Living Radical Polymerization - American Chemical Society

Chapter 30 Progress in RAFT/MADIX Polymerization: Synthesis, Use, and RecoveryofChain Transfer Agents 1

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1

Sébastien Perrier , Pittaya Takolpuckdee , Steven Brown , Thomas M. Legge , Debashish Roy , Murray R. Wood , Steven P. Rannard , and David J. Duncalf 1

3

1

3

1

Department of Colour and Polymer Chemistry, The UniversityofLeeds, Leeds LS2 9JT, United Kingdom Chemistry Program, FacultyofScience and Technology, Valayalongkorn Rajabhat University, Phathumtani 13180, Thailand Unilever Research and Development, Port Sunlight Laboratories, Quarry Road East, Bebington, Wirral L63 3JW, United Kingdom

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3

W e report the synthesis of n o v e l c h a i n transfer agents f o r

RAFT/MADIX

p o l y m e r i z a t i o n , a n d their

use to m e d i a t e the p o l y m e r i z a t i o n of a variety of m o n o m e r s , in order to p r o d u c e a range of functional materials. A f t e r p o l y m e r i z a t i o n , the c h a i n transfer agent i s r e c o v e r e d in a one p o t reaction, w h i c h also p e r m i t s t o introduce functional groups at the c h a i n - e n dofthe p o l y m e r s .

438

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

439

Introduction Since

the first

polymerization RAFT/MADIX

reports

i n the early

techniques have

3 , 4

such

received

as

1990's,

living

ATRP,1

a strong

radical

NMP2

and

from

both

interest

a c a d e m i c a n d i n d u s t r i a l laboratories. R A F T a n d M A D I X are b o t h based o n a r a d i c a l l y i n d u c e d degenerative c h a i n transfer r e a c t i o n between

a

thiocarbonyl-thio

containing

compound

and

a

p r o p a g a t i n g r a d i c a l , f o l l o w i n g the p r i n c i p l e o f degenerative c h a i n Downloaded by UNIV OF OKLAHOMA on March 9, 2013 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch030

transfer first reported b y Z a r d ' s g r o u p i n the late 8 0 ' s . 5 T h e process offers a l l advantages

o f living polymerization

techniques ( e . g .

p r o d u c t i o n o f c o m p l e x p o l y m e r i c architectures) w i t h o u t s u f f e r i n g from

traditional

downfalls

attached

t o s u c h techniques ( l o w

temperatures, ultra-pure reactants, etc.). M o r e o v e r , the v e r s a t i l i t y o f R A F T / M A D I X towards f u n c t i o n a l groups p e r m i t s the synthesis o f a w i d e range o f f u n c t i o n a l m a c r o m o l e c u l e s . 6 F o r the last f e w years,

our group

has f o c u s e d

o n the d e s i g n

of

alternative

techniques f o r the synthesis o f p o l y m e r i z a t i o n mediators

(chain

transfer agents, C T A ' s ) , a n d their use i n p o l y m e r i z a t i o n reactions. W e report here o u r latest results, a n d p r o v i d e s i m p l e techniques o f CTA

synthesis,

a n d their use to p r o d u c e

specific

functional

m a t e r i a l s . T h e last s e c t i o n o f this chapter c o v e r s a straight f o r w a r d t e c h n i q u e to replace the t h i o c a r b o n y l - t h i o m o i e t y present at the c h a i n - e n d o f the p o l y m e r b y a f u n c t i o n a l g r o u p , w h i l s t r e c y c l i n g the c h a i n transfer agent.

Experimental Section Materials. A l l s o l v e n t s , m o n o m e r s , a n d other c h e m i c a l s w e r e p u r c h a s e d from A l d r i c h at the highest p u r i t y a v a i l a b l e a n d u s e d as


440 r e c e i v e d unless o t h e r w i s e stated. A l l m o n o m e r s

were

filtered

b e f o r e u t i l i z a t i o n t h r o u g h a basic a l u m i n a ( B r o c k m a n n I) c o l u m n , to e l i m i n a t e the r a d i c a l i n h i b i t o r . T e t r a h y d r o f u r a n ( T H F , R i e d e l d e H a ë n , H P L C grade) w a s d r i e d o v e r m o l e c u l a r sieves 4 Â . 2 , 2 Azobisisobutyronitrile

(AIBN,

9 9 % , Fisher)

was recrystallized

t w i c e from e t h a n o l . D i e t h y l ether, e t h y l acetate a n d n-hexane w e r e p u r c h a s e d f r o m R i e d e l - d e H a ë n ( A R grade). M a g n e s i u m t u r n i n g (AnalaR)

was purchased

from


methoxycarbonyl-a-phenylmethyl Merrifield

supported

dithiobenzoate

8,9

BDH.

C e l l u l o s e supported

dithiobenzoate7

and

S-

silica

/

S-methoxycarbonyl-a-phenylmethyl

w e r e synthesised as p r e v i o u s l y reported. A i r a n d

m o i s t u r e sensitive c o m p o u n d s w e r e m a n i p u l a t e d u s i n g standard S c h l e n k techniques under a n i t r o g e n atmosphere.

General synthesis method of trithiocarbonates / xanthates using Ι,Γ-thiocarbonyl diimidazole - Typical synthesis of 2ethylsulfanylthioearbonyl-sulfanylpropionic acid ethyl ester (ETSPE, 14, Figure 2). D r y toluene ( 6 0 m L ) w a s added to a stoppered

three-necked

flask.

Ι,Γ-Thiocarbonyl

diimidazole

( T C D I ) ( ( 9 5 % ) 3 . 6 0 g , 19.2 m m o l ) a n d p o t a s s i u m h y d r o x i d e ( 0 . 0 5 g , 0 . 8 6 m m o l ) w a s a d d e d , i n o n e p o r t i o n , under n i t r o g e n . T o the solution w a s added ethyl 2-mercaptopropionate ((95%) 2.63 m L , 2.71 g , 19.2 m m o l ) d r o p w i s e . T h e r e a c t i o n m i x t u r e w a s heated to 6 0 ° C a n d r e f l u x e d f o r 7 h . A f t e r this t i m e , the m i x t u r e w a s a l l o w e d to c o o l to r o o m temperature a n d left to stand, u n d e r n i t r o g e n , o v e r n i g h t . E t h a n e t h i o l ( ( 9 9 % ) 1.34 m L ,

1.49 g , 19.2

m m o l ) w a s a d d e d d r o p w i s e , under n i t r o g e n , to the stirred s o l u t i o n . T h e r e a c t i o n m i x t u r e w a s heated to 6 0 ° C , r e f l u x e d f o r 6 h , t h e n left to c o o l to r o o m temperature o v e r n i g h t . T h e s o l u t i o n w a s filtered,

concentrated

under

vacuum

a n d subjected to

flash

c h r o m a t o g r a p h y ( s i l i c a , 5 % e t h y l acetate i n hexane as eluent). T h e d e s i r e d fraction w a s concentrated under v a c u u m a n d subjected to K u g e l r o h r d i s t i l l a t i o n , a f f o r d i n g the p r o d u c t as a b r i g h t y e l l o w o i l . (62 % ) . ' H - N M R ( 4 0 0 M H z , C D C 1 3 ) δ 1 . 2 6 - 1 . 3 0 ( 3 H , t, J = 7 . 2 H z ,

SCH2CH3), 1.34-1.38 ( 3 H , t, J= 4 . 2 H z , OCH2CH3), 1.57-1.61


441 (3H,

d , J = 7.7 H z , S C H C H 3 ) , 3 . 3 4 - 3 . 4 0

( 2 H , q , J= 6.5 H z ,

SCH2CH3), 4 . 1 7 - 4 . 2 3 ( 2 H , q , J = 3 . 8 H z , O C H 2 C H 3 ) , ( 1 H , q , J= 7.4 H z , SCHCH3) 61.9,

4 8 . 0 , 31.5, 31.1,

1 3

4.78-4.83

C - N M R ( 1 0 0 M H z ) δ 2 2 1 . 9 , 171.1,

17.0,

14.3, T O F - M S

( E S + ) m/z=

239.023(MH ) +

General polymerization and end-group modification method.


To

a n ampoule

w a s added

mmol),

bromophenyl

mmol)

and A I B N

methyl

methacrylate

acetic a c i d dithiobenzoate (0.002g,

0.012 mmol).

(6.10g, 6 0 . 9

(0.035g, 0.122

The solution w a s

degassed w i t h N2 f o r 10 m i n s then heated to 6 0 ° C f o r 8 h . T h e s o l u t i o n w a s c o o l e d a n d the p o l y m e r precipitated b y d r o p w i s e addition

o f the s o l u t i o n into

poly(methyl

methacrylate)

c o l d hexane

to y i e l d 3 4 % o f

a n d characterized b y size e x c l u s i o n

c h r o m a t o g r a p h y ( M n = 13 5 0 0 , P D I = 1.18). T h e i s o l a t e d p o l y m e r was added to a n ampoule (0.063g,

0.380 mmol)

(0.256g,

0.019 mmol)

with

AIBN

a n d toluene (5 m L ) . T h e s o l u t i o n w a s

degassed w i t h N2 f o r 10 m i n s then heated to 8 0 ° C f o r 2 . 5 h . T h e s o l u t i o n w a s c o o l e d a n d the p o l y ( m e t h y l methacrylate) precipitated b y d r o p w i s e a d d i t i o n o f the s o l u t i o n into c o l d hexane. T h e p o l y m e r was

filtered

off

and

characterized

by

size

exclusion

c h r o m a t o g r a p h y ( M n = 13 4 0 0 , P D I = 1 . 1 7 ) .

Results and Discussions Synthesis of chain transfer agents. Synthesis

of

functional

chain

transfer

agents.

Living

r a d i c a l p o l y m e r i z a t i o n techniques offer the potential t o introduce specific

functionalities

within

instance, b y using tailor-made

polymeric

architectures.10 F o r

c h a i n transfer agents, o n e c a n

i n t r o d u c e c h a i n - e n d f u n c t i o n a l i t i e s into p o l y m e r i c c h a i n s . W e a n d others have

s h o w n that derivatives o f a m e t h y l

phenylacetate

r a d i c a l ( R , F i g u r e 1) are attractive l e a v i n g / r e - i n i t i a t i n g ( R ) g r o u p 1


442 f o r c h a i n transfer a g e n t s . 1 1 Indeed, their potential f o r from

fragmentation

the t h i o c a r b o n y l - t h i o m o i e t y i s p r o m o t e d b y the s t a b i l i z a t i o n

o f the r e s u l t i n g r a d i c a l b y the p h e n y l g r o u p o n the α - C o f the r a d i c a l , w h i l s t its a b i l i t y to reinitiate p o l y m e r i z a t i o n i s e n h a n c e d b y the l o w steric h i n d r a n c e o f the secondary r a d i c a l . F u r t h e r m o r e , the presence o f a c a r b o x y l i c m o i e t y a l l o w s the i n t r o d u c t i o n o f a v a r i e t y o f f u n c t i o n a l i t i e s . W e h a v e s y n t h e s i z e d a series o f dithiobenzoates a n d trithiocarbonates s h o w i n g a range o f f u n c t i o n a l groups v i a


e s t e r i f i c a t i o n o f a m o l e c u l e b e a r i n g the relevant f u n c t i o n a l i t y w i t h 2-chloro-2-phenylacetyl

c h l o r i d e o r α - b r o m o p h e n y l acetic a c i d ,

i n c l u d i n g ( F i g u r e 1) m e t h y l ester (1 a n d 2 ) , c a r b o x y l i c a c i d (3 a n d 4) a n d its s o d i u m salt (5 a n d 6 ) , a m i d e (7), h y d r o x y l (8) a n d a l s o m u l t i f u n c t i o n a l C T A ' s (e.g. 9 ) .

Figure 1. Functional CTA's producedfrom 2-chloro-2-phenylacetyl chloride or a-bromophenyl acetic acid and derivatives.


443

Synthesis of trithiocarbonates via one pot reaction from 1 , 1 ' thiocarbonyl diimidazole. T h e syntheses o f trithiocarbonates a n d xanthates f o l l o w i n g t r a d i t i o n a l routes h a v e s i g n i f i c a n t d r a w b a c k s f o r laboratories that are u n f a m i l i a r w i t h c o m p o u n d s o f c o n s i d e r a b l e safety concerns ( e . g . h i g h f l a m m a b i l i t y , risks o f e x p l o s i o n , water s e n s i t i v i t y , a n d so o n ) a n d have contributed t o the l i m i t e d spread o f RAFT

and M A D I X

i n academic

and industrial

laboratories

w o r l d w i d e . T h e r e have b e e n m a n y reports o f alternative synthetic


routes f o r the p r o d u c t i o n o f C T A ' s , b u t v e r y f e w o f t h e m c o n c e r n s trithiocarbonate

derivatives.6

W e have

developed

a

synthetic

m e t h o d based o n the use o f Ι , Γ - t h i o c a r b o n y l d i i m i d a z o l e ( T C D I , 10) t o p r o d u c e i n a one-pot r e a c t i o n a range o f trithiocarbonates a n d xanthates. S u c h syntheses present clear advantages i n terms o f y i e l d , reaction time and simplicity (Figure 2).

Figure 2. Synthetic schemes for the production of trithiocarbonates and xanthates using 7,1 '-thiocarbonyl diimidazole (10). W i t h t w o equivalents o f a p r i m a r y t h i o l reacted onto T C D I , the symmetrically Furthermore,

disubstituted

product

(11)

is readily

obtained.

w h e n T C D I i s reacted w i t h a secondary t h i o l o r

secondary a l c o h o l , o n l y the intermediate 5/O-ester o f i m i d a z o l e - N t h i o n o c a r b o x y l i c a c i d i s p r o d u c e d (12 a n d 13). 12 a n d 13 m a y b e further reacted w i t h a p r i m a r y t h i o l , to g i v e a trithiocarbonate (14) o r a xanthate (15).


444 ιοοΗ • 80-Ι

•

• • •

100:1:1.5 500:1:1.5

À J

0


0

25

50

75

100

125

150

175

200

time (min)

Figure 3. Conversion vs. time for the polymerization of methyl acrylate mediated by 2-ethylsulfanylthiocarbonylsulfanylpropionic acid ethyl ester (ETSPE) at various Monomer: CTA ratios.

T r i t h i o c a r b o n a t e s h a v e the added advantages b y c o m p a r i s o n to other C T A ' s (e.g. dithiobenzoates) i n that they i n d u c e v e r y l i t t l e , o r n o , retardation i n R A F T p o l y m e r i z a t i o n . 6 Indeed, p o l y m e r i z a t i o n s mediated

by

2-ethylsulfanylthiocarbonylsulfanyl-propionic

acid

e t h y l ester ( E T S P E ) s h o w s i m i l a r k i n e t i c s , despite i n c r e a s i n g the m o n o m e r c o n c e n t r a t i o n b y a factor 5 ( F i g u r e 3 a n d 4).

RAFT/MADIX polymerization process. In a d d i t i o n to a l l o w i n g the

introduction

architectures,

of

specific

RAFT/MADIX

functionalities polymerization

into

polymeric

benefits

from

a

straight f o r w a r d process. Indeed, the p o l y m e r i z a t i o n relies o n the s i m p l e i n t r o d u c t i o n o f a s m a l l a m o u n t o f c h a i n transfer agent into a c l a s s i c a l free r a d i c a l system ( m o n o m e r + initiator). T h e transfer o f the C T A

b e t w e e n g r o w i n g r a d i c a l c h a i n s , present at v e r y

c o n c e n t r a t i o n , a n d dormant

polymer

c h a i n s , present at

low

higher

c o n c e n t r a t i o n (three or f o u r orders o f m a g n i t u d e ) , w i l l regulate the growth

of

the

molecular

weight,

and

limit

the

termination


445

40000

30000

τ

ô Ε σ> 20000


ϊ 10000-

0 0

10 20

30 40

50 60

70

80

90 100

Conversion / %

Figure 4. Molecular weight and PDI evolution with conversion for the polymerization of methyl acrylate mediated by 2ethylsulfanylthiocarbonylsulfanyl-propionic acid ethyl ester (ETSPE) at various Monomer: CTA ratios. reactions. In the s y s t e m , the c o n c e n t r a t i o n o f r a d i c a l i n i t i a t o r i s a k e y parameter.

Free r a d i c a l s are r e q u i r e d at the start o f

the

p o l y m e r i z a t i o n i n order to trigger the degenerative c h a i n transfer reactions

which

control

the

polymerization.

However,

these

r a d i c a l s also generate ' d e a d ' p o l y m e r i c c h a i n s , w h i c h affect the level o f control over molecular weight distribution. Therefore, an increase i n the c o n c e n t r a t i o n o f free r a d i c a l initiator leads to faster polymerizations,

but

increases the

probability

of

irreversible

terminations between growing chains, resulting i n a worse control over molecular weight distribution.6 I n order to p r o m o t e faster p o l y m e r i z a t i o n s w h i l s t k e e p i n g g o o d c o n t r o l o v e r the m o l e c u l a r w e i g h t d i s t r i b u t i o n , w e investigated microwave-assisted R A F T

polymerizations.

Microwave-assisted

p o l y m e r i z a t i o n s have b e e n s h o w n to p e r f o r m faster, a n d to h i g h e r y i e l d s , t h a n e q u i v a l e n t reactions p e r f o r m e d under m o r e t r a d i t i o n a l h e a t i n g c o n d i t i o n s , a n d this effect i s g e n e r a l l y attributed to

a

temperature effect. B u l k R A F T p o l y m e r i z a t i o n o f m e t y l acrylate,


446 m e t h y l methacrylate a n d styrene w a s attempted i n the presence o f 2-ethylsulfanylthiocarbonylsulfanyl-propionic

acid

ethyl

ester

( E T S P E ) as the c h a i n transfer agent. W e o b s e r v e d f o r m e t h y l acrylate

and

methyl

methacrylate

that

the

kinetics

of

p o l y m e r i z a t i o n w e r e greatly e n h a n c e d , w i l s t k e e p i n g a n e x c e l l e n t c o n t r o l o v e r the m o l e c u l a r w e i g h t (PDI < 1.10) ( F i g u r e 5 a n d 6 ) . O n the other h a n d , the p o l y m e r i z a t i o n o f styrene d i d n o t s h o w a n y increase i n rate. S i m i l a r p o l y m e r i z a t i o n k i n e t i c s w e r e reported f o r


the

free

radical

polymerization

o f these

monomers,12"14

but

p r e v i o u s studies h a v e s h o w n n o n o t i c e a b l e effect o f m i c r o w a v e h e a t i n g o n other L R P t e c h n i q u e s . 1 5

Synthesis of functional polymeric architectures. T h e versatility o f the R A F T

process towards

f u n c t i o n a l groups p e r m i t s the

synthesis o f a range o f f u n c t i o n a l m a t e r i a l s a n d i n d e e d p o l y m e r i c architectures.

T h e use o f m e t h y l

phenylacetate

d e r i v a t i v e s as

l e a v i n g groups does not o n l y p e r m i t to introduce f u n c t i o n a l e n d g r o u p s , but a l s o to synthesize m o r e c o m p l e x architectures, as illustrated b e l o w .

Telechelic polymers and A B A triblock copolymers. T h e use o f c o m p o u n d (9) as c h a i n transfer agent to mediate the p o l y m e r i z a t i o n o f methacrylate o r acrylate d e r i v a t i v e s l e a d to the p r o d u c t i o n o f t e l e c h e l i c h o m o p o l y m e r s w i t h a dithiobenzoate f u n c t i o n a l i t y . T h e r e s u l t i n g t e l e c h e l i c p o l y m e r s c a n b e further reacted w i t h radical

initiators

(see

below)

to

produce

free

α,ω-functional

h o m o p o l y m e r s . P o l y m e r i z a t i o n o f m e t h y l methacrylate a n d b u t y l methacrylate y i e l d e d t e l e c h e l i c h o m o p o l y m e r s w i t h w e l l c o n t r o l l e d m o l e c u l a r w e i g h t s . P o l y m e r i z a t i o n o f b u t y l acrylate, o n the other hand,

l e d to p o l y m e r s

distribution.

We

showing multimodal molecular

explained

these

observations

by

the

weight initial

f r a g m e n t a t i o n o f the R group o c c u r r i n g at different rates o n e a c h side d u e to o n e side o v e r c o m i n g s i n g l e p o l y m e r u n i t i n h i b i t i o n s i g n i f i c a n t l y earlier that the other. S u c h a situation m a y e x i s t to a lesser

extent

f o r methacrylate

derivatives,

d u e to the faster

f r a g m e n t a t i o n o f methacrylate p o l y m e r u n i t s . 1 6


447


100

Time (mins)

Figure 5. Conversion vs. time for the microwave-assisted polymerization of methyl acrylate and methyl methacrylate mediated by ETSPE using a ratio Monomer :ETSPΕ :AIBN = 500:1:0.25.

Conversion / %

Figure 6. Molecular weight and PDI evolution with conversion of the microwave-assisted polymerization of methyl acrylate and methyl methacrylate mediated by ETSPE using a ratio Monomer:ETSPE:AIBN = 500:1:0.25.


448

Table 1. (Co)polymers synthesized via RAFT in toluene at 60 °C M

1

CTA-.LM.S

BMA

f

1:0.2:500:100

BMA

f

1:0.4: 500:100

810

83.4

1:0.4:500:100

810

88.8

1:0.4:500:100

2868

79.8

MMA BA Downloaded by UNIV OF OKLAHOMA on March 9, 2013 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch030

time com / min /%

f

t

810

82.5

M„ 37,900°

Mi/Aeo

PDf

59,500

1.18

48,400

e

59,300

1.23

30,000

e

44,500

1.26

51,200

e

51,200

1.70

d

95,800

1.36

d

39,800

1.20

BA*

1:0.4:800:1200

1030

96

53,300

BA*

1:0.4:800:1200

175

40

15,700

BA*

1:0.25:800:1200

860

18

21,700

d

19,800

1.55

73,900

d

82,000

1.73

90,700

d

100,700

1.24

BA*

1:0.25:800:1200

3000

80

BMA*

1:0.25:800:1200

4180

91

CTA = 9, Figure 2; * CTA = P M M A a

b

CTA = chain transfer agent, I = initiator, M = monomer, S = solvent; determined by gravimetric analysis; determined by SEC using poly(methyl methacrylate) as standards; determined by H NMR by comparing the integrations of known proton signals from each block, and standardized using the molecular weight of the original macroCTA (determined by SEC). c

d

!

B M A = Η-butyl methacrylate, M M A = methyl methacrylate, B A = w-butyl acrylate. B l o c k e x t e n s i o n o f the p o l y ( m e t h y l methacrylate) w i t h b u t y l methacrylate a n d b u t y l acrylate l e d to the f o r m a t i o n o f A B A t r i b l o c k c o p o l y m e r s . W h i l s t the use o f the methacrylate d e r i v a t i v e s yielded block copolymers o f w e l l controlled molecular weights for c o n v e r s i o n s a b o v e 9 0 % , the b l o c k e x t e n s i o n u s i n g b u t y l acrylate s h o w e d the presence o f a s e c o n d p e a k at h i g h m o l e c u l a r w e i g h t s f o r c o n v e r s i o n a b o v e 4 0 % . I n s p e c t i o n o f the m o l e c u l a r w e i g h t d i s t r i b u t i o n data suggested that c r o s s - t e r m i n a t i o n s b y c o m b i n a t i o n m a y e x p l a i n the presence o f these h i g h m o l e c u l a r w e i g h t p e a k s .

Cellulose supported RAFT polymerization. T h e treatment o f c e l l u l o s e fiber (cotton f a b r i c a n d paper substrate) w i t h 2 - c h l o r o - 2 phenylacetyl chloride f o l l o w e d b y reaction w i t h a dithiobenzoate salt leads t o the f o r m a t i o n o f c e l l u l o s e supported c h a i n transfer agents ( C e l l u l o s e - C T A ) , attached to the c e l l u l o s e v i a the R g r o u p ( R - s u p p o r t e d C T A ' s ) . W e use these R - s u p p o r t e d C T A ' s t o m e d i a t e


449 the p o l y m e r i z a t i o n o f styrene a n d d i m e t h y l a m i n o e t h y l methacrylate ( D M A E M A ) . T h e use o f free c h a i n transfer agent i n s o l u t i o n w a s f o u n d to i m p r o v e the graft r a t i o , a n d l i m i t the p r o d u c t i o n o f free polymeric chains i n solution.

Table 2. Graft polymerizationa of styrene (Sty) and dimethyl aminoethylmethacrylate (DMAEMA) mediated by a celluloseC T A in a molar ratio Monomer/Cellulose-CTA/AIBN = a


100/l/0.1 Monomer Sty

DMAEMA

Monomer Time/ h Graft ratio /wt Monomer concentration % conversion% 0.5M 0.5M 0.5M 0.5M 0.5M 0.5M 2M 2M 2M 2M

14

8 16 24 4 20 48 4 20 48 20

b

7 11 17 13 60 75 29 78 94 66"

16 195 1 7 11 8 12 25 20

b

b

a

7

b

Solvent = toluene; Temperature =60 °C. See reference for expérimentais; Undertaken in the presence offreeCTA (MCPDB) in a ratio cellulose-CTA/free CTA = 1/1

R e c o v e r y o f c h a i n t r a n s f e r a g e n t s . A k e y feature o f l i v i n g r a d i c a l p o l y m e r i z a t i o n systems i s the presence o f a f u n c t i o n a l group at the polymeric chain-end (e.g. halogen, alkoxyamine, thiocarbonyl-thio group

for A T R P ,

NMP

and R A F T ,

respectively),

which is

r e s p o n s i b l e f o r the l i v i n g character o f the p o l y m e r i z a t i o n .

There

has b e e n a variety o f studies a i m i n g to m o d i f y this e n d g r o u p f o r ATRP,17'18

NMP19'20

and

RAFT.6

In

the

case

of

RAFT

p o l y m e r i z a t i o n , w e f o u n d that the i n - s i t u a d d i t i o n o f a r a d i c a l to the reactive C = S b o n d o f the t h i o c a r b o n y l - t h i o p o l y m e r e n d - g r o u p leads to the f o r m a t i o n o f a n intermediate r a d i c a l , w h i c h c a n t h e n either fragment b a c k to the o r i g i n a l a t t a c k i n g r a d i c a l o r t o w a r d s the p o l y m e r i c c h a i n r a d i c a l . I n the presence o f a n excess o f free r a d i c a l s , the e q u i l i b r i u m i s d i s p l a c e d t o w a r d the f o r m a t i o n o f the


450 p o l y m e r i c c h a i n r a d i c a l , w h i c h c a n then r e c o m b i n e i r r e v e r s i b l y w i t h one o f the free r a d i c a l s present i n excess i n s o l u t i o n , thus f o r m i n g a d e a d p o l y m e r i c c h a i n . T h e process w a s s h o w n to w o r k i n a variety o f c o n d i t i o n s a n d i s easy to undertake ( F i g u r e Besides

the

color

removal,

the

system

also

7).

21

allows

the

p r o d u c t i o n o f e n d - f u n c t i o n a l p o l y m e r s a n d the r e c o v e r y o f the c h a i n transfer agent

(up

to

three p o l y m e r i z a t i o n

cycles

were

u n d e r t a k e n u s i n g the same r e c o v e r e d C T A e a c h t i m e ) .


T h e r e c o v e r y o f the C T A m i g h t h o w e v e r be a p r o b l e m f o r large scale p r o d u c t i o n . W h i l s t i n a t y p i c a l laboratory set-up o n the m u l t i g r a m s c a l e , the p o l y m e r c h e m i s t w o u l d precipitate the after r a d i c a l m o d i f i c a t i o n a n d r e c o v e r the C T A

from

polymer

the s o l u t i o n ,

larger p r o d u c t i o n s require a n alternative technique o f r e c o v e r y . p o t e n t i a l s o l u t i o n i s the use o f a s o l i d - s u p p o r t e d C T A ,

A

covalently

l i n k e d to a s o l i d support v i a its Ζ group. S u c h Z - s u p p o r t e d

CTA

c a n be e a s i l y i s o l a t e d after r a d i c a l cleavage b y s i m p l e f i l t r a t i o n f r o m the p o l y m e r product. T h e process a l s o a l l o w s r e m o v a l o f a l l side

products,

terminated),

including dead

and

non-reacted

polymeric monomer

chains by

a

(irreversibly

filtration

after

p o l y m e r i z a t i o n . T h i s a d d i t i o n a l feature m a k e s this p r o c e s s u n i q u e , as it i s the o n l y r a d i c a l p o l y m e r i z a t i o n system able to p r o v i d e c h a i n s that are

100%

living (All

non-living

chains, i.e.

not

t e r m i n a t e d b y the t h i o c a r b o n y l - t h i o m o i e t y , are w a s h e d o f f after p o l y m e r i z a t i o n - o n the other h a n d , 1 0 0 % o f the p o l y m e r i c c h a i n s recovered

have

kept

their

end-group

functionality,

and

are

therefore able to re-initiate p o l y m e r i z a t i o n ) . 8 , 9 A direct o u t c o m e o f s u c h s y s t e m i s the p o s s i b i l i t y to p r o d u c e ' p u r e ' b l o c k c o p o l y m e r s via

radical polymerization.

Indeed,

homopolymers

impurities

a r i s i n g from t e r m i n a t i o n reactions are separated from the ' l i v i n g ' (controlled) c h a i n s b y f i l t r a t i o n . A f t e r

filtration,

the i s o l a t e d l i v i n g

c h a i n s , attached to the support, c a n be c h a i n extended to p r o d u c e b l o c k c o p o l y m e r s ( F i g u r e 8). In this s y s t e m , the c o n c e n t r a t i o n i n dead p o l y m e r i c c h a i n s (non-attached to the s o l i d support) i s n o t i c e a b l y h i g h e r than that o b s e r v e d i n the case o f h o m o g e n o u s R A F T p o l y m e r i z a t i o n .


We


451

Figure 7. Schematic of the RAFT/MADIXpolymerization and CTA recovery cycle. (Reproducedfrom reference 8. Copyright 2005 American Chemical Society.)

attributed

this

effect

to

the

presence

of

two

competing

p o l y m e r i z a t i o n reactions i n the system. Free r a d i c a l p o l y m e r i z a t i o n o c c u r s i n s o l u t i o n , l e a d i n g to the f o r m a t i o n o f u n c o n t r o l l e d , h i g h m o l e c u l a r w e i g h t p o l y m e r s that r e m a i n free i n s o l u t i o n . O n

the

other h a n d , R A F T p o l y m e r i z a t i o n o c c u r s i n presence o f the s o l i d support, a n d leads to attached l i v i n g p o l y m e r i c c h a i n s . F o l l o w i n g these o b s e r v a t i o n s , a n d i n order to i m p r o v e

the c o n t r o l

over

attached p o l y m e r i c c h a i n s a n d reduce the amount o f dead c h a i n s left i n s o l u t i o n , w e i n t r o d u c e d a free c h a i n transfer agent into the s y s t e m (ratio free C T A

: Z-supported C T A

= 1 : 1 ) . After cleavage,

the m o l e c u l a r w e i g h t f o r b o t h attached a n d free p o l y m e r i c c h a i n s i s greatly r e d u c e d b y c o m p a r i s o n to a system w i t h o u t free c h a i n transfer agent,

as the p o l y m e r i z a t i o n

presence o f a free C T A

is slowed d o w n

i n solution. Moreover,

by

the

the m o l e c u l a r

w e i g h t o f the attached c h a i n s i s c l o s e to that expected a n d the P D F s r e m a i n b e l o w 1.2 f o r b o t h types o f chains. q u a n t i f i c a t i o n o f the ratio o f attached:

Furthermore,

free p o l y m e r i c

r e v e a l e d that the p r o p o r t i o n o f free c h a i n s has decreased.


chains


452

Figure 8. Schematic of the (co)polymerization process using Z-supported RAFT polymerization.

Conclusion R A F T / M A D I X p o l y m e r i z a t i o n is one o f the m o s t versatile l i v i n g r a d i c a l p o l y m e r i z a t i o n techniques to date. W e h a v e demonstrated the use o f s i m p l e synthetic schemes i n order to p r o d u c e e f f e c t i v e c h a i n transfer agents able to c o n t r o l p o l y m e r i z a t i o n , the use o f these

c h a i n transfer agents

to

synthesize

complex

functional

p o l y m e r i c architectures ( b l o c k c o p o l y m e r s a n d graft c o p o l y m e r s ) , the o p t i m i z a t i o n o f the p o l y m e r i z a t i o n processes to increase the p o l y m e r i z a t i o n rate w h i l s t k e e p i n g g o o d c o n t r o l o v e r the p r o d u c t s , a n d the r e c o v e r y o f the c h a i n transfer agent f o r further use.

We

b e l i e v e the s i m p l i c i t y o f this synthetic c y c l e m a k e s it p o s s i b l e f o r n o n - s p e c i a l i s t s scientists to p r o d u c e c o m p l e x f u n c t i o n a l p o l y m e r i c architectures w h i c h , to date, w e r e o n l y a v a i l a b l e to the w e l l - t r a i n e d polymer chemists.

Acknowledgement W e a c k n o w l e d g e the support o f U n i l e v e r , I C I , E P S R C , the R o y a l Thai

Government,

the

Green

Chemistry

Centre

for

Industrial

C o l l a b o r a t i o n a n d Y o r k s h i r e F o r w a r d , the U n i v e r s i t y o f L e e d s a n d the D e p a r t m e n t o f C o l o u r a n d P o l y m e r C h e m i s t r y f o r f u n d i n g .


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Living Radical Polymerization - American Chemical Society

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