Water-Soluble Polymers - American Chemical Society

8 Metal-Activated Redox Initiation for the Synthesis of Low Molecular Weight

Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch008

Water-Soluble Polymers Kathleen Hughes and Graham Swift Rohm and Haas Research Laboratories, Spring House, PA 19477

Metal-activated redox initiation in aqueous solution, employing aqueous peroxide solutions and water-soluble transition metal salts, provides one interesting industrial route to low molecular weight water-soluble polymers. Synthesis of low molecular weight poly (acrylic acids) (M 1,000-20,000), using hydrogen peroxide and copper salts as the initiator system, is described. Molecular weight control is dependent upon reaction temperature, hydrogen peroxide level, and hydrogen peroxide to copper salt molar ratios. Reaction temperatures of 80-100 °C and hydrogen peroxide levels of approximately 1-20 wt % (on monomer) are employed. Hydrogen peroxide to copper molar ratios of approximately 50:1 to 100:1 provide the lowest molecular weight poly(acrylic acids) for a given hydrogen peroxide level. Inclusion of low levels (Fe 2

2

3 +

+ OH"+ O H

O u r analyses i n d i c a t e that h y d r o x y l free radicals initiate o u r p o l y m e r c h a i n s ; h o w e v e r , t h e c o m p l e t e m e c h a n i s m o f i n i t i a t i o n is n o t e s t a b l i s h e d . I n this c h a p t e r , w e p r e s e n t i n t e r e s t i n g f i n d i n g s r e g a r d i n g o n e i n d u s t r i a l a p p r o a c h t o the synthesis o f p o l y ( a c r y l i c a c i d s ) . R e s u l t s r e g a r d i n g p o l y m e r i z a t i o n u s i n g this t y p e o f m e t a l - a c t i v a t e d i n i t i a t i o n as w e l l as t h e dependence of polymer molecular weight control o n hydrogen peroxide to c o p p e r salt m o l a r r a t i o s a r e d i s c u s s e d .

Experimental Section Synthesis. T h e poly(acrylic acid) polymers described were all prepared at 55 w t % solids i n deionized water b y using gradual addition processes at the temperatures indicated under Results and Discussion. Water, the metal salt, and the amine (if included) were charged to a 2 - L , four-necked flask equipped w i t h a mechanical stirrer, a condenser, a thermometer, and addition funnels for the gradual additions of the acrylic acid monomer and hydrogen peroxide solutions were completed linearly and separately during 2-3 h . H y d r o g e n peroxide concentrations of 30 w t % were used. Sulfate salts of the metals were used, and the amount of metal used is expressed as parts per million of metal (not metal salt) on monomer. A m i n e and hydrogen peroxide charges are expressed as weight percent active ingredient on the monomer throughout the tables. M o l a r ratios of hydrogen peroxide to metal salt are based on the total charges o f each (i.e., not on the molar ratio at each point throughout the gradual addition o f hydrogen peroxide). Characterization. Characterization of the p o l y (acrylic acid) samples include Brookfield viscosity measurements o n samples equilibrated at 25 ° C , gas-liquid chromatographic analysis for residual acrylic acid monomer, iodometric titration for residual peroxide, and gel permeation chromatography for molecular weight measurements.

Results and Discussion S u r v e y o f M e t a l A c t i v a t o r s . T a b l e I d a t a illustrate the r e l a t i v e e f f i c i e n c i e s o f c o p p e r , i r o n , a n d m a n g a n e s e as m e t a l a c t i v a t o r s i n 9 8 ° C p o l y m e r i z a t i o n s o f a c r y l i c a c i d w h e n 5% a c t i v e h y d r o g e n p e r o x i d e w a s u s e d as t h e i n i t i a t o r . I n e a c h c a s e , the s u l f a t e salt w a s u s e d , a n d also i n e a c h e x p e r i m e n t , 2 ? ( d i m e t h y l a m i n o ) e t h a n o l w a s i n c l u d e d . O n the basis o f m o n o m e r c o n v e r s i o n a n d u s e o f h y d r o g e n p e r o x i d e i n this l i m i t e d s u r v e y o f m e t a l a c t i v a t o r s , c o p p e r is t h e p r e f e r r e d m e t a l f o r these a q u e ous p o l y m e r i z a t i o n s . A l t h o u g h m o l e c u l a r w e i g h t m e a s u r e m e n t s o n these T a b l e I s a m p l e s w e r e n o t d o n e , the v i s c o s i t y at 508? s o l i d s i n d i c a t e s that the c o p p e r s y s t e m y i e l d s the l o w e s t m o l e c u l a r w e i g h t s u n d e r the c o n d i -

In Water-Soluble Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986.

8.

147

Metal-Activated Redox Initiation

H U G H E S A N D SWIFT

Table I. Efficiency of Hydrogen Peroxide as an Initiator for Poly(acrylic acid) Synthesis in the Presence of Several Transition Metals Metal (on Monomer) 380ppmofCu 380ppmof F e 380ppmofMn

AA Monomer Conversion (%) a


b C

2

100 95 80

2 +

a

H0

2 + 2 +

2

Residue (%)

25 °C Brookfield Viscosity (cP) b

220 8200 NA

5 25 50

C

AA represents acrylic acid. At m solids. NA is not available.

tions s t u d i e d . H y d r o g e n p e r o x i d e i n i t i a t i o n o f a q u e o u s a c r y l i c a c i d p o l y m e r i z a t i o n s , u s i n g the s a m e l e v e l o f h y d r o g e n p e r o x i d e , i n the absence of metal activation under similar reaction conditions, yields p o o r u t i l i z a t i o n o f h y d r o g e n p e r o x i d e (>70% r e s i d u a l ) a n d a h i g h e r molecular weight product. O n t h e basis o f the m o n o m e r c o n v e r s i o n a n d h y d r o g e n p e r o x i d e u t i l i z a t i o n , the c o p p e r r e d o x t r a n s i t i o n is m o r e f a c i l e t h a n the i r o n o r m a n g a n e s e e l e c t r o n transitions: Cu+ — >

Cu

2 H h

Cu

+

T h i s o b s e r v e d o r d e r o f m e t a l e f f i c i e n c y is consistent w i t h the o x i d a t i o n p o t e n t i a l s f o r these m e t a l s (6): Cu

+

—>

Cu

2 +

+ e~

£° = -0.153 V

Fe

2 +

—•

Fe

3 +

+ e"

E° = - 0 . 7 7 1 V

Mn

2 +

—>

Mn

3 +

+ e"

E° = - 1 . 5 1 0 V

Effect of C o p p e r to H y d r o g e n Peroxide Molar Ratio Variations. M e t a l - a c t i v a t e d r e d o x i n i t i a t i o n w a s t h e n p u r s u e d b y u s i n g c o p p e r salt a n d h y d r o g e n p e r o x i d e as the i n i t i a t i n g s y s t e m . T h e e f f e c t o f the h y d r o g e n p e r o x i d e to c o p p e r m o l a r r a t i o o n p e r o x i d e u t i l i z a t i o n , m o n o m e r conversion, a n d molecular weight control i n poly(acrylic acid) synthesis w a s i n v e s t i g a t e d . T w o h y d r o g e n p e r o x i d e l e v e l s w e r e s t u d i e d (1 a n d 5 w t % o n the m o n o m e r ) ; a l l e x p e r i m e n t s w e r e r u n at 9 5 ° C , a n d 1 w t % t r i e t h a n o l a m i n e w a s c h a r g e d to t h e r e a c t o r w i t h the c o p p e r salt. D a t a f o r these e x p e r i m e n t s are p r o v i d e d i n T a b l e s I I a n d I I I . T h e T a b l e I I d a t a illustrate the e f f e c t o f h y d r o g e n p e r o x i d e t o c o p p e r m o l a r ratios o n p e r o x i d e utilization, m o n o m e r conversion, a n d m o l e c u l a r w e i g h t i n the 1% h y d r o g e n p e r o x i d e e x p e r i m e n t s . I n t e r m s o f m o n o m e r c o n v e r s i o n , p e r o x i d e u t i l i z a t i o n , a n d m o l e c u l a r w e i g h t , the 60:1 t o 120:1 m o l a r r a t i o r a n g e s e e m s o p t i m u m f o r these r e a c t i o n conditions. ^

Library 1155 16th St.. H.W. In Water-Soluble Polymers; Glass, J.;

Washington, 200 36 Advances in Chemistry; American Chemical D.C. Society: Washington, DC, 1986.

148

WATER-SOLUBLE POLYMERS

Table II. Effect of Hydrogen Peroxide (1%) to Copper Salt Molar Ratios on Poly(acrylic acid) Synthesis


molojHzOz: Cu (ppm) mol of Cu 3600 1500 600 300 150 30

5:1 12:1 30:1 60:1 120:1 600:1

AA H0 Residue (%) Conversion (%) a

2

2

0 0 0 0 11 60

23 54 82 100 100 100

Viscosity (cP) at 25 °C NA NA NA 1000 3000 72000

M

M„

w

NA NA NA NA NA NA 9800 4140 17350 7800 49900 13500

AA is acrylic acid.

fl

T o prepare lower molecular weight poly (acrylic acids), w e perf o r m e d t h e T a b l e I I I e x p e r i m e n t s at t h e 5% h y d r o g e n p e r o x i d e l e v e l using the reaction conditions listed i n T a b l e III. I n these T a b l e I I I e x p e r i m e n t s , t h e 60:1 t o 150:1 h y d r o g e n p e r o x i d e t o c o p p e r m o l a r r a t i o s , as i n t h e 1% h y d r o g e n p e r o x i d e e x p e r i m e n t s , a p p e a r t o b e i n a n o p t i m u m r a n g e f o r these r e a c t i o n c o n d i t i o n s i n t e r m s of molecular weight control a n d peroxide utilization. E f f e c t o f H y d r o g e n P e r o x i d e L e v e l V a r i a t i o n at C o n s t a n t H y d r o gen P e r o x i d e to C o p p e r M o l a r R a t i o . T a b l e I V data show the effect of h y d r o g e n p e r o x i d e l e v e l o n m o l e c u l a r w e i g h t at a c o n s t a n t h y d r o g e n p e r o x i d e t o c o p p e r m o l a r r a t i o (60:1) u n d e r t h e s a m e r e a c t i o n c o n d i t i o n s as i n t h e T a b l e I I a n d I I I e x p e r i m e n t s . T h e s e T a b l e I V d a t a d e m o n s t r a t e the d e p e n d e n c e o f m o l e c u l a r w e i g h t o n i n i t i a t o r l e v e l at a c o n s t a n t H2O2 to c o p p e r j m o l a r r a t i o . F i g u r e 1, a p l o t o f M v s . 1 / [ H 0 ] , illustrates t h e T a b l e I V M data. Synthesis o f the lowest m o l e c u l a r w e i g h t products requires h i g h l e v e l s o f c o p p e r as w e l l as h y d r o g e n p e r o x i d e . O v e r t h e r a n g e s t u d i e d , the p r e s e n c e o f c o p p e r at e v e n 6000 p p m d o e s n o t p r e s e n t a s o l u b i l i t y o r storage s t a b i l i t y p r o b l e m . H o w e v e r , i f d e s i r a b l e , c o p p e r c a n b e e f f i n

2

2

M

n

Table III. Effect of Hydrogen Peroxide (5%) to Copper Salt Molar Ratios on Poly(acrylic acid) Synthesis molofH2C>2: Cu (ppm) mol of Cu 1500 600 300 150 50

60:1 150:1 300:1 600:1 1800:1

AA H2O2 Residue (%) Conversion (%) 0 8 26 27 52

100 99 98 99 100

Viscosity (cP) at 25 °C 185 320 750 780 3000

M

w

4700 6250 8150 8700 13300


M„ 2160 2820 3620 2820 5500

8.

149



T a b l e I V . V a r y i n g H y d r o g e n Peroxide Levels at 60;1 H y d r o g e n Peroxide to C o p p e r Salt M o l a r Ratios H 0 (wt%)

Cu (ppm)

1 5 10 20

300 1500 3000 6000


2

H 0 AA Residue (%) Conversion (%)

2

2

2

0 0 0 0

Viscosity (cP) at 25 "C 1000 185 110 40

100 100 100 100

M„ 9800 4700 3520 2450

4140 2160 1600 1120

ciently r e m o v e d p o s t p o l y m e r i z a t i o n b y using w e l l - k n o w n ion-exehange techniques. E f f e c t o f A m i n e L e v e l . A n o t h e r v a r i a b l e e x a m i n e d w a s the e f f e c t o f a m i n e i n c l u s i o n a n d a m i n e l e v e l o n these m e t a l - a c t i v a t e d r e d o x p r o cesses. D a t a f o r these e x p e r i m e n t s a r e p r o v i d e d i n T a b l e V . T h e T a b l e V e x p e r i m e n t s w e r e r u n at 95 ° C , the c o p p e r l e v e l w a s 600 p p m , the H 0 l e v e l w a s 5% o n m o n o m e r ( h y d r o g e n p e r o x i d e t o c o p p e r m o l a r r a t i o o f 150:1). T h e s e T a b l e V d a t a s h o w that i n c l u s i o n o f l o w l e v e l s o f a t e r t i a r y a m i n e , s u c h as t r i e t h a n o l a m i n e , tends t o l o w e r the m o l e c u l a r w e i g h t a n d i m p r o v e p e r o x i d e u t i l i z a t i o n . T h e r o l e o f the a m i n e i n these 2

2

1000

2000

1500

2500 _ GPC M

3000

3500

4000

n

Figure 1. A plot of 1/[H202]' is an illustration of the Table IV gel permeation chromatographic data. These data show the dependence of poly (acrylic acid) M on the hydrogen peroxide level at a constant hydrogen peroxide to copper salt molar ratio. 2

n


150

WATER-SOLUBLE POLYMERS

Table V . E f f e c t of A m i n e Inclusion on Poly(acrylic acid) Synthesis TEA (wt %) 0

H0 Residue (%)

AA Conversion (%)

Viscosity (cP) at 25 °C

M ,

M„

33 8 0

95 99 99

655 325 275

7000 6250 5600

3280 2820 2500

2

0 1 2

b

2


fTEA represents triethanolamine. AA is acrylic acid.

r e a c t i o n s is n o t c o m p l e t e l y u n d e r s t o o d at this t i m e ; h o w e v e r , its f u n c t i o n m a y b e that o f a c h e l a t e . E f f e c t o f T e m p e r a t u r e . T e m p e r a t u r e also is a n i m p o r t a n t f a c t o r i n controlling molecular weight, efficiency of peroxide utilization, a n d m o n o m e r conversion. T a b l e V I contains data for experiments done i n the 8 0 - 9 5 ° C r a n g e : h i g h e r t e m p e r a t u r e f a v o r s l o w e r m o l e c u l a r w e i g h t a n d m o r e e f f i c i e n t p e r o x i d e u t i l i z a t i o n . I n the m e c h a n i s m p r o p o s e d (see n e x t s e c t i o n ) , f o r this i n i t i a t i o n , the h y d r o g e n p e r o x i d e serves as b o t h the o x i d i z i n g a n d r e d u c i n g agent f o r the m e t a l ; p r e s u m a b l y , the C u — > C u transition requires higher temperatures. 2 +

+

P r o p o s e d M e c h a n i s m . T h e complex mechanism of initiation i n our p r o c e s s e s is n o t f u l l y u n d e r s t o o d . P e r o x i d e s , i n c l u d i n g h y d r o g e n p e r o x i d e , are c o m m o n l y used to initiate a d d i t i o n p o l y m e r i z a t i o n s of acrylate a n d v i n y l monomers. H y d r o g e n peroxide p r o b a b l y functions b y redox d e c o m p o s i t i o n to the h y d r o x y l free r a d i c a l (7). A m e c h a n i s m f o r the f o r m a t i o n o f the h y d r o x y l free r a d i c a l that initiates these p o l y m e r i z a tions is H 0 2

M*

+

2

+ M

+ H 0 2

+

2

—>

HO- + OH" + M

—>

H0 - + M

H 0 - + H2O2 — > 2

2

+

+ H

H 0 + HO- + 2

2

+

+

0 f 2

Table V I . E f f e c t of Temperature o n Poly(acrylic acid) Synthesis Synthesis Temp (°C)

H2O2 Residue (%)

AA Conversion (%)

Viscosity (cP) at 25 °C

95 90 80

8 10 44

99 98 90

320 500 1200

M

w

6250 7150 11400


M» 2820 3240 4730

8.


151


H O - is a n e f f e c t i v e i n i t i a t i n g s p e c i e s , a n d N M R a n a l y s i s o f these p o l y (acrylic acid) polymers verifies h y d r o x y l end groups. Also, monitoring o f the r e a c t i o n a t m o s p h e r e v e r i f i e s the p r e s e n c e o f o x y g e n .

Acknowledgments W e t h a n k the R o h m a n d H a a s C o m >any f o r a l l o w i n g us to p u b l i s h this w o r k . W e a c k n o w l e d g e the c o n t r i b . tions o f S t e v e n E d w a r d s a n d B e n j a m i n K i n e i n this w o r k a n d the a n a l y t i c a l s u p p o r t of A l W e i s s a n d B e t t y Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch008

Ginsberg

i n the

gel

permeation

chromatographic

analysis of

our

polymers.

Literature Cited 1. Song, D. S.; Duffy, R. J.; Witschonke, C. R.; Schiller, A. M . ; Higgins, M . A. U.S. Patent 4 001 161, 1973. 2. Muenster, A.; Rohmann, M . U.S. Patent 4 301 266, 1981. 3. Hughes, K. A.; Kine, B. B.; Swift, G. U.S. Patent 4 314 044, 1982. 4. Fenton, H . J. H . J. Chem. Soc. 1894, 65, 899. 5. Barb, W. G.; Baxendale, J. H . ; George, P.; Hargrave, K. R. Trans. Faraday Soc. 1951, 47, 462. 6. Handbook of Chemistry and Physics, 44th ed.; C R C Pr: Boca Raton, F L , p 1743. 7. Wallace, J. G. Hydrogen Peroxide in Organic Chemistry, E . I. d u Pont de Nemours: Wilmington, D E , 1962; p 107. R E C E I V E D for review September 28, 1984. A C C E P T E D August 14, 1985.


Water-Soluble Polymers - American Chemical Society

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