Esterification reactions on syndiotactic poly(methallyl alcohol)—III. Esterification with cyclic carboxylic acid anhydrides

Eur. Polym. J. Vol. 22, No. 9, pp. 745-753, 1986 Printed in Great Britain

0014-3057/86 $3.00+0.00 Pergamon Journals Ltd

ESTERIFICATION REACTIONS ON SYNDIOTACTIC POLY(METHALLYL ALCOHOL) III ESTERIFICATION WITH CYCLIC CARBOXYLIC ACID ANHYDRIDES* W. FREY, B. DEDERICHS and E. KLESPER Lehrstuhl fiir Makromolekulare Chemic der Technischen Hochschule Aachen, Worringer Weg 1, 5100 Aachen, BRD (Received 4 February

1986)

Abstract--The esterification of syndiotactic poly(2-methallyl alcohol) by aromatic and aliphatic cyclic acid anhydrides, including N~-protected (L)-aminodicarboxylic acid anhydrides is reported. Conditions are chosen such that defined partial or complete conversions are obtained. The structures of the products, including the degree of conversion, are determined by ~H- and *3C-NMR. By potentiometric titration, the solubility in water of some of the polymers and their dissociative and conformational behaviour are studied.

INTRODUC'I'ION Cyclic carboxylic acid anhydrides possess advantages for on-polymer reactions when compared to their parent dicarboxylic acids. First, it is not necessary for the preparation of, for instance, a monoester to block one of the carboxyl groups; second, no condensation agent or catalyst, e.g. carbodi-imides or protons are necessary; third, a free carboxyl group is available immediately for a further on-polymer reaction. A disadvantage can be the 2 possibilities for opening the anhydride ring. Depending on the carboxyl group involved in the cleavage of the ring, 2 different products are possible for structurally asymmetric cyclic anhydrides. Previously, we reacted syndiotactic poly(2methallyl alcohol) (s-PMA) with phthalic anhydride [1, 2] to obtain poly(2-methallyl hydrogen phthalate). This communication describes the reaction of s-PMA with the aromatic cyclic anhydrides, 2,3-pyrazinedicarboxylic acid anhydride (PDCA), pyromellitic acid anhydride (PMAA) and diphenic acid anhydride (DPAA), with the aliphatic cyclic anhydrides, succinic acid anhydride and glutaric acid anhydride, and with the N~-protected (L)-amino dicarboxylic acid anhydrides, aspartic acid anhydride and glutamic acid anhydride. The latter anhydrides are protected on their ~t-NH2 by either the t-butyloxycarbonyl (Boc) or the benzyloxycarbonyl (Z) group. The overall reaction sequence is depicted in the reaction scheme. Syndiotactic poly(methyl methacrylate) is first reduced to s-PMA, which in turn is esterified with a cyclic anhydride. The esterification of the s-PMA can be carried out partially, i.e. to a copolymer, or completely, i.e. to a homopolymer. The polymer structure, including the extent of

*Dedicated to Professor Dr G. Manecke on the occasion of his 70th birthday.

esterification, has been characterized by ~H- and 13C-NMR. The polymers have also been studied by potentiometry for their solubility and conformational behaviour in aqueous solution. EXPERIMENTAL

Syndiotactic poly(methyl methacrylate) [3, 4] and syndiotactic poly(2-methaUyl alcohol) (s-PMA) [5, 6] have been prepared as previously described, both containing 92-93% syndiotactic triads. For the complete esterification of sPMA by the aromatic cyclic acid anhydrides, PCDA, PMAA, and DPAA, 300mg (4.17 retool) s-PMA are dissolved in 40 ml (60 ml for PMAA) pyridine. The anhydride (5mmol) is added and kept at 50°C for 168hr. After cooling, the homopolymer is precipitated in 300 ml diethylether, redissolved in pyridine and again precipitated in diethylether. After thorough washing with ether, it is dried at 50°C in vacuo. With reaction times considerably shorter than 168 hr, partial conversion is found, even if double the stoichiometric amount of anhydride is used. Several charges prepared according to this general procedure, for which time or stoichiometry has been varied, are given in Table 1. The complete conversion of s-PMA by the aliphatic cyclic acid anhydrides, succinic and glutaric acid anhydride, is carried out by dissolving 200 nag (2.78 mmol) s-PMA in 25 ml pyridine, adding 15 mmol of the anhydride and reacting at 50°C for 144 hr. After cooling, the polymer is precipitated in ether, reprecipitated by this pyridine--ether combination, washed thoroughly, and dried at 50°C /n vacuo. For complete esterification of s-PMA by the N~-protected aspartic acid anhydride, 3 mmol of the anhydride is added to 200 mg (2.78 retool) s-PMA dissolved in 25 ml pyridine, and the solution kept at 50°C for 168 hr. After cooling, the polymer is precipitated in 300 ml diethylether, reprecipitated by pyridind--ether, washed repeatedly, and dried at 50°C/n oacuo. The complete esterification with N~-protected giutamic acid anhydride is carried out by the same procedure, except for using 40 ml of pyridine. With smaller volumes of pyridine, erosslinking of the polymer has been observed. The partial conversion of s-PMA by the N~-protected aspartie acid anhydrides is carded out with 500 mg (7 retool) s-PMA in 50 ml pyridine, adding between 5 to 100% of the stoichiometrically required amount of

745

W. FREY et al.

746

,o,

Q\o /C

/ ,c,

0 CH2"cC

= L

)

=

r

CH31

Jn

CH2

CN z ¢ OH

I O'CH 3

I

0! / C=O

©

\COON

o g o

0

~CX~-

,

I R-NH-CH ICH~

R-NH-CHi

,

cx~ ,o R

,,

i

CH~-C-O-CI CHs

CH2 XCXr

0

>'CNI0-8@

Scheme 1

anhydride (based on complete conversion of s-PMA). Further conditions for a number of runs are listed in Table 2. Partial esterification with N~-protected glutamic acid anhydrides is carried out similarly, but using a higher dilution, i.e. 100 ml pyridine as a solvent. Moreover, the amount of the anhydride was always kept at a level sufficient for complete esterification, the partial esterification being obtained by shorter reaction times. A set of runs is presented in Table 3. The aromatic acid anhydrides and the N~-protected amino acid anhydrides are obtained according to known procedures: 2,3-Pyrazinedicarboxylic acid anhydride (PDCA) F p f 4 4 3 K [7], pyromeUitic acid anhydride (PMAA) Fp = 559 K [8], diphenic acid anhydride (DPAA) Fp-- 490 K [9], N~(t-butyloxycarbonyl)-(L)aspartic acid anydride (Boc-Asp-O) Fp = 406--407 K [10], N~(benzyloxy carbonyl)-(L)aspartic acid anhydride (Z-Asp-O) Fp = 384 K [11], N~(t-butyloxycarbonyl-(L)glutamic acid anhydride (Boc-Glu-O) Fpf388-389K [12], and N~(benzyloxy carbonyl)-(L) glutamic acid anhydride (Z-GIu-O) Fp=

364-365 K [13]. Other chemicals were obtained from commercial sources; solvents were rigorously purified. 1H-NMR spectra were recorded on a Bruker CXP-200 FT-NMR spectrometer at 200 MHz, pulse angle 90 °, delay time sufficient to prevent saturation, solvent DMSO-d6 temperature 100°C, polymer concentration 5%, internal standard TMS, measurement of peak areas by planimeter. The error in determination of P(A) is approx 5%. The ~3C-NMR broadband ~H-decoupled spectra were obtained at 50 MHz on the same instrument. The recording conditions were also similar, except for permitting partial saturation, using polymer concentrations of 10-20% and 10,000-20,000 pulses for an acceptable signal-to-noise ratio. Infrared spectra were recorded on a Perkin-Elmer 283 B spectrometer using the KBr press technique. Potentiometry utilized a Radiometer Copenhagen PMH pH-meter in combination with a TTT-60 Titrator, an Abu-80 autoburette, a glass electrode and a calomel reference electrode. The polymer concentration was 30mg/10ml, the speed of titration 0.125 ml/min using 0.I N reagents.

Esterification reactions on syndiotactic poly(methallyl alcohol)--III Table 1. Reaction conditions for the esterification of s-PMA by aromatic, cyclic acid anhydrides, PDCA, PMAA, DPAA Conversion Molar ratio Time Temperature of s-PMA No. s-PMA PDCA (hr) (K) (%) 1 I 2 120 323 94 2 1 2 168 323 100 3 1 1 120 323 95 4 1 1 168 323 I00 Molar ratio s-PMA PMAA 1 2 3 4

1 2 3 4

1 2 1 2 1 1 1 1 Molar ratio s-PMA DPAA 1 1 1 1

2 2 1 1

120 168 120 168

323 323 323 323

99 100 93 100

120 168 120 168

323 323 323 323

80 100 94 100

nESUCTS AND mSCUSSION The aromatic dicarboxylic acid anhydrides P C D A , P M A A , and D P A A are employed for complete esterification of s-PMA, obtaining the h o m o polymers poly((2,3-pyrazine dicarboxylic acid)-2methallylmonoester) (PCDA-s-PMA), poly((benzene1,2-dicarboxylic acid anhydride-4, 5-dicarboxylic acid)-2-methallylmonoester) OaMAA-s-PMA), and Table 2. Reaction conditions for the esterificationof s-PMA by the N'-protected (L)-aspartic acid anhydrides Boc-Asp-Oand Z-Asp-O Conversion Boc-Asp-O Time Temperature of s-PMA No. (tool %) (rag) (hr) (K) (%) I 25 378 48 353 24 2 50 753 48 353 49 3 75 1128 48 353 75 4 100 1506 48 353 99 5 25 378 144 323 25 6 50 753 144 323 49 7 75 1128 144 323 73 8 100 1506 144 323 97 Z-Asp-O (mol %) (mg) 1 2 3 4 5 6 7 8

20 50 80 100 25 50 75 100

345.8 864.6 1383.3 1729.2 432.3 864.6 1128.0 1729.2

24 24 24 24 168 168 168 168

323 323 323 323 323 323 323 323

7 34 46 72 25 50 75 100

747

poly((diphenic acid)-2-methallylmonoester) ( D P A A s-PMA). The conversion of the s-PMA was determined by the peak areas of the 1H-NMR spectra. In Fig. la the spectra are shown for the 3 homopolymers, together with their assignments. F o r the homo- and copolymers derived from P C D A equation (1) has been used for the determination

P(A) =

F4+F5

7(&+ Fs)

F3

2(F, + F2 + F3)

(1)

where P(A) is the probability of finding an esterified methallyl alcohol m o n o m e r unit (mol %/100) and the F are the areas o f peaks identifiable by their subscripts (Fig. la). F o r polymers derived from P M A A , equation 1 can be used also, while for DPAA-derived homo- and copolymers equation 2 has been employed

P(A) =

7(F4 + F5 + F6 + F7) 8(V, + F2 + F3)

(2)

Apart from equations 1 and 2, the complete esterification of the polymers of Fig. 1 is already visually apparent, because their ---CHz--O--~H-resonance is found about 0.7 ppm at lower field than for unreacted s-PMA (e.g. 4.18 cf. 3.3 ppm). The same conclusion can be derived from the ' 3 C - N M R spectra of the homopolymers in Fig. lb. The -----CH2-----O-- '3C-resonance is located by about 2 - 4 p p m to lower field (e.g. 73.55cf. 70.00ppm). Assignment of the individual resonances in Fig. I b has been carried out by means of the educt spectra and known chemical shift data [14]. The i.r. spectra show the expected ester band o(C-----O) at 1730-1740cm -~ and, in the case of the P M A A - d e r i v e d polymers, also the anhydride band 0(C-------O) at 1775 cm -I. The homo- and copolymers are soluble in D M S O , pyridine, and D M F ; their solubility in water is significant. F o r instance, the P C D A - s - P M A is soluble over the entire p H range down to p H = 3 and the D P A A - s - P M A is soluble down to p H = 7. The reaction of s-PMA with the cycfic acid anhydrides is slow; even in pyridine (Tables l-3). Pyridine possesses a catalytic effect, based on the formation o f an acyl pyridinium cation, which in turn reacts with the O H group of s-PMA. F o r the N*-protected cyclic amino acid anhydrides, 2 different modes of ring opening may occur, depending on which carbonyl, ~ or o~, is reacting with the alcohol. It has been shown earlier that for the N'-protected aspartic acid anhydrides only the ~t-carbonyl is attached to the polymeric alcohol [15]. The complete esterification of the s-PMA by the N'-protected amino acid anhydrides yields the homopolymers poly(2-methallyl-N'(t-butyloxy-

Table 3. Reaction conditions for the esterification of s-PMA by the N~-protected (L) glutamic acid anhydrides Boc-Glu-O and Z-GIu-O Conversion Molar ratio Time Temperature of s-PMA No. s - P M A Boc-Glu-O* (hr) (K) (%) 1 24 296 2 1 36 296 9 1 48 296 I1 2 72 296 19 2 120 323 94 2 144 323 97 2 168 323 100 *Or Z-GIu-O.

W. FREYet al.

748

carbonyl)-(L)-hydrogen-g-aspartate) (Boc-Asp(OH)s-PMA), poly(2-methallyl-N~(benzyloxy-carbonyl)(L)-hydrogen-a -aspartate) (Z-Asp(OH)-s-PMA), poly(2 - methallyl) - N~(t - butyloxycarbonyl) - (L) hydrogen-a-glutamate) (Boc-Glu(OH)-s-PMA), and poly(2-methallyl)-N~(benzyloxycarbonyl)- (L)-hydro gen-a-glutamate) (Z-GIu(OH)-s-PMA). The ]HN M R spectra of the homopolymers are shown in Fig. 2a. The conversion was obtained for BocAsp(OH)-s-PMA by

P(A) =

5F6 9(F, + F~)

For Z-Asp(OH)-s-PMA by

P(A) =

0

(4)

5F6 = FT+Fs 9(F1 + F2) F2+F3 And for Z-Glu(OH)-s-PMA by P(A)

(5)

= -

F, _&+F9

P(X) =

H~ : H3 H H 4*H

PDCA-s-PHA

_F6+F s

For Boc-GIu(OH)-s-PMA by

(3)

-

F,

F,+& F2+F3

F,+F2

(6)

F2+F 3

1.10 ires 1.65 Nil 4.11 Ires 5 : 8.60 R m 4 . 6 7 INto

~-o Y(~"I' /~N ~ s

!

COOH

ms

4*6

3

÷

1.01 I ~

PNAA*s-PMA 0 ' C:04

H

Hs H

0

1.37 4.13 7.1R 7.1~

INto plm fill plm

/L,,~.c/0

!

COON 0 3

4*S

A -fc.|c[ q.L |Jn

H~ tt 3 s H 4-H

DPAA-S-PMA

,cH2 0t

0.85 Mm 1.37 INto 3.10 Rm 7 : 6 . ~ IkU1-7.81 I)llll

C:04 6

COOH

I

4-7 3

A_ *

|

|

9

8

7

Fig. l(a)

G

5

4

3

2

Esterification reactions on syndiotactic poly(methallyl alcohol)--III

.~

6-|

'CtH 3

c'.,,c':4,C'H/"

749

om4,

PDCA o S - PHA

c;

14.11i I.i.36 41.23 73.SS iM.~J 165.56

C

0

C

~S:O

c ,s C

I m C 6 o C 7J C m Dim m Im m

$

C10OOH lO,S

L_

,

m

1H

r ;e ~.,31 t C H f C 4 "--']'n

PHAAos-PHA

C I : 24.88 ppl

c H2

C C 4s C

0

,

C : 166.45 Ippm ?,12 C 1 166.76 pp~ C I. C 8, C I1. C 13.. 135 ppm

oJ ,.1 °

IT

',

U 11,13

s

[.

47.73 Pl~ 41.81 pl~m 73.30 plm

2

!

_ _ --

1

10 = 162

|

4

/ I

L

II

__1 ~. -__ __

__JL I

I

~L

C1H3. cZHfc 3 ,~ 'H2 ' n

DPAA-s-PHA

c

I

II, C II 24.M p ~ ¢ i , ¢ 7 ¢ | 171 li' ,e.,~. c'S c 't~ m 4o.*s It" "llS

..

0

= c ':c

16S.11 ppm 13

142.Z$ ppm 142.H ppm 167,$7 ppm

11L"--'Ja

"oo.

i

175

l

I

i

l

i i

i

i

150

125

100

75

50

25

j t

0

Fig. 1. (a) ~H-NMR spectra of the homopolymers PDCA-s-PMA, PMMA-s-PMA, and DPAA-s-PMA, together with listing of chemical shifts and assignment. Co) L3C-NMR of the same homopolymers. The first parts of equations 3-6 rely on the resonances of the amino acid proper, but the second parts of equations 4-5 on its N * protecting groups. An unintentional loss of the protecting group should, therefore, be noticeable. The ~3C-NMR spectra o f the same 4 homopolymers are given in Fig. 2b, together with the expected structure, With the N~-protected amino acid anhydrides, a series of copolymers, distributed over the range o f P(A), have also been obtained. The IH- and

Table 4. The pH ranges of the water solubility of the copolymers [Z-Asp(OH)-MA]-s-co-(MA)between pH 12 to 2, as determined by titration Titration by HCI Titration by KOH Soluble Insoluble Insoluble Soluble P(A) 12.0-6.6 Further Further 8.2-12.0 0.25 0.40 12.0-6.3 down to down to 7.8-12.0 o.5o 12.0-6.0 pH = 2 pH = 2 7.7-12.0 0.60 12.0-5.7 7.5-12.0 12.0-5.5 7.4---12.0 0.75 12.0-5.2 7.2-12.0 1.00

W. FREYet al.

750

13C-NMR spectra of the copolymers in DMSO.d6 showed no splitting of resonances by different sequences of monomer units. The spectra were superpositions of the corresponding homopolymers, i.¢. of s-PMA and of fully cstcrified s-PMA, if the small

shifts with P ( A ) , which appear for part of the peaks, are disregarded. For instance, the --CH2--10--H and - - - C H 2 - - O - - ~ - - resonances are separate and are seen side on side in the copolymers. The homo- and copolymers are soluble in DMSO, pyridine, DMF,

c.;]

[.

CH|~H "~n

: 1.13 HH• 1.51 =

Boo-Asp(; OHm)- , - , H A .

4.43 =

?

H7

1.38=

2.75 p~

CH,q~-C-O-C-CH 3 I

I

COOH S

-

0I C=O

HHI 2 H3 4

1.05 m 1.50 I ~ 4.00 p~ 4.32 m 7.17 Ima 4.9~ Imm

H

2.75 pc,,

5 H6 H H 87

0 ?

4

6

7.27 lira

~H2

COOH

! 6

O C:O

0

CH;

1 H2 H3 H4 H5 H6 H7 H8 H

CH~

3

0

2

1.12 1.52 plm 3.99 pea 4.22 m 5.65 plm 1.38 ppa 1.~ pp,, 2.48 p ~

CHl

COOH

3

$

Ht I 2 3 H4 H5 6 7 8 g

[..

o

~ ~

0

1.06 1.49 4.02 4.19 5.92 5.01 7.29 1.98 2.46

e

.

ZJ

IHm Plm IHm Pm Im IHm Plm PN I

COOH i B

Fig. 2(a)

Esterification reactions on syndiotactic poly(methallyl alcohol)--Ill and in water at higher pH. By potentiometric titration, it was found, for instance, that the homopolymers Z-Asp(OH)-s-PMA and Boc-Asp(OH)-sPMA are soluble down to pH 5, as seen in Figs 3 and IH

q-

d4Hz "n I o* c'--o I

1

o

6"H , 2

-I-PMA

C 6 C C C C

H3

3

CH3

C

C~iOOH

'rr

_ IH f,.2.. ~ 3 $1 ~"-M1r~," "[ ~:4H ~ m 2 --

Z-Asp(OH)-s-PHA

u_

. _

,

C

Boc-Glu(OH)-s-PMA

0

0

2

CH3

C 8

2_L z-oz.(o.>-,-PM^

~13H '1 2

•

171.95 pp.

lit

_*

il

1 2 10-12 C C 3 C C 4 C S C G C 7 C 8 9 C 13 C 14 ¢¢ 15

24.18 48.26 40.28 72.27 171.51 53.54 15S.60 6S,64 136.85 26.52 30.13 173.34

~.

I

,

PI~ pp. ppm Pl~ pl)m pp. pp. pplu pp. pp.

24.42 4ll.lr 40.34 72.13 171.62 53.33 155.12 78.46 28.23 26.49 30.20 172.67

8

pp. II

?

oI Cs:O 0 IO 11 *H-~H-~7-O-C o H ~ ' ~ , z c 4H2 4:~OH

c 4Z C c C 5

cC 67

C 1 H 31 '

c%'"

51.23pp,

1.5.5.56 ppl 6.5.97 pp. I 36.85 PI~ 36.28 pp.

,

c lO9 c i1 CC 12 S

pp. C 10, C I1° C 12= 128 pp. pp. pp. pp. ppim

23.53 48.46 ,0.30 72.69 171.00

mq

C~

.7 , -C~'.I-NH-C .-O-Cq-CH3

r

I " 43 5 ~

c 1

c'

~:mH~ ClIH , 2 C r2OOH

10-42 C C C C C

2_2

,

~-~

4H~n

C:s:O i --

r

,

C ~ C • C .= C ::

"~',b4"~ 2

c OOH

7 8 9 10 11

23.8.5 pp. 48.71 pp. 40.32 pp. 72.79 pp. 170.8.5 pp. 50.67 pp. 154.96 pp. 78.75 28.26 pp. 36.31 pp. 171.9.5 pp.

c ;

C~O 0 m ml , L 4 ~ . . , ~ . . ~ ,,.L,-eu~/"~-~..~ ~, ..~ m ~ m m ~.. - ~ n2~kkJ/,,m . . ,---~-'~ ~ '

C'H,C'

C 43 C C C 5

9

c

C~t-NH,,C ~ - ~ H

L

4. The water solubility of the copolymers depends on the degree of esterification, because s-PMA is not water soluble at any pH. Table 4 gives the pH ranges for the solubility of the copolymers (2-methallyl-

C 2

Boc-ksp(~OH)

751

I

~

I

I

ILj__

pp. C 10o C I1, C 12 pp. pFa PP. pp. pp. pp. pp. p~ PP. ~

128 pp.

8

7

Fig. 2. (a) IH-NMR spectra of the homopolymers Boc-Asp(OH)-s-PMA, Z-Asp(OH)-s-PMA, BocGIu(OH)-s-PMA, and Z-Glu(OH)-s-PMA, together with listing of chemical shifts and assignment. (b) ~3C-NMR spectra of the same homopolymers.

m

752

W. F~Y et al. 12

12

11 ~a-1

11 10

9

~

8

9

I ~ \ dissolution

"t

7

~,~

z='6"

8

--.,.m.,,

"I" Q.

% ,?olutlon

7 6

~

5

-

~

precipitation

a'O

4 3

?f

,

o.o 2.0

,

0.5

1.o 1.0

1.5

2

,

I 2.0ml HCI O.Oml KOH

1.5 0.5

I

0.0 2.0

I

0.5 1.5

1.0 1.0

I

1.5 0.5

/7'

8

÷

/+ 7

++s

~,,+.+.+~+.+,+

insoluble , ,~.~+/ . + ~." A..+1"~ , I o.o o.1

I 0.3

I

2.0ml HCl O.Oml KOH

6

Insoluble I .1.-~ ~'t''4~'~'~t-~'t~'+~

~++ - ~ , + I, 0.5

I 0.7

I 0.9

I 1.0

I 0.0 0.1

I 0.3

I 0..5

I 0.7

1

0.9

I 1,0

a

Fig. 3. Potentiometric titration curves, pH vs reagent, for the homopolymer Z-Asp(OH)-s-PMA (upper part). Titration with N/IO HC1 [ ], backtitration by N/IO KOH [- . . . . . . . ]. Points of precipitation and redissolution [& V]. Apparent acid dissociation constants, pK,, versus degree of ionization of carboxyl groups, ~, as derived from [ ] (lower part).

N=(benzyloxycarbonyl)-(L)-hydrogen-a-aspartate)-s co-(2-methallylalcohol) (Z-Asp(OH)-MA-s-co-(MA). Starting the titration from a solution of pH = 11, the homopolymer precipitates at pH = 5.2. For a copolymer with only 25% esterified monomer units, precipitation occurs somewhat sooner at pH = 6.6. Already 20% of esterified units suffice to effect considerable solubility. By back titration by N/IO KOH, a hysteresis effect becomes apparent. (Table 4, and Figs 3 and 4). The hysteresis is caused mainly by the time requirement of the redissolution process. It should be noted also that the ester bond of the polymers is not stable in the strongly alkaline region for prolonged periods. Using the Henderson-Hasselbalch equation, the apparent acid dissociation constants, pK,. have been calculated over the range of pH, using only the curve of "forward" titration with HCI. p K . --- pH + log 1 - ¢ tv

(7)

Where ~t is the degree of ionization of the unesterified ---COOH group. The curves for pK~ are also shown in Figs 3 and 4. The maximum in the curves at ~t = 0.5 indicates a change of the conformation of the dissolved polymer. Up to • = 0.5 the increasing negative volume charge density reduces the dissociation of

Fig. 4. Potentiometric titration and p / ~ curves for BocAsp(OH)-s-PMA similar to Fig. 3.

the ---COOH group. Then a change in conformation sets in, leading to a lower charge density and possibly increased hydration of the polymer, causing increased dissociation. With further increasing of ~t, the charge density resumes its increase, leading to steeply decreasing dissociation at = > 0.8. The conformation at ~t <0.45--0.5 is possibly a relatively compact coil conformation of the polymer chain which is held together to a great extent by hydrophobic forces and hydrogen bonding. This is in accord with a more pronounced maximum at a higher level of p K , for Z-Asp-s-PMA than for Boc-Asp-s-PMA, because this behaviour may be expected, considering the more hydrophobic character of the benzyl compared with the t-butyl group. The conformation at a > 0.8, on the other hand, is an extended chain, caused by the high repulsive interaction of the carboxylate groups. Acknowledgement--We thank the Deutsche schungsgemeinschaft for financial support.

For-

REFERENCES 1. J. Klein and E. Klesper, Polym. Bull. 6, 291 (1982). 2. J. Klein and E. Klesper, Makromolek. Chem. 5, 701 (1984). 3. H. Abe, K. Imai and M. Matsumoto, J. Polym. Sci. B3, 1053 (1965). 4. H. Abe, K. Irnai and M. Matsumoto, J. Polym. Sci. C23, 469 (1968). 5. H. L. Cohen and L. M. Minsk, J. Org. Chem. 24, 1404 (1959).

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