RING OPENING POLYMERIZATION (r o p)

RING OPENING
POLYMERIZATION
R O P

oxirane

I/ GENERAL CHARACTERISTICS
* Polymerizability
3,4 > 8 > 5,7
6-membered ring ??

cis- and trans-8-oxabicyclo[ 4.3.0]-nonane
FREE OF STRAIN  I does not polymerize
II

tetrahydrofuran ring is twisted

highly strained

* Polymerization Mechanism and Kinetics
Cationic ROP
Anionic ROP

II/ Cyclic ether :
* Anionic Polymerization of Epoxides

Propylene oxide : head to tail structure

Ex : ferric chloride–propylene oxide adduct ClFe[OCH(CH3)CH2Cl]2
Anionic coordination initiators

Reaction rate :
Degree of polymerization

Exchange Reactions
Epoxide polymerizations taking place in the presence of protonic substances
such as water or alcohol involve the presence of exchange reactions
Dormant
the addition of alcohol or other protonic substance is useful for control of
polymer molecular weight
Number-average
degree of pol.

The use of protonic compounds such as HCl or RCOOH in place of ROH
or H2O yields a different result in most systems. When such
substances
take part in the exchange reaction, the result is not exchange as
described above but inhibition or retardation since an anion, such as
Cl
-
or RCOO
-
, possesses little or no nucleophilicity. Reinitiation does
not occur or is very slow. The polymeric alcohols are no longer dormant
but are dead. Both the polymerization rate and polymer molecular
weight decrease along with a broadening of the polymer molecular
weight

Chain Transfer to Monomer
Species VII and VIII reinitiate polymerization of propylene oxide

B
O
R
B
O
R
B
O
O
R
R
n
EX: EtO , K +
O
CH
3
EtO
O K
CH
3
O
CH
2
CH
CH
3
O CH
2
CH

CH
3
O
+
O
CH
2
H
O CH
2
CH O CH
2
CH OH
CH
3
CH
3
+
O
CH
2
CH
2
CH CH
2
O
CH
2
CH CH
2

O CH
2
CH O
CH
3
O
CH
3

* Cationic Polymerization
Propagation


oxonium ion
A
-
:

counterion
The a-carbon of the oxonium ion is electron-deficient because of the
adjacent positively charged oxygen. Propagation is a nucleophilic attack of
the oxygen of a monomer molecule on the a-carbon of the oxonium. The
nucleophilic reaction is an SN
2
reaction for most cyclic ethers [Saegusa et
al., 1976]. However, SN
1
ring opening has been suggested for a monomer
with two alkyl substituents at the a-position (e.g., 2,2-dimethyloxetane
[Dreyfuss and Dreyfuss, 1987; Kops and Spangaard, 1982]

The species present in cationic ring-opening
polymerizations are covalent ester (IX), ion pair (X),
and free ion (XI) in equilibrium. The relative amounts of
the different species depend on the monomer,
solvent, temperature…

Methyl triflate CF
3
SO
2
-OCH
3


(Trifluorosulfonic acid, methyl ester)
Covalent propagation

in the polymerization of THF by triflic esters.
Covalent propagation is slower than ionic propagation
When covalent species are the major species present, they act effectively
as dormant species because of their lower reactivity compared to ionic
species.
 Living system

Initiation
Strong protonic acids such as trifluoroacetic, fluorosulfonic,
and trifluoromethanesulfonic (triflic) acids initiate
polymerization via the initial formation of a secondary
oxonium ion

 tertiary oxonium ion

Lewis acids such as BF
3
and SbCl
5
, almost always in
conjuction with water or some other protogen, initiate
polymerization of cyclic ethers
BF
3
. H
2
O, H
+
(SbCl6)
-
Initiator + Coinitiator 
Alkyl or acyl halide with a Lewis acid generates carbocations
and acylium ions :

Termination and Transfer Processes
Transfer Reactions.
Common mode

Chain transfer to polymer
 broadening of the MWD

Competition between propagation and chain transfer to polymer
Propagation is favored on steric grounds since attack by

monomer is less hindered than attack by the ether oxygen
in a polymer chain.
Cationic ROP of ethylene oxide is not useful for the synthesis of linear
polymer, but is used to produce crown ethers.

Propylene oxide gives less cyclic dimer than does ethylene
oxide for steric reasons; cyclic tetramer predominates
[propylene oxide]
inst.

mol.l
-1
Tetramer %
2.0 45
0.3 2.3
0.05 0.95

Activated Monomer Polymerization
ACE
AMM
ACE : active chain-end
AMM : activated monomer mechanism
 Conventional ROP

The ratio of the rates of AMM-to-conventional ROP depends on
[ROH]/[M] and the ratio of the rate constants for the two
reactions. Assuming that the two rate constants are comparable,
AM ROP becomes the dominant process at high [ROH] and low
[M]. Thus, AMM is carried out under monomer-starved
conditions. The instantaneous monomer concentration is very

low, but monomer is continuously added to the reactor at a rate
equal to its rate of consumption.
Cyclic oligomer is minimized by minimizing the extent of ACE
For example, conventional ROP of propylene oxide with BF
3
in
CH
2
Cl
2
at 25C gives mostly cyclic oligomers, the yield of cyclic
tetramer alone is 50%. The cyclic tetramer drops below 1% when
the reaction is changed to an AM polymerization by including an
alcohol and slowly adding monomer

AM polymerization has been successfully
demonstrated for the synthesis of telechelic
polymers of propylene oxide and epichlorohydrin
with number-average molecular weights of
1000–4000

Termination Reactions
- Transfer of an anion from the counterion
- Chain transfer with the initiator (e.g., water or alcohol)
Deliberate termination of growth is carried out to produce polymers
with specific molecular weights or, more often, telechelic polymers
with specific end groups.