Spring 2008

Chap 10. Non-Radical Addition Polymerization

Anionic Polymerization

the growing chain end bears a

negative

charge The mechanism of anionic polymerization is a kind of repetitive conjugate addition reaction .

(the "Michael reaction" in organic chemistry)

Cationic Polymerization

the growing chain end bears a

positive

charge The mechanism of cationic polymerization is a kind of repetitive alkylation reaction.

Hanyang Univ.

Anionic

Polymerization

General Scheme

Initiation: B-Z + CH 2 =CHX B-CH 2 -CH Z + X Propagation: M Z + + M Termination: M Z + + HT MM Z + MH + ZT

Spring 2008 Hanyang Univ.

Anionic Polymerization

Styrene Polymerization

Initiation: CH 3 CH 2 CH CH 3 Li + CH 2 CH Propagation: CH 2 CH Li + CH 2 CH CH 3 CH 2 CH CH 3 CH 2 CH Li CH 2 CH CH 2 CH Li

Spring 2008

Termination: CH 2 CH Li + H OH CH 2 CH 2 + Li OH

Hanyang Univ.

Spring 2008

Anionic Polymerization

Characteristics of an Ideal Anionic Polymerization

 Negative centers repel one another and thus termination by recombination is not possible. An ideal polymerization is “living”, which does not terminate until a terminator is added.

  Initiation is normally very fast relative to propagation and all chains grow simultaneously. This leads to polymers with low polydispersity or monodispersity.

Theoretically:

M w

 1  1

M n x n

 The rate of polymerization for methacrylates and styrenes is high even at -78 o C. This is partly for the high concentration of the anion centers.

 The degree of polymerization

x n



K I

  0 0 

K

=1 or 2 depending on initiator used.

Hanyang Univ.

Anionic Polymerization

Initiation by Electron Transfer

- .

THF + K -78 o C

- .

K + + CH 2 CH 2

.

CH 2

-

CH K + K + K +

-

CH CH 2 CH 2

-

CH K + +

.

CH 2

-

CH K + • • Polymerization mostly done in THF and not nonpolar solvents like cyclohexane or benzene for the solubility the complex in THF.

The degree of polymerization is given by

x n

 2     0 0

Hanyang Univ.

Spring 2008

Anionic Polymerization

Initiation by Nucleophilic Attack

CH 3 (CH 2 ) 2 CH 2 Li + + CH 2 CH THF -78 o C CH 3 (CH 2 ) 4 CH Li + N Li + CH 3 THF CH 2 C COOCH 3 -78 o C N CH 2 CH 3 C Li + COOCH 3 • Polymerization can be done in both polar and nonpolar solvents.

• The degree of polymerization is given by

x

n



    0 0

Hanyang Univ.

Spring 2008

Anionic Polymerization

Initiation by Living Polymer

CH 2 CH Li + + CH 3 CH 2 C THF COOCH 3 -78 o C But not CH 2 CH 3 C Li + COOCH 3 + CH 2 CH CH 2 CH CH 2 CH 3 C Li + COOCH 3 Because the starting anion has to be a stronger Lewis base than the resulting anion.

x n

     0 0

Hanyang Univ.

Spring 2008

Anionic

Polymerization

Propagation

M Z Covalent Bond M Z+ Contact Ion Pair M Z + Solvent Separated Ion Pair Solvent polarity increases k P increases M + Z + Free ions Polymer tacticity decreases • K p can vary by orders of magnitude.

• The polydispersity remains low because the rate of inter-conversion between the different forms is much faster than the polymerization.

Spring 2008 Hanyang Univ.

Anionic Polymerization

Termination

By proton M Z + By CO 2 H + CO 2 M Z + MH + Z + MCOO Z + By using a limiting amount of 1,2-dibromoethane 2 M Z + CH 2 Br CH 2 Br M K=2 By using a much excess of 1,2-dibromoethane M M Z + CH 2 Br CH 2 Br M Br

Hanyang Univ.

Spring 2008

Spring 2008

Anionic Polymerization

(1) proton donor H 2 O or ethanol Strong base is not enough for initiation.

(2) H 2 O C tr,s =10 H 2 O low MW polymer No living polymer

H CH 2 C: + C 2 H 5 OH

Strong base is not enough for initiation.

CH 2 CH 2 + C 2 H 5 O -

EtOH ethoxide C tr,s =10 -3 (small chain transfer constant) high MW product no longer living.

Hanyang Univ.

Anionic Polymerization

(3) Termination can occurred by hydride elimination without impurities.

a) Spring 2008 b) anionic species(active center) react with chain ends to form inactive allylic anion.

C H 2 .

C .

H + C H 2 C H C H C H C H 2 C H 2 + C H 2 .

C .

H C H C H

1,3 diphenylallyl anion is very unreactive, highly resonance stabilized Hanyang Univ.

Spring 2008

Anionic Polymerization

Termination of polar monomer In this case, although the initiator or active center attacks the monomer, that results the non-polymerization.

CH 2 CH 3 C: Li + COOCH 3 + CH 2 CH 3 C COOCH 3 CH 2 CH 3 C O C COOCH 3 CH 3 C CH 2 + Li + CH 3 O -

Hanyang Univ.

Spring 2008

Anionic Polymerization

Backbiting or intramolecular reaction Cyclic trimer at the end of chain 4) Hugginson-Wooding System J.Chem. Soc. 1952 Polymerization of styrene conducted in liq. NH3 at bp -33



C (1) reaction rate ↑ as [I] and [M] 2 I=K+NH 2 (2) MW



rate ↑ as [NH 2 ] ↑ but as [K + ] ↓ [K + ] and [NH 2 ] (3) Polymer is formed without unsaturation.

Hanyang Univ.

Anionic Polymerization

Initiation step

k

KNH 2 K

k

 [

K

 ][

NH

2 [

KNH

2 ]  ] NH 2 + NH 2

Dissociation of initiator

CH CH 2

k 1

H 2 N CH 2 H C: -

Spring 2008 R i



k i

[ 

N H

2 ][

M

] 

k i K

[

M

[ ][

KNH

2

K

 ] ]  

If [K + ] , then R i



Hanyang Univ.

Spring 2008

Anionic Polymerization

Propagation R p



k p

[

M

 ][

M

]

Termination Occurs by chain transfer

H 2 N C H 2 C H C H 2 H C : + N H 3 k tr,s H 2 N C H 2 C H H C H 2 C H + : N H 2 -

R

p n

Overall Rate using Steady state assumption. (R i



R t ).

n 

K



k i



k p

[

M k tr

,

s

[

K

 ] 2 [

KNH

][

NH

3 ] 2 ] 

k i



k p

[

M

] 2 [

H

2

N

 ]

k tr

,

s

[

NH

3 ]

R i R p

 1 1

k i K

2 [

M

][

KNH

2 ] 2  1

k i K

2

k p

[

M

1 ] 2 [

KNH

2 ] 2

k tr

,

s

[

NH

3 ] If KCl is added  [K + ]=[NH 2 ] Rtr=

ktr,s

[M-][NH3+] R p decreases

Hanyang Univ.

Anionic Polymerization

In dehydrate state,

X n



k p

[

M

]

k tr

,

s

[

NH

3 ]  [

M

]

C s

[

NH

3 ] Chain transfer constant for solvent

Activation energy for X n E x n



E p



E



temp overall E R



E i rate



E p tr

   

E tr

4

kcal DP



n

9 /

mole kcal

 /

Rate mole Spring 2008 Hanyang Univ.

Spring 2008

Anionic Polymerization

In Flory If there is no termination rxn, the narrow MW distribution can be obtained.

M w M n

 1  1

X n

 if

X n

 

M w M n

 1

5) Base Initiated Polymerization - a strong nucleophile is required as the initiator

NO 2 > C O > SO 2 > CO 2   CH CH 2 >>> CH 3 CN > SO > C 2 H 5

Hanyang Univ.

Spring 2008

Anionic Polymerization

6) Practical Comments purity import!

If we use metal as an initiator, the propagation rate is fast.

7) Propagation Kinetics Comparing to the radical polymerization, the propagation doesn’t occur too fast

R p



k p

[

M

 ][

M

]

For most of the living polymers

[M: ] = [I] [M



] = is about 10 -9 [M: ] = 10 -3 to 10 -7 to 10 -2 molar

conc. of anion = conc. of initiator

molar

k p for free radical case is 5



10 3 K p l/mole



sec : depends on solvent and counter ion Counter ion and active center can be separated by changing the solvent



then reaction rate increases Hanyang Univ.

Anionic Polymerization

(1) Evaluation of Individual Propagation Rate Constants R p



k p

 [

P

 ][

M

] 

k p

 [

P

 (

C

 )][

M

]

Propagation rate constant for free ion and ion pair.

[P ]: conc. of free ion [P (C + )]: conc. of ion pair

k p at app



Eq

.

k p

 [

P

 ] 

k

 [

P

 (

C

 )] [

M

 ]

P

 (

C

 ) K

P

 

C

 

R p



k p app

[

M

 ][

M

]

Spring 2008

1

K

 [

P

 (

C

[

P

 ][

C

 )]  ] if more ions have been added , [

P

 ]  [

C

 ] [

P

 ]  (

K

[

P

 (

C

 )]) 1 2

Hanyang Univ.

Anionic Polymerization

*

How to measure kp



, kp, K ?

C

log [

M



C

] 0

slope



k p app

.

t

k



app slope

 (

k intercept



p



k



p

)

K

1 2 

k p

 [

M

 ] 2 1

Hanyang Univ.

Spring 2008

Spring 2008

Anionic Polymerization

A salt that must be soluble in THF with common ion to gegen ion is added to reaction mixture.

[

P

 ] 

K

[ [

M C

  ] ]

The salt was added at high conc.

Conc. of the added salt is [CZ] [C + ]



[CZ]

 [

P

 ] 

K

[

M

 ] [

CZ

]

the conc. of living and the conc. of free ion

[

P

 (

C

 )]  [

M

 ] 

K

[

M

 [

CZ

] ]

Hence k app p



k



p

 (

k



p



k



p

)

K

[

CZ

] 

originally k app p



k



p

[

P

 ] 

k

 [

P

 (

C

 )] [

M

 ]

k app p slope

 (

k



p

int 

k



p



k



p

)

K

[CZ ]

Then able to get k p , k p



, K from the two graphs.

Hanyang Univ.

Spring 2008

Anionic Polymerization

Effect of gegen ion on Anionic Polymerization of Styrene

Li + Na + K + R b + C S + k p  160 80 60~80 50~80 22 THF K  10 7 2.2

1.5

0.8

0.1

0.02

k p 6.5

 10 4 Dioxane k p  0.94

3.4

19.8

21.5

24.5

- Why kp- is the same value?

; kp- is much more larger than kp



Thus we can say that reactivity of free ion is much greater than that of ion pairs.

- In the case of dioxane?

;In dioxane which is tend not to be solvated, it has reverse tendency comparing to the case of THF. Solvation is not important in dioxane.

Cs is too high and there is no difference. Hanyang Univ.

Spring 2008

Anionic Polymerization

Li + R genenion in aromatic hydrocarbon p



k p

[

M

: 

Li

 ][

M

]

R i



k i

[

RLi

][

M

]

Let’s say we are using the BuLi initiator.

Look at difference.

Unassociated species solvation as well as



is important!

Although, the 1,2 diethoxyethane reduce the



, k p solvating ether .

varies 1~1000 fold because of highly Reactivity of free ion < Reacitivity of ion pair In aromatic hydrocarbon, unassociated species dominate rate.

Depends on the unassociated species in very low conc. Covalent character Hanyang Univ.

Anionic Polymerization

Evidence — the viscosity measurement before and after term, we find that living polymer is associated after termination, viscosity drops.

[

RLi

]  1

K

6 1 [(

C

4

H

9

Li

) 6 1 ] 6

R i

 [ ] 1 6 [

M

: 

Li

 ]  1

K

2 2 [(

M

: 

Li

 1 ) 2 ] 2 1

order in the R

 2 1 6

order in initiation rate Spring 2008 Because initiators and ion pairs are reduced, Polymerization reaction in Aliphatic HC is lower than inaromatic HC. Hanyang Univ.

Spring 2008

Anionic Polymerization

Lenz P.437 Table 13-9 Effect of solvent and gengenion on Copolymerization of Styrene and isoprene at 25



C Solvent Nonsolvent Benzene Triethyl ether Ethyl ether THF(highly saturating solvent) % Styrene in copolymer Na + counter ion Li + counter ion 66 66 15 15 77 75 80 59 68 80 Generally sodium is more ionic than lithium Hanyang Univ.

Cationic Polymerization

The growing chain bears a positive charge. The active sites are either carbenium ions or oxonium ions.

Electron donating groups are needed as the R groups because these can stabilize the propagating species by resonance.

Ex) Spring 2008 Hanyang Univ.

Cationic Initiators

Proton acids with unreactive counterions Lewis acid + other reactive compound: * To use Lewis acid effectively as initiators, use the co-initiator.

F

:

F + C 2 H 5 Cl

cationogen

C 2 H 5 + [BF 3 Cl]

Hanyang Univ.

Spring 2008

Cationic Polymerization

Typical Initiator Systems Co-initiator Initiator SnCl 4 AlCl 3 H 2 SO 4 H HCl H 2 2 O SO 4 More acidic initiators Order of reactivity AlCl 3 > AlRCl 2 > AlR 2 Cl >AlR 3



are the effective in initiating polymerization most HCl > CH 3 COOH > C 6 H 5 NO 2 > > H 2 O >> CH 3 OH > CH 3 COCH 3 Ex)

BF 3 + H 2 O k e BF 3 OH H +

Spring 2008

BF 3 OH H + + C  C C  C H 3 C C C C isobutylene C + BF 3 O H + H 3 C C C + B F 3 OH C  C C C  C k p

Hanyang Univ.

Spring 2008

Cationic Polymerization

Termination

C C C B F 3 OH C H C C C

Problem : temination reactions occur randomly.

+ HB-F 3 OH

Kinetics

R R p i   k k p i     H B

 

 F 3 O  BF 3 H   OH



R i  R t



S S



R t   k i K e   H 2 O   3 k t    BF 3 OH   k i K e   H 2 O   3    BF 3 OH   k i K e   H 2 O   3 k t

[ * ] can control rxn

R p  k p k i K e



H * 2 O

  

3 2 k t

Hanyang Univ.

Spring 2008

Cationic Polymerization

 X R p  k  n R  R k p    t tr t   k tr   C 1 15 C 3 C C 16 7 C C 6 B 17 8 C F 3 OH X n  k t k  p k     tr k tr

k

t

k

tr



0

X

n



0

X

n



const

.

,



22 C C 21 27 C + 30 C 33 C C 32 B F 3 OH C 34

Hanyang Univ.

Chain Transfer Reactions

-Cationic vinyl polymerization is plagued by numerous side reactions, which lead to chain transfer mostly.

Ex)

• Difficult to achieve high MW (*initiator can give rise to many separate chains because of chain transfer) • These side reactions can be minimized But ! not eliminated by running the reaction at low temperature

Spring 2008 Hanyang Univ.

Cationic Polymerization

1) Ring opening polymerization (1) Mechanism

O R CH 2 + O CH 2 + R

carbon type polymzn. . .

CH 2 R ORCH 2 ORCH 2 + O CH 2 R

Example of ROR : cyclic amides, sulfides, acetals, esters, lactam, alkanes, … (2) Polymerizability - unstable ring or the ring which cannot be cyclized easily are very reactive * 3,4 and 7-11 membered ring is the most reactive ring 5,6 membered rings are stable and polymerize slowly, but, it still possible to be polymerized.

** 3-membered ring is the most easiest to be polymerized Spring 2008 Hanyang Univ.

Spring 2008

Cationic Polymerization

(3) polymerization of THF(Polytetrahydrofuran)

2 PF 5 PF 4 + (PF 6 ) PF 4 + (PF 6 ) + O PF 4 + O PF 6 gegenion

if H 2 O exist in the co-catalyst, the polymerization rate increases.

If the living polymerization is possible to occur, thus the termination or transfer also could be occurred.

O (CH 2 ) 4 + O A + O (CH 2 ) 4 (CH 2 ) 4 O(CH 2 ) 4 O(CH 2 ) 4 O + O A (CH 2 ) 4 (CH 2 ) 4 O(CH 2 ) 4 O(CH 2 ) 4 + + O A (CH 2 ) 4 O(CH 2 ) 4

Hanyang Univ.

Spring 2008

Cationic Polymerization

(4)Kinetics Initiation

I + ZY K Y + (IZ) initiator

R i

Y + (IZ) 

k i

[

Y

coinitiator  + (

IZ

M )  ][

M

] k i  YM + (IZ) -

Kk i

[

I

][

ZY

][

M

]

ex) styrene, stannic-chloride-H 2 O System [SnCl 4 OH ]H + Propagation – can has a low activation energy and can be polymerized rapidly

M n + O +  O M n O (CH 2 ) 4 O 

or Simple propagation reaction

H CH 2 C + [SnCl 4 OH] + H 2 C CH CH 2 CHCH 2 R R R

R p



k p

[

M

 strong initiator ][

M

]

The total rate of polymerization may actually increases by decreasing the temperature, which means that the termination has a high activation energy.

Hanyang Univ.