NOT electron deficient monomers

NOT methacrylate, acrylates, Vinyl ketones, acrylamides

NOT basic monomers


Radical polymerizationPropagation and initiation

Two aspects 1) Reactivity of the alkene to the radical2) Stability of the radical

Initiation

Propagation

I-I 2 I.

I.

+ M IM .

Termination IM.2 IMMI or IM IM’

Rp = fkd

kt( )

1/2

[M][I]1/2

k

p

rate constantfor propagation

rate constant

for termination

Monomer concentration

initiator concentration


R.

R=R

SOMO HOMO

σ-bondingHOMO

n-bondingSOMORRR .

Radical polymerization

LUMOπ -anti bonding

π -bonding

Can be either SOMO-HOMO

ie if SOMO is low energySOMO-HOMO

LUMO

1) Reactivity of the radical to alkene


R.

R=R

HOMO

HOMO

n-bondingSOMORRR

.


Or SOMO-LUMO

ie if SOMO is high energySOMO-LUMO

LUMO


BUT BE CAREFUL!

THE NEW SOMO NEEDS TO PERTURB THE π* LUMO

R.

SOMO HOMO

σ-bondingHOMO

n-bondingSOMO

π -bonding

Very high EnergyNon-stabilized

O

R

O

So homo polymerization isVery difficult

Can be trapped by lowEnergy LUMO of Electron deficient monomer

LUMO of MVE


Also,

OO

O1,2 substitution-sterics prevent propagation

OO

OOO

O

R

V. Electron deficient-low energy π*

R.

R=R

HOMOHOMO


LUMO


So can get alternating MVE-MA polymers

1) MVE is initiated R

O

But unstable and e-richDoes not perturb HOMO of MVEMVE π* too high energy

2) MA does not propagate due to sterics but π* is low energy

R

O OO

O New SOMO low energyStabilized radical

3) MVE HOMO is perturbed and addition occurs

R

O OO

O

O

Also Vinyl acetate


R.

R=R

HOMO

HOMO

In general both SOMO-HOMO and SOMO-LUMO perturbation occurs


LUMO

SOMO

SOMO

So monomers add randomly into the chain with a TENDENCY for

e-rich radicals (high energy SOMO) to add to e-deficient monomers (low energy LUMO)

e-deficient radicals (low energy SOMO) to add to HOMOs


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