Initiation of Polymerization - American Chemical Society

6 Esterolytic Reactions of Active Esters Using Heterogeneous Polymeric Catalysts Containing Imidazole Groups Downloaded by NANYANG TECH UNIV LIB on April 28, 2015 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch006

C. G. OVERBERGER and BYONG-DO KWON The University of Michigan, Department of Chemistry and the Macromolecular Research Center, Ann Arbor, MI 48109 The preparation of polymeric catalysts and substrates containing imidazole groups and nitrophenyl esters, respectively, grafted onto crosslinked polystyrene beads has been described and the effects of the acyl chain length in the substrate in the aqueous alcohol solvent systems on the rate of hydrolysis of 3-nitro-4-acyloxybenzoic acid substrates catalyzed by insoluble catalysts were determined. Over the last decade we have studied extensively the esterolytic activity of polymers that contain pendant imidazole groups. (1-12) In the poly-4(5)-vinylimidazole catalyzed reaction, three major factors which contribute to large rate enhancements have been defined; cooperative effects(13,14), electrostatic effects (15,16), and hydrophobic effects.(17,18) The hydrophobic effect can be described i n a broad sense as a type of non-specific apolar bonding between catalyst and substrate. These hydrophobic interactions, especially i n an aqueous environment, have been predominant in determining the efficiency of the catalysts.(8,10,12) Toward this end, considerable effort on our part has been directed toward the preparation of polymeric catalysts that contain pendant imidazole groups and apolar bonding sites that are soluble in highly aqueous solvent systems. The efficiency of our recently investigated catalysts was enhanced by apolar bonding in an aqueous environment between substrate and the apolar polymer backbone of the catalysts,(8,12,13) a pendant apolar group associated with the catalyst,(10,18) or a long-chain N-acyl-imidazole intermediate that occurs during the solvolysis reaction.(17) Other workers have also used the hydrophobic effect in imidazole-catalyzed esterolytic reactions. Recently, Nango and Klotz (20,21) reported esterolytic catalysis by polyethylenimine derivatives that contain pendant imidazole groups, diethylamino groups, or functionalized triazine residues which increased catal y t i c effectiveness due to cooperative effects and increased 0097-615 6/ 8 3/0212-0065$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

66

INITIATION OF

POLYMERIZATION

Downloaded by NANYANG TECH UNIV LIB on April 28, 2015 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch006

apolar b i n d i n g . Mirejovsky(22) i n v e s t i g a t e d the optimal e n v i r o n ment f o r imidazole-bound polyethylenimine c a t a l y s t s that contained pendant apolar l a u r y l groups as binding s i t e s . In an e f f o r t to design a more e f f e c t i v e macromolecular c a t a l y s t , Shinkai and Kunitake(23) e l u c i d a t e d the c o r r e l a t i o n between binding c a p a c i t y and c a t a l y t i c a c t i v i t y i n phenylimidazole-containing copolymers. More r e c e n t l y , we turned our a t t e n t i o n toward i n s o l u b l e polymer c a t a l y s t s to provide a d d i t i o n a l i n s i g h t i n t o t h i s area. This a r t i c l e describes the synthesis of i n s o l u b l e polymeric c a t a l y s t s and substrates and a p r e l i m i n a r y study on t h e i r e s t e r o l y t i c r e a c t i v i t y towards v a r i o u s long-chain p - n i t r o p h e n y l e s t e r s . Experimental M a t e r i a l s . C r o s s l i n k e d polystyrene beads were purchased from Bio-Rad L a b o r a t o r i e s and were d r i e d under vacuo a t 40 C. M e r r i f i e l d r e s i n (chloromethyl group content 0.9 meq/g) was purchased from P i e r c e Chemical Co. A s e r i e s of 3-nitro-4-acyloxybenzoic a c i d s ( 7 ) were prepared by methods described i n the l i t e r a t u r e . The long-chain p - n i t r o p h e n y l e s t e r substrates were purchased from Sigma Chemical Co. p-Nitrophenyl acetate was a product of P i e r c e Chemical Co. and sublimed i n vacuo before being used. Polystyrene (average MW 22,000) was purchased from A l d r i c h Chemical Co. General Procedure. The elemental analyses were determined by Spang L a b o r a t o r i e s , Eagle Harbor, Michigan, or G a l b r a i t h Labora t o r i e s , Inc., K n o x v i l l e , Tennessee. The NMR s p e c t r a on low molecular weight compounds were recorded on a Varian T-60A spectrometer. NMR of polymers were recorded on a JEOL JNM-FX902 F o u r i e r Transform NMR Spectrometer equipped with a FAFT50 FG/BG d i s c u n i t , and WM-360 FT NMR spectrometer equipped with ASPEC 2000 computer system manufactured by Bruker Instruments, Inc. Rates of h y d r o l y s i s were measured on a V a r i a n Cary 219 UV spectrophotometer equipped with a Haake constant temperature bath. A s t i r r e d batch tank r e a c t o r was placed i n a water bath maintained a t 26.0 ± 0.1 C and the s o l u t i o n was transported to a micro flow UV c e l l l o c a t e d i n the spectrophotometer v i a s i l i c o n e rubber tubing using a p r e c i s i o n flow p e r i s t a l t i c pump (MHRE 22) manufactured by the New Brunswick S c i e n t i f i c Company. Preparation of Polymer C a t a l y s t s ( I - V I ) . C r o s s l i n k e d p o l y styrene beads with 1% divinylbenzene mesh s i z e 200-400 (SX1) purchased from Bio-Rad L a b o r a t o r i e s , Inc., were chloromethylated by the method of Pepper, P a i s l e y , and Young(24). The r e s i n contained 4.59 meq/g of chloromethyl groups according to the modified Volhard method.(25) The chloromethylated r e s i n was allowed to swell i n dioxane-ethanol and reacted with histamine; p y r i d i n e or t r i e t h y l a m i n e was used as an a c i d captor. The r e a c t i o n mixture

In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


6.

OVERBERGER AND KWON

67

Estewlytic Reactions of Active Esters

was s t i r r e d and kept at 80°C f o r a p e r i o d of 7 days. When the r e a c t i o n was completed, the r e s i n ( I ) was washed with dioxane:H2Û ( l : l / v : v ) dioxane:methanol ( l : l / v : v ) , dioxane, s u c c e s s i v e l y . The r i n g s u b s t i t u t i o n r a t i o was 44.4%, histamine and 17.0% remaining -CH2CI u n i t s . Polystyrene bead c r o s s l i n k e d with 2% d i v i n y l b e n z ene mesh s i z e 200-400 was chloromethylated followed by s u b s t i t u t i o n with histamine using the same method described above.(II) The aromatic r i n g s are s u b s t i t u t e d by histamine (41.7%) and 9.93% with -CH2CI groups. Parts of I and I I were t r e a t e d with potassium a c e t a t e i n dioxane f o r 48 hr at 120°C, then with d i l u t e d 2N NaOH;dioxane (l:2/v:v) at room temperature overnight to remove the remaining c h l o r i d e residue ( I I I , I V ) . M e r r i f i e l d r e s i n purchased from P i e r c e Chemical Co. which cont a i n s 0.9 meq/g of chloromethyl groups was t r e a t e d with histamine f r e e base i n the presence of t r i e t h y l a m i n e . ( V ) The r i n g s u b s t i t u t i o n r a t i o was 2.8% by histamine, 4.5% by chloromethyl group. Some M e r r i f i e l d r e s i n and potassium phthalate i n dry DMF was heated at 120 C overnight. When the r e a c t i o n was completed, the r e s i n was f i l t e r e d , washed and t r e a t e d with NH2NH2Ή2Ο followed by 5% NaOH to give an orange-colored resin· No c h l o r i n e was detected i n the r e s i n . The second polymer d e r i v a t i v e prepared f o r use as a p o l y meric c a t a l y s t i s of the type

XPS-C(CH ) -Νw 0

.

(VI)

One of these was synthesized by the F r i e d e l - C r a f t s a c y l a t i o n of ω-chloroacid c h l o r i d e ( 2 6 ) (0.1 mole) and A1C1 (0.15 mole) with Bio-beads SX1, SX2, SX12^ and SM2 (0.1 mole) i n 200 ml of n i t r o benzene f o r 2 hours at 5 C. The r e s i n was washed with a c e t i c a c i d , 6N HClrdioxane (1:1), H 0-dioxane (1:1), dioxane, CH C1 and MeOH. The r e s i n was d r i e d under vacuo at 45°C and t r e a t e d with the sodium d e r i v a t i v e of imidazole i n DMF. S t r u c t u r a l c h a r a c t e r i z a t i o n of these c a t a l y s t s i s under i n v e s t i g a t i o n using i n f r a r e d , elemental a n a l y s i s and NMR s p e c t r o s copy using equipment s t a t e d e a r l i e r . A summary of the polymeric c a t a l y s t s i s i n Table I. 3

2

2

2

Preparation of Polymeric Substrates. C r o s s l i n k e d polystyrene c o n t a i n i n g carboxyl groups was prepared by the L e t s i n g e r method. (27) Diphenylcarbamyl c h l o r i d e (18.5 g, 80 mmole) i n 40 ml n i t r o benzene was added over a p e r i o d of 15 min to a w e l l - s t i r r e d mix ture of 8.3 g (80 mmole) Bio-Bead SX2 and 15.0 g (0.113 mole) of aluminum c h l o r i d e i n 350 ml of dry nitrobenzene. The dark mixture was then warmed at 80 C f o r 2.5 hr, cooled and t r e a t e d with 200 ml of water. The r e s i n was separated and washed i n succession with d i l u t e h y d r o c h l o r i c a c i d , methanol, and ether. For h y d r o l y s i s , the carboxamido polymer was heated at 103-138 C f o r 32 hr with a



C XN η

Β XN η

VI

IV

III

II

Catalyst

0

Bio-Beads SX1, SX2 SX12, SM

Merrifield Resin (Pierce)

Bio-Beads SX2 (Bio Rad)

Bio-Beads SX1 (Bio Rad)

Backbone

0

XPS-C(CH,

?

xps-jj(CH tri

2

N ^ H H

SXl-CH -histamine

2

Site

10.6%

4.5%

2.8%

9.95%

44.7%

9.93%

3

4

3

mole/g

mole/g

mole/g

mole/g

mole/g

bead

9.76xl0~

2.518x10

3.01xl0"

2.7x10

2.7x10

Catalyst Concentration

XPS = c r o s s l i n k e d polystyrene

Histamine

2

-CH C1

Histamine

2

-CH OH

Histamine

2

-CH C1

Histamine

44.7%

17.0%

-CH OH 2

44.2%

17.0%

44.4%

Histamine

2

-CH C1

Histamine

Degree of Substitution

Catalysts

SXl-^Q^--CH -histamine

Active

Table I.


δ 2

H

r

Ο

δ ο

Η > Η

6.

OVERBERGER AND KWON

Estewlytic Reactions of Active Esters

69

mixture of 335 ml of a c e t i c a c i d , 250 ml of s u l f u r i c a c i d , and 150 ml of water. F i l t r a t i o n and washing with water and ether a f f o r d e d polymer-containing c a r b o x y l groups as a pale greenish m a t e r i a l showing the c h a r a c t e r i s t i c carboxyl a b s o r p t i o n i n the i n f r a r e d at 1730 cm and 1680 cm" . T i t r a t i o n was e f f e c t e d by suspending 0.500 g of polymer i n 25 ml of 95% ethanol, adding 25 ml 0.1N sodium hydroxide, warming the mixture to r e f l u x , c o o l i n g , and back t i t r a t i n g with h y d r o c h l o r i c a c i d : 1.4 g mequivalent of carboxyl per gram of polymer was found. Warming a p o r t i o n of carboxyl polymer with excess t h i o n y l c h l o r i d e i n benzene f o r 5 hr a f f o r d e d the a c i d c h l o r i d e and Downloaded by NANYANG TECH UNIV LIB on April 28, 2015 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch006

1

1

on r e a c t i o n with p - n i t r o p h e n y l and p y r i d i n e . ^ A strong band at 1530 cm and a weak C-N s t r e t c h i n g (1345 cm ) i n d i c a t e s the nitrobenzene moiety c l e a r l y . The second polymeric s u b s t r a t e with a longer chain l e n g t h was prepared by the F r i e d e l - C r a f t s a l k y l a t i o n of Bio-Bead SX1 and A1C1 with 4 - c h l o r o b u t y r i c a c i d i n nitrobenzene f o r 5 hours at 80°C. The r e s i n was washed with AcOH, 6N HCl-dioxane (1:1), H2O:dioxane (1:1), dioxane and CH2CI2. T i t r a t i o n was e f f e c t e d as s t a t e d e a r l i e r ; 0.9 mequiv of carboxyl per gram of polymer was determined. 1

1

3

K i n e t i c Measurement. A p r e l i m i n a r y k i n e t i c study has been c a r r i e d out as f o l l o w s : Solvent and c a t a l y s t were added to a 50 ml 3-neck round-bottom f l a s k equipped with mechanical s t i r r e r , and the f l a s k was placed i n a constant temperature bath a t 26 C ± 0.02. Time was recorded as s u b s t r a t e was added i n the f l a s k with s t i r r i n g . An a l i q u o t of r e a c t i o n mixture was taken out using a s y r i n g e equipped with a m i l l i p o r e f i l t e r and the absorp t i o n was measured. A t y p i c a l example of r e a c t i o n was the f o l l o w i n g : (catalyst) = 5.185 χ 15 M (substrate) = 3.085 χ 10 M pH = 9.1 T r i s 0.02M μ = 0.02 Τ = 26 C r a t i o = [ c a t a l y s t ] / [ s u b s t r a t e ] = 16.8. Absorbance versus time curves were obtained. A blank on each k i n e t i c run was prepared and the blank value was subtracted from the c a t a l y t i c v a l u e . A l l data obtained under c o n d i t i o n s of [ c a t a l y s t ] »[substrate] were, unless otherwise noted, t r e a t e d as pseudo f i r s t - o r d e r k i n e t i c s by p l o t t i n g £n(A -A ) versus time. &max used i n s t e a d of A^. The slope of the s t r a i g h t l i n e was taken as k-obs d i n the case of a c c e l e r a t i v e behavior the slope at 75% of the maximum absorption was taken; k t was c a l c u l a t e d from k b s cat = k /[catalyst]. Recently a continuous flow type measurement was set up as s t a t e d e a r l i e r i n the M a t e r i a l s S e c t i o n . 3

-lf

w

oo

a

t

a n

:

c a

k

o b s


Q

s

70

INITIATION OF POLYMERIZATION


Results and

Discussion

K i n e t i c Studies. The previous work of Overberger(1,7) pro vided a r a t i o n a l e to t h i s r e s e a r c h . These i n v e s t i g a t o r s u t i l i z e d the hydrophobic backbone of p o l y - 4 ( 5 ) - v i n y l i m i d a z o l e i n conjunc t i o n with hydrophobic substrates (Sn ) to demonstrate enhanced rates of e s t e r o l y s i s . In p a r t i c u l a r , a buildup of long-chain a c y l a t e d imidazole r e s i d u e s , which increased h y d r o l y s i s r a t e s d r a m a t i c a l l y , i n s p i r e d the concept of using hydrophobic polymer s i d e chains. In t h i s study, i n s o l u b l e polymers c o n t a i n i n g imidazole groups were t e s t e d f o r e s t e r o l y t i c a c t i v i t y with s e v e r a l sub s t r a t e s of d i f f e r i n g hydrophobic chain l e n g t h s . The substrates used were 3-nitro-4-acyloxybenzoic a c i d (Sn ). The c o n c e n t r a t i o n of substrate used does not allow the formation of substrate m i c e l l e s , the concentration being below the c r i t i c a l m i c e l l e con c e n t r a t i o n f o r these e s t e r s (28,29). A t e n - to twenty-fold c o n c e n t r a t i o n excess of polymeric imid a z o l e residues over that of substrate molecules was u s u a l l y employed. This allowed a pseudo f i r s t - o r d e r p r e s e n t a t i o n of the k i n e t i c dataIn many cases, curvature i n the p l o t s of l n ( A - A ) versus time was found. Observation of complex k i n e t i c s i n h y d r o l y s i s of f u n c t i o n a l groups on polymer chains i s not uncommon. (30,31) L e t s i n g e r and Klaus(32) have observed some phenomena i n the study of s y n t h e t i c polymeric c a t a l y s t s and s u b s t r a t e s . In t r e a t i n g the data, they used the e m p i r i c a l r e l a t i o n m a x

Α-A /A -A = k't ο

0 0

A , A^, and A represented the absorbancies at i n i t i a l time, at completion of the r e a c t i o n , and at the time of measurement, r e s p e c t i v e l y » k was considered to be a pseudo f i r s t - o r d e r r a t e constant defined by the expression -dc/dt = k C ( C / C ) , where C/C represents the r e l a t i v e r e a c t i v i t y of the e s t e r groups on a polymer chain as a f u n c t i o n of extent of r e a c t i o n . A reasonably good f i t to t h i s was obtained f o r r e a c t i o n s conducted i n the absence of c a t a l y s t s as w e l l as i n those c a t a l y z e d by N-methylimidazole and p o l y - ( N - v i n y l i m i d a z o l e ) . An e m p i r i c a l equation was a p p l i e d to the data to o b t a i n non l i n e a r p l o t s . Therefore, i n the case of a c c e l e r a t i v e behavior, the slope at 75% of the maximum absorption was taken. A summary of the second-order r a t e constant are provided i n Table I I . f

f

0

Q

1 3

NMR Studies. Previous C NMR i n v e s t i g a t i o n of some polymers at temperatures w e l l above the Tg and i n solvent-swollen g e l s have shown that high r e s o l u t i o n s p e c t r a may be obtained i n good s o l vents without use of c r o s s - p o l a r i z a t i o n , magic angle spinning, or d i p o l a r decoupling.(33-35) The only published C NMR spectra of c r o s s l i n k e d polystyrenes show high r e s o l u t i o n i n s w e l l i n g solvents and the technique has been used to estimate the degree 1 3


6.

OVERBERGER AND KWON

Table I I .

Esterolytic

Reactions

Apparent Second-Order Rate Constants f o r the H y d r o l y s i s of S n

k

< cat>

-

k


Catalysts

S

_ cat S

2~

71

of Active Esters

. m" 1min

-1

S

5~

12~

I

28.92

19.37

24.75

II

15.80

10.73

23.05

III

27.03

IV

12.56

V

18.4

Solvent EtOH = H 0 = 50:50 2

μ = 0.02

pH = 9.1 T r i s

Table I I I .

Comparison of Nuclear Overhauser E f f e c t s

With NOE

r

No NOE

Linear polystyrene

0.381

0.289

1.32

Funetionalized ^ Linear Polystyrene

0.589

0.493

2.24

0.788

0.621

1.27

Catalyst

a:

A

C

Average MW 22,000; 5000 scans. Π

CH2C1; 4000 scans. 0

b:

L-PS-C-CH

2

r=\

C H ^ H ^ N y ^ N : 10,000 scans. c:

X-PS J ] - C H

2

In order to get a n a l y t i c a l and s t r u c t u r a l i n f o r m a t i o n from c r o s s l i n k e d polystyrenes by solvent s w e l l i n g , extensive H - and C NMR study i s under i n v e s t i g a t i o n . 1


1 3

72


of f u n c t i o n a l i z a t i o n of r e s i n s f o r s o l i d phase peptide s y n t h e s i s (36) by Horowitz, Horowitz, and P i n n e l l . They reported NMR a n a l y t i c a l r e s u l t s on s o l v e n t - s w o l l e n , c r o s s l i n k e d polystyrenes which demonstrated a new approach f o r o b t a i n i n g a n a l y t i c a l and s t r u c t u r a l i n f o r m a t i o n on f u n c t i o n a l ! z e d polystyrenes that a r e important as the backbone f o r many s o l i d - s t a t e syntheses and reagents. They obtained the extent o f -CH C1 and -CH 0H i n chloromethylated p o l y s t y r e n e r e s i n using proton n o i s e decoupled spectra. In p r i n c i p l e , NMR s p e c t r a with a Nuclear Overhauser E f f e c t (NOE) cannot be used q u a n t i t a t i v e l y , because the Nuclear Overhauser E f f e c t cannot be the same f o r a l l pendant groups. Therefore, a p r e l i m i n a r y NMR study of the Nuclear Overhauser E f f e c t on l i n e a r polymers and c r o s s l i n k e d polymer was made. The NMR s p e c t r a were taken on r e s i n s l u r r i e s i n deuterochloroform. The s l u r r i e s were prepared by i n t r o d u c i n g a q u a n t i t y o f r e s i n i n t o an NMR tube; a p o r t i o n of solvent was p i p e t t e d i n and the r e s i n was allowed to s w e l l while the tube was a g i t a t e d . A d d i t i o n a l solvent was added with f u r t h e r a g i t a t i o n (sometimes an u l t r a s o n i c device helped the a g i t a t i o n ) as the s w e l l i n g continued so that the s l u r r y remained mobile. A v o r t e x plug was used to c o n t a i n the swollen r e s i n . Complete proton n o i s e decoupled spectrum were obtained under the c o n d i t i o n s : 45 pulse a t 0.9 sec r e p e t i t i o n ; CDCL3 solvent; i n t e r n a l deuterium l o c k . Gated decoupling spectra without Nuclear Overhauser E f f e c t were obtained under the c o n d i t i o n s : 90° pulse a t 4 sec r e p e t i t i o n ; CDCL3 s o l vent i n t e r n a l deuterium lock. The r a t i o of height of peak i n the aromatic r e g i o n was compared to that of peak a t a l i p h a t i c r e g i o n (Table I I I ) .


2

2

Acknowledgments The authors a r e g r a t e f u l f o r f i n a n c i a l support from the Department of Chemistry and the Macromolecular Research Center, The U n i v e r s i t y of Michigan, Ann Arbor, Michigan 48109 USA.

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Overberger, C. G; Salamone, J . C. Acc. Chem. Res., 1969, 2, 217. Overberger, C. G.; Morimoto, M.; Cho, I.; Salamone, J . C. J . Am. Chem. Soc., 1971, 93, 3228. Overberger, C. G.; Morimoto, M. J . Am. Chem. Soc., 1971, 93, 3222. Overberger, C. G.; Okamoto, Y. Macromolecules, 1972, 5, 363. Overberger, C. G.; Okamoto, Y. J . Polym. S c i . , Polym. Chem. Ed., 1972, 10, 3387. Overberger, C. G.; Glowaky, R. C. J . Am. Chem. Soc., 1973, 95, 6014. Overberger, C. G.; Glowaky, R. C.; Vandewyer, P.-H. J . Am. Chem. Soc., 1973, 95, 6008.


6. OVERBERGER AND KWON 8. 9. 10. 11. 12.


13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

Esterolytic Reactions of Active Esters

73

Overberger, C. G.; Smith, T. W. Macromolecules, 1975, 8, 401, 407, 416. Overberger, C. G.; Kawakami, Y. J . Polym. Sci., Polym. Chem. Ed., 1978, 16, 1237, 1248. Overberger, C. G.; Guterl, A. C., Jr. J . Polym. Sci., Polym. Symp., 1978, 62, 13. Overberger, C. G.; Smith, T. W.; Dixon, K. W. J . Polym. Sci. Polym. Symp., 1975, 50, 1. Overberger, C. G.; Dixon, K. W. J . Polym. Sci., Polym. Chem. Ed., 1977, 15, 1863. Overberger, C. G.; Salamone, J . C.; Cho, I.; Maki, H. Ann. N.Y. Acad. Sci., 1969, 155, 431. Overberger, C. G.; St. Pierre, T.; Vorchheimer, N.; Lee, J.; Yaroslavsky, S. J . Am. Chem. Soc., 1965, 87, 296. Overberger, C. G.; Corett, R.; Salamone, J . C.; Yaroslavsky, S. Macromolecules, 1968, 1, 331. Overberger, C. G.; Salamone, J . C.; Yaroslavsky, S. J . Am. Chem. Soc., 1967, 89, 6231. Overberger, C. G.; Glowaky, R. C.; Vandewyer. P.-H. J . Am. Chem. Soc., 1973, 95, 6008. Pacansky, T. J . Ph.D. Thesis, The University of Michigan, Ann Arbor, Michigan, 1972. Overberger, C. G.; Mitra, S. Pure Appl. Chem., 1979, 51, 1391. Nango, M.; Klotz, I. M. J . Polym. Sci., Polym. Chem. Ed., 1978, 16, 1265. Nango, M.; Gamson, E. P.; Klotz, I. M. J . Polym. Sci., Polym. Chem. Ed., 1979, 17, 1557. Mirejovsky, D. J . Org. Chem., 1979, 44, 4881. Shinkai, S.; Kunitake, T. J.Am. Chem. Soc., 1971, 93, 4256. Pepper, K. W.; Paisley, H. M.; Young, M. A. J . Chem. Soc., 1953, 4097. Stewart, J . M.; Young, J . D. "Solid Phase Peptide Synthesis," W. H. Freeman, San Francisco, 1969, p. 54. Tilak, M. A.; Hollinden, C. S. Tetrahedron Lett., 1968, 1297. Letsinger, R. L . ; Kornet, M. J.; Mahedevon; Jerina, D. M. J. Am. Chem. Soc., 1964, 86, 5163. Glowaky, R. C. Ph.D. Thesis, The University of Michigan, 1972. Smith, T. W. Ph.D. Thesis, The University of Michigan, 1973. Moens, J.; Smets, G. J . Polym. Sci., 1957, 23, 931. Gaetjens, E . ; Morawetz, H. J . Am. Chem. Soc., 1961, 83, 1738. Letsinger, R. L . ; Klaus, I. J . Am. Chem. Soc., 1964, 86, 3884. Yokota, K.; Abe, A.; Hosaka, S.; Sakai, I.; Saito, H. Macromolecules, 1978, 11, 95. Schaefer, J . Macromolecules, 1971, 4, 110.


74 35. 36.


Torchia, D. A.; VanderHart, D. L. "Topics in Carbon-13 NMR Spectroscopy," Vol. 3, Levy, G. C., Ed., John Wiley and Sons, Inc., New York, N.Y., 1979, pp. 325-360. Horowitz, D.; Horowitz, R.; Pinnell, R. P. Anal. Chem., 1980, 52, 1529.


RECEIVED August 24, 1982


Initiation of Polymerization - American Chemical Society

Recommend Documents