ionic and group transfer polymerizations - kmutt ionic... · side reaction in anionic...

Ionic and Group Transfer Polymerizations

Dr. Jatuphorn Wootthikanokkhan

School of Energy, Environment and Materials, KMUTT

Introduction

Fewer polymers are produced industrially via ionic

polymerizations,compared to free radical polymerization

Monomers used in ionic polymerization must contain

substituted groups capable of stabilizing carbocations or carbanions.

Commercial polymers prepared by ionic polymerizations

Cationic Polymerization (CP.) Polyisobutylene

Butyl rubber (copolymer of isobutylene and isoprene),

Anionic Polymerization (AP.) cis-1,4-Polyisoprene

cis-1,4-Polybutadiene

SBS(block copolymer)

Anionic polymerization

Monomers having substituent groups capable of stabilizing a carbanion (through either resonance or

inductive effects) are suitable for anionic polymerization Examples of the substituent groups are

- CN - C=C - COOR -

Anionic polymerization

Two EWGs are so effective in stabilizing anions that even water can initiate cyanoacrylate ("Super Glue"). Weak bases

(such as those on the proteins in skin) work even better.

Anionic Initiation

If the propagating anion is not very strongly stabilized, a powerful nucleophile is required as initiator.

Anionic initiators

Most widely used inititors for anionic polymerization include Organometallic compounds e.g. BuLi Alkaline metal in ammonia (from KNH2) Complex of alkali metals and aromatic compounds (electron

transfer precess)

Na + Na+ +

-Radical anion

Na+

-

CH2 CH

+

CH2 CH-

Na+

+

CH2CH CH2 CHNa+ Na+--

+

CH2CH CH2 CHNa+ Na+--

d i - a n i o n

s t y r y l r a d i c a l a n i o n

Dimerization

Interactions between ions

UV U+V- U+// V- U+ + V-

Covalent bonding

Contact ion pair

Solvent separated ion pair

Solvated free ions

Increasing strength of interaction

The tighter the ion pair, the slower the rate of propagation

Effects of solvent types on reaction rates

Solvent Relative Rate

Benzene 1

Dioxane 2.5

THF 225

1,2-Dimethoxyethane 1900

http://chem.chem.rochester.edu/~chem421/anionic.htm

This is related to polarity of the solvent.

Effects of counter ion types on reaction rates

Counterion Relative Rate

Li+ 1

Na+ 3.6

K+ 21.1

Rb+ 22.9

Cs+ 26.1

Larger counter-ions usually form looser ion pairs, so there is

an increase in rate

http://chem.chem.rochester.edu/~chem421/anionic.htm

Side reaction in anionic polymerization

Acrylates have problems in anionic propagation because of chain transfer to polymer. The hydrogen atoms adjacent to the ester groups are slightly

acidic, and can be pulled off by the propagating anion. The new anion thus

created can reinitiate, leading to branched polymers.

Termination

When carried out under the appropriate conditions, termination reactions do not occur in anionic polymerization. One usually adds

purposefully a compound such as water or alcohol to terminate the process. The new anionic species is too weak to reinitiate.

Functionalize the chain end

Initiation and termination in the

anionic polymerization with KNH2

KNH2 K+NH2

-+

NH2

-CHCH2+ NH2

CHCH2

-

NH3CHCH2

-+ NH2CH2CH2 +

-

Kinetic of anionic polymerization

Consider potassium amide-initiated polymerization in ammonia

Initiation KNH2 K

+ + NH2-

NH2- + M H2NM -

Ri = ki[NH2-][M]

Kinetic of anionic polymerization

Propagation Rp = kp[M][M-]

Termination Termination is resulted primarily by transfer

of anion to the solvent

Rtr = ktr[M- ][NH3]

Kinetic of anionic polymerization

Using the Steady State Assumption (SSA) Ri = Rtr

ki[NH2-][M] = ktr[M

-][NH3] and [M-] = ki[NH2

-][M] ktr[NH3] substituting [M-] in Rp we obtain

Rp = kpki[M]2[NH2-]

ktr[NH3]

Kinetic of anionic polymerization

Kinetic chain length ( v ) v = Rp/Rtr = kp[M][M-] ktr[M-][NH3]

v = kp[M] ktr[NH3]

Kinetic chain length and DP

In the case of a simple anionic initiator (e.g. BuLi) v = DP

In the case of electron transfer initiator such as Na/naphthalene

2v = DP (because polymer grow simultaneously from each end of a di-anion, a growing

specie)

“Living” anionic polymerization

In the absence of impurities or any proton sources, anionic polymerization could proceeds via the “Living” mechanism

Consequences of the living polymerization - low polydispersity GPC standards can be prepared - quenching with CO2 leads to carboxyl-terminated polymer - addition of a second monomer leads to block copolymer

Living anionic polymerization

Chains are initiated all at once (fast initiation)

Little or no termination (except purposeful).

Little or no depolymerization.

All chains grow under identical conditions.

MW of polymer prepared via living anionic polymerization

Narrow MW distribution (PD approaches 1.0, typically 1.05 - 1.2).

The MW is predictable (unlike other polymerizations) by using data from

Conversion Formula weight of the repeating units Mole ratio between monomer and initiator

Kinetic of the LIVING anionic polymerization

-d[M]/dt = kp [I]o[M]

d[M]/[M] = -kp [I]odt

ln [M] - ln [M]o = - kp [I]o t

ln [M]/[M]o = - kp [I]ot

[M]/[M]o = e- kp [ I ]o t

Kinetic chain length of the LIVING anionic polymerization

v = ([M]o - [M]) / [ I ]o

v = ( [M]o / [ I ]o ) – ( [M] / [ I ]o)

= ( [M]o / [ I ]o) - [M]oe-kp [I]ot / [ I ]o

v = [M]o (1 - e-kp [I]ot ) / [ I ]o

Group Transfer Polymerization (GTP)

Initiators used are organosilicon compounds. These

include methyl trimethylsilyl acetal of dimethylketene

R OR C=C R OSiR3

(bifluoride ion is used as a catalyst)

Group Transfer Polymerization

The mechanism of

GTP is not known with certainty. In the case of

anionic catalysis, the

mechanism has been

porposed by Webster (J. Am. Chem. Soc, 105 (1983) 5703) as below

R

C

R

C

OR

OSiR3

H2C

CH3

O

OCH3

C

C

R

C

R

C

OR

O

H2C

CH3

OSiR3

OCH3

C

C

Group Transfer Polymerization

Advantages of GTP include the following Wider temperature range (-100 - 100 degree C) Polydispersity close to 1 is obtainable

It does not polymerize other monomers except (meth)acrylate and acrylonitrile.

Monomers reactivity in CP.

For aliphatic monomers, the more stable the carbocation intermediates, the greater the rate of polymerization.

CCH 2 CHCH 2 CH 2CH 2

CH 3

CH 3 CH 3

> >

Monomers reactivity in CP.

For para-substituted styrenes OCH3 > CH3 > H > Cl For ortho-substituted styrenes,

substitutents retard the propagation regardless of the type of substituents due to the “steric effect”

CH 2CHCH 2CH CH 2CH CH 2CH

OCH 3 CH 3 Cl

> > >

Others cationic polymerizable monomers

CH2 CH OR CH2 CH

OR

+

CH2 CH

OR+

N

H2C CH

N

H2C CH

+

N

H2C CH

+

Initiating systems for CP.

Acids such as H2SO4, H3PO4

Lewis acids in combination with proton sources (or cation sources), for example,

BF3 / H2O AlCl3 / RCl

Initiation in CP

Initiation in CP

RCl + AlEt 2Cl R+ AlEt2Cl2 -

M

RM+ AlEt2Cl -

Initiation in CP

BF3OH-

H+ CH2 C

R

R

+ CH3 C+

R

R

+ BF3OH-

BF3 H2O+ BF3OH-

H+

CH2 C

R

R

CH3 C+

R

R

+ BF3OH-

H2O

BF3OH-

H3O+

Propagation in CP

Cationic polymerization is normally a non-living polymerization

Chain transfer reactions are common in CP.

Transfer to polymer chains (Hydride abstraction)

Transfer to monomers (Proton transfer)

Transfer to solvents (electrophilic substitution)

Chain transfer to polymer

CH 2 CH +

CH 3

CH 2 C H

CH 3

X-

CH 3+

CH 2 CH

CH 3

CH 2 C

CH 3

X-

CH 3+

H

+

Transfer to monomer

Chain transfer to water

CH2 C+

R

R

+ H2O CH2 COH

R

R

H+X

-+x-

Types of Terminations in Cat. Polymn.

1. Reacting with counter-ion

Proton transfer from chain end to the counter-ion

Chain end chlorination

2. Sometime, the above chain transfer reactions are

considered to be the termination.

Kinetic in cationic polymerization

Initiation Ri = ki[I][M] Propagation Rp = kp[M][M+] Termination with counter ion Rt = kt[M+] Transfer reaction to monomer

Rtr = ktr[M][M+]

Kinetic in cationic polymerization

Steady State Assumption (SSA)

Ri = Rt ki[I][M] = kt[M+] [M+] = ki[I][M]/kt

Substituting [M+] in Rp

Rp = kpki[I][M]2

kt

Kinetic in cationic polymerization

In the absence of chain transfer, the kinetic chain length, v, is equal to DP

v = DP = Rp/Ri = Rp/Rt

= kp[M][M+] kt[M+]

v = kp[M]/kt

(Note that v is independent with [I]) Why?