Control of Radical Polymerizations by Metalloradicals - American

Chapter 19

Control of Radical Polymerizations by Metalloradicals 1

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Downloaded by NORTH CAROLINA STATE UNIV on September 28, 2012 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch019

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B. B. Wayland , S. Mukerjee , G . Poszmik , D. C . Woska , L . Basickes , A . A . Gridnev , M . Fryd , and S. D. Ittel 2

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Department of Chemistry, University of Pennsylvania, Philadelphia, P A 104-6323 DuPont Marshall Laboratory, Philadelphia, P A 19146 Central Research and Development, DuPont Experimental Station, Wilmington, D E 19880-0328 2

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Cobalt(II) porphyrin complexes ((por)Co •) are used to illustrate how metalloradicals (Μ•) can function to control radical polymerization through both chain transfer catalysis and living polymerization. Chain transfer catalysis ( C T C ) is best achieved when there are minimal steric demands. This allows β-hydrogen abstraction from oligomer radicals by Μ•, as illustrated by the radical polymerization o f methyl methacrylate in the presence of tetraanisylporphyrinato cobalt(II). When β - Η abstraction from the oligomer radical is precluded by sterics, then a metalloradical mediated living radical polymerization ( L R P ) can occur. Radical polymerization initiated and mediated by organo-cobalt tetramesitylporphyrin complexes manifest high l i v i n g character as shown by the linear increase in M with conversion, formation of block copolymers and relativity low polydispersity homo and block copolymers. Kinetic studies provide rate and activation parameters for the living radical polymerization process. n

B o n d homolysis of an organometallic complex ( M - C ( C H ) ( R ) X ) in solution proceeds through the intermediacy o f a caged radical pair ( M » C ( C H ) ( R ) X ) that can recombine, separate into freely diffusing radicals, or react by Μ · abstracting a β-Η from the organic radical to form a metal hydride ( M - H ) and an olefin ( 7). 3

e

3

Μ · + •C(CH )(R)X 3

M—C(CH )(R)X 3

Μ · •C(CH )(R)X 3

M - H + CH =C(R)X 2

In the absence of events that irreversibly terminate radicals and metal hydride, the homolytic dissociation of an organo-metal complex can potentially provide a constant equilibrium source of both an organic radical and a metal hydride. The broad objectives of this program are to evaluate the kinetic and thermodynamic factors that

© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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govern the bond homolysis and subsequent radical and metal hydride reactions, and to apply this information in exercising control over the radical polymerization of olefins. Chain transfer catalysis ( C T C ) and quasi-living radical polymerization (LRP) are two important processes that can be mediated and controlled by metalloradicals. Chain transfer catalysis occurs when the metalloradical abstracts a β-Η from the growing polymer radical to form a metal hydride that reinitiates polymerization by reaction with the olefin monomer (2-5). C T C is useful for controlling the polymer molecular weight and introducing terminal alkene functionality (6). ^ W ^ C H

2

C ( C H

3

) X » + M -

M-H + CH =C(CH )X 2

3

•

^ A W C H

2

C ( X ) = C H

M"C(CH ) X 3

2

+ M - H

Μ· + 5x 10 ) and low polydispersities ( M / M < 1.15) of acrylate polymers have been prepared using this approach (Table V ) (22, 27). D#

2

3

3

3

2

3

3

s

n

5

n

w

n

Table III: Polymerization of M A using ( T M P ) C o - C H C ( C H ) 2

3

Time (hours)

% Conversion

0.5

5.5

7,856

1.10

1.5 5 15 38

11.5 20 42

18,930 38,120 91,930 143,600

1.16 1.21 1.17 1.21

M

66 3

n

M

w

/ M

3

n

II

[ ( T M P ) C o - C H C ( C H ) ] = l.OxlO" M ; [MA]j = 2.5 M ; [ ( T M P ) C o ] = 2.0x ΙΟ" M ; solvent = C D ; Τ = 60 °C. 2

3

3

i

i

4

6

6


310

Table I V : Polymerization of M A using ( T M P ) C o - C H ( C H ) C 0 C H 3

2

Time (hours)

% Conversion

Mn

Mw/Mn

0.5 6 12

3 21

5,900 40,300

1.12 1.17

40 50 85

76,500

1.21 1.21 1.10

20 96

99,000 163,000 3

[ ( T M P ^ o - C H i C H O C O . Œ ^ = 1.15xl0 M ; [MA]j = [ ( Τ Μ Ρ ) 0 ) ] ; = 2.5x10^ M ; solvent = C D ; Τ = 60 °C.

2.5

3

M;


π

6

6

200,000.

% Conversion Figure 4. Polymerization of M A in C D CH(CH )C0 CH . 6

3

2

at 60 °C using ( T M P ) C o -

6

3

Table V : Bulk Polymerization of M A by ( T M P ) C o - C H ( C Q C T ) C H 2

Time (hrs.)

% Conversion

0.75

3.5 10.5

2.0

3

M 176,800

M /M

554,300

1.13

N

W

3

N

1.15

4

[(TMP)Co-CH(C0 CH )CH ] = 1.75xlO" M; [CH^CHiCO.CH,)]; = 10.5 Μ ; [ ( Τ Μ Ρ ) Ο / ] ; = 3.1xl0" M ; solvent = Μ Α ; Τ = 60 °C. 2

3

3

i

5

The l i v i n g nature of the M A polymerization induced by ( T M P ) C o organo complexes is also illustrated by formation of acrylate block copolymers. Reaction of ( T M P ) C o - C H ( C H ) C 0 C H with M A ( [ M A ] / [ ( T M P ) C o - C H ( C H ) C 0 C H ] = 2,174) in benzene at 60 C is used to form a block of P M A attached to ( T M P ) C o ( M = 39,500, M / M = 1.15) (Figure 5a). Removal of unreacted M A followed by addition of butyl acrylate ( B A ) and benzene to the preformed P M A - C o ( T M P ) complex and heating at 60 °C results in Β A polymerization (Figure 5b) to form an ( M A ) ( B A ) block copolymer ( M = ( 8 2 - 2 7 2 ) x l O \ M / M = 1.15-1.23). 3

2

3

3

2

3

0

n

w

n

n

n

w

n


m


311

% Conversion of M A 300,000.

% Conversion of ΒΑ Figure 5. B l o c k c o p o l y m e r i z a t i o n of M A and B A by ( T M P ) C o C H ( C H ) C O C H in C D at 60 ° C . [ ( T M P ) C o - C H ( C H ) C 0 C H ] = 1.15x1a M . " a) ( T M P ) C o - P M A block [MA]j = 2.5 M ; M / M 1) 1.14 2) 1.15; b) ( T M P ) C o - P M A - P B A block copolymer [ Β Α ] = 2.5 M ; M / M 3) 1.15 4) 1.17 5)1.23. 3

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3

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3

i

3

w

n

(

w

n

Polymerization of acrylates induced by ( T M P ) C o - R complexes is envisioned to occur by the reaction sequence given by equations 1-5 ( X = C 0 R \ R ' = C H , ( C H ) C H ) . Bond homolysis of ( T M P ) C o - R produces a carbon centered radical (R») (equation 1) that initiates polymerization by reacting with an acrylate monomer to form R C H C H X » (equation 2) which either combines reversibly with ( T M P ) C o * (equation 3) or reacts with additional acrylate monomers to form an oligomer radical (equation 4) that subsequently combines reversibly with ( T M P ) C o (equation 5). 2

2

3

3

3

11

2

u #


312

n

(TMP)Co-R R*

( T M P ) C o - + R«

+ CH =CHX

(D

RCH CHX«

2

(2)

2

R C H C H X * + (TMP)Co"*

RCH CHX-Co(TMP)

2

R C H C H X - + (n+l)CH =CHX 2

(3)

2

RCH

2

2

C H X ( C H C H X ) CH 2

n

2

CHX-

(4)

11

R C H C H X ( C H C H X ) C H C H X * + (TMP)Co * RCH C H X ( C H C H X ) CH CHX-Co(TMP) 2

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n

2


2

2

n

(5)

2

Repetition of these events without radical termination or chain transfer would result in a fully living radical polymerization process. The real polymerization process cannot be fully living because of inherent bimolecular radical termination processes (equations 6 and 7) and Η · transfer reactions with monomer, polymer, solvent (equation 8) and ( T M P ) C o (equation 9) which result in non-living polymer chains. II#

2 ^ w c H

C H X .

•

^ w v \ r c H C H X - C H X C H ^ w

(6)

2 ^wwcH CHX«

•

^ W W ^ C H C H X + ^ww>CH=CHX

(7)

•

(8)

2

2

' w w ^ C H C H X * + HT 2

2

2

2

2

' w w ^ C H C H X + T* 2

n

^ w w ^ C H C H X * + (TMP)Co -

n

•

2

2

' ^ w ^ C H = C H X + (TMP)Co-H

(9)

In spite of the processes that can limit polymer growth (equations 6-9), observation of linear increases in M with conversion, formation of block copolymers, and relatively small polydispersities clearly demonstrate that ( T M P ) C o - R complexes initiate and control effective living radical polymerization of acrylates. Representative results from rate studies for the polymerization of M A that is initiated and controlled by ( T M P ) C o - R complexes are illustrated in Figures 6, 7, and 8. In a radical polymerization, the rate of conversion of monomer (M) is first order in both the radical and monomer concentrations with the radical propagation constant (k ) as the rate constant (-d[M]/dt = £.,[R»][M]). In a living radical process, the concentration of radicals is maintained constant at a value determined by an equilibrium with a dormant species. In the organometallic mediated polymerization of M A , the radical concentration is determined by the equilibrium constant for the homolytic dissociation of ( T M P ) C o - R ( K = [(TMP)Co] [R«] / [ ( T M P ) C o - R ] ; [R«] = K [(TMP)Co-R] / [(TMP)Co-]). The rate of M A polymerization is thus given by the expression such that the slope of the plot of n

p

e q

-d[MA] = ^ K [ΜΑ]Λ

M

[(TMP)Co-R] [(TMP)Co«]

l n ( [ M A ] / [ M A ] ) versus time (t) gives the product of the propagation constant, k , and the equilibrium constant ( K ) at temperature, T. Temperature dependence of the rate of M A polymerization gives the overall activation parameters (ΔΗ*, AS*) which are composed of activation parameters for the radical propagation (ΔΗ *, A S *) and the thermodynamic parameters ( Δ Η ° , AS°) for the dissociation of ( T M P j C o - P M A . Kinetic analysis of the ( T M P ) C o - R controlled living polymerization of M A gives effective activation parameters of ΔΗ* = Δ Η * + ΔΗ° = 28 kcal mol" and AS* = AS * + AS° = 4.4 cal K" mol" . Thermodynamic values for the homolytic dissociation of 0

t

p

e q

1

ρ

1

1


p

313

1

A

1

( T M P ) C o - P M A (ΔΗ° = 24 kcal m o l , AS° = 29 cal °K mol" ) are estimated by assuming that the activation parameters for radical propagation of M A are comparable to those determined for butylacrylate ( A H * « 4 kcal mol" ; ASp* « -25 cal ""K" mol" ) (75, 18). The thermodynamic values obtained for dissociation of ( T M P ) C o - P M A (AH° = 24 kcal m o l \ AS° = 29 cal K" m o l ) compare favorably with those determined for ( T A P ) C o - C H ( C H ) C 0 C H (AH° = 25.0±0.4 kcal m o l \ AS° = 34±1 cal Κ"' mol" ) (9). 1

p

1

1

1

3

0

2

1

3

1


2.5

Time (hours) Figure 6. First order rate plots for polymerization of a 2.50 M solution of M A in C D at 60 °C using 1.15xlO" M ( T M P ) C o - C H ( C H ) C 0 C H and varying initial concentrations of (TMP)Co» (•) [(TMP)Co»]j"= 5 . 8 χ 1 ά M (#)[(TMP)Co«]i = 1.38X10" M (A) [(TMP)Co«]i = 2.53x10^ M . 3

6

6

3

2

3

5

4

Conclusions. Cobalt(II) porphyrins are prototype metalloradicals that illustrate the control of radical polymerizations through both chain transfer catalysis ( C T C ) and living radical polymerization ( L R P ) . Major challenges for the continued development of this area are to expand the range of both metalloradicals and monomers that can be incorporated into systems that accomplish C T C and L R P .

Acknowledgments This research was supported by DuPont research fellowships and the National Science Foundation through NSF-CHE-95-27782.



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Ο

10

20

30

40

50

60

70

4

Time χ ΙΟ" s Figure 7. Kinetics for radical polymerization of ( T M P ) C o - P M A at various temperatures. Slope equals (([Co«]/[Co-R])ln(M /M ) = ^ K ^ t ) . 0

-44

t

I I ' ' • ' I ' • ' » I ' » ' ' I ' ' '

' ι ι

3.00

3.20

3.05

3.10

3.15

1000/T(°K) Figure 8. The plot of l n ( K Κ *) versus 1/T yields the effective activation parameters for the polymerization of M A by ( T M P ) C o - P M A ( K * = kp(MT)).


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Control of Radical Polymerizations by Metalloradicals - American

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