pure appi. chem., vol. 51, pp. 1391—1404. pergamon press ltd....

Pure Appi. Chem., Vol. 51, pp. 1391—1404.Pergamon Press Ltd. 1979. Printed in Great Britain.

.

CATALYSIS OF ESTEROLYTIC REACTIONS BY WATERSOLUBLE IMIDAZOLE CONTAININGPOLYMERS :

C G Overberger and Smarajit Mitra(5)

Department of Chemistry and the Màcromolecular Research Center,The University of Michigan Ann Arbor Michigan 48109 USA

Abstract: In a review article [Accounts Chem. Res. , 2, 217 (1969)] is asummation of our work up to 1968. In this review we summarized theactivity of polyvinylimidazole and copolymers in esterolytic reactions.We explained the increased reactivity of these polymeric reactants in termsof cooperative effects and electrostatic factors. In 1967, we blundered

. into the extremely interesting discovery that long-chain esters providedus with enormous acceleration of rates at room temperature and we haveattributed this in 'general terminology to apolar bonding as a thirdfactor to understand the high reactivity of thesesynthetic macromolecules.Reference 1 describes our recent published work. We are heavily emphasizingthe apolar bonding concept. Related work which conceptually can be regardedas similar to our own discovery was carried out by the group of ProfessorIrving M. Klotz at Northwestern(2), the group of Professor Kabanov at theUniversity of Moscow(3), and the group of Professor Kunitake(4). Thislecture will briefly review the background of the problem and discuss indetail the catalysis of esterolytic reactions by water-soluble copolymers

of vinylimidazoles and vinylamines. Synthetic macromolecules containingpendant imidazole groups have been used as catalysts for the hydrolysis ofesters. This report extends that study to copolymérsof 4(5)-vinylimidazoleand vinylamine. The latter component renders these polymers water solubleand, therefore, their catalytic activity could be studied in a totally

.

aqueous environment. The precursor for the vinylamine monomer used forthese copolymerizations was N-vinyl-t-butylcarbamate which, in turn, wasprepared'from vinylisocyanate. Extremely large rate enhancements wereobserved with these catalysts for hydrolyses of activated esters and thesewere attributed to cooperative, electrostatic and hydrophobic effects. Thelarge cooperative effects weredue to tight coiling of these copolymers inwater, increasing the proximity of imidazole groups for interaction. Thehighly aqueous media in which these esterolysis could be carried out withthese copolymers resulted in very large hydrophobic effects with long chainester substrates. Less aqueous environments led to dramatic rate decreases.Esterolytic studies were also carried out under conditions of excess sub-strate which showed typical steady-state kinetics from which rates ofdeacylation of the catalyst could be determined. The fraction of imidazolesin these copolymers that were acylated under steady-state conditions werequite low.

INTRODUCTION

The utilization of synthetic macromolecules as catalysts for reactions has received con-siderable attention(6-9), especially because these reactions can serve as models for complexenzymatic processes. Though less efficient than enzymes, these polymeric catalysts haveseveral anaio9ous characteristics such as higher reactivities than the corresponding mono-meric systems(6-9), specificity towards substrates(6-ll), competitive inhibition(l2-l4),bifunctional catalysis(6-9,ll,l5-l7) and saturation phenomena(l2,18-24).

We have been interested in polymeric catalysts where the catalytically active site is acovalent part of the macromolecular species. One special interest has been concerned withesterolytic reactions in the presence of synthetic imidazole-containing macromolecules(7-9).Three factors have been attributed to the large rate increases generally observed withthese catalysts: cooperative effects, electrostatic effects and hydrophobic effects. Theserate enhancing factors have been demonstrated(25,lf)to be predominant in highly aqueoussolvent systems, but due to the limitations on the solubility of poly[4(5)-vinylimidazole]in water, it was found necessary to synthesize copolymers of 4(5)-vinylimidazole which couldbe dissolved in solvents of high water content. This study was directed towards the synthe-sis and esterolytic activity of copolymers of 4(5)vinylimidazole with vinylamine where thelatter component would impart water solubility to the catalytic polymer

1391

1392 C. G. OVERBERGER and S. MITRA

RESULTS AND DISCUSSION

Monomer Snthesis: 4(5)-Vinylimidazole was synthesized by the method of Overberger(26).The monomer chosen for the vinylamine units in the copolymers was N-vinyl-t-butyl carbamate(NVBC). The t-butyl carbamate was chosen because Hart(27) has demonstrated that while it isdifficult to hydrolyze the polymers of methyl or ethyl carbamates, the corresponding t-butylcarbamate can be easily hydrolyzed by acids to give polyvinylamine. Both 4(5)-vinylimida-zole and N-vinyl-t-butyl carbamate were purified by sublimation.

Copolymerizations: The purified monomers were copolymerized in benzene solvent using 1 mole% of AIBNäs a free-radical initiator. The polymers precipitated from the benzene solutionand were purified by repeated precipitations from methanol into acetone. The composition of

AIBN / NH

N,NH + NCOOC(CH3)3 N,NH oH CH

6 6OC(CH3)3

(1) (2) (3)

the copolymers could be determined from both the N-analysis of the copolymers and their NMRspectra. In the latter method, the ratio of proton signals for the imidazole and the t-butyl group in the copolymers were used to calculate the polymer composition. The agreementbetween the values obtained by the two methods was satisfactory. An examination of thesecopolymerization results (Table I) show the low reactivity of N-vinyl-t-butyl carbamate toenter into copolymerization with 4(5)-vinylimidazole. Increased amounts of the carbamatemonomer in the feed resulted in drastic reductions c copolymer yield and, even then, nomore than 46% N-vinyl-t-butyl carbamate could be introduced into the copolymers.

TABLE 1. Copolymerization of 4(5)-VIm and NVBCa

Sample

MoleNVBC

Percin

entFeed

ofTime

(hr)

CopulymerYield

%

Copolymer CompositionMole Percent of NVBC

NMR N-Analysis

IC-l 9 24 39.0 24.7 23IC-2 13 24 72.2 18IC-3 32 24 54.1 33IC-4 49 55 65.0 32 31

IC-5 50 32 50.5 28 32IC-6 50 24 49.2 36.5IC-7 72 30 24.7 35 34IC-8 72 24 15.2 46

a Polymerizations were carried out on 1.5 molar monomer solutions inbenzene with 1 mole % AIBN at 65°C.

H drol ses of Copolymers: Two of the above copolymers (IC-i) and (IC-7) were chosen forhydro yses of the carbamates to obtain the free amines. These hydrolyzed copolymers wereused for the study of their catalytic action on esterolysis reactions. The hydrolyses werecarried out with 10(N) hydrochloric acid(28,29) or 48% hydrobromic acid(30) in ethanolsolution. The hydrolyzed copolymers were purified by repeated precipitations from methanol

10(N) HC1 or

/\ NH48HBr

+ (m+n)P

N,,NH COEtOH

x = Cl or Br

OC(CH3)3(3) (4)

+ n CO2 + n(CH3)2CCH2

solutions into acetone followed by drying at 60°C under a vacuum until constant weights wereobtained. The compositions of these copolymers as determined from their NMR spectra andelemental analyses were consistent and agreed well with the compositions of the unhydrolyzedcopolymers from which they were derived. Thus the polymer derived from IC-l was found tocontain 25% vinylamine. units (IA-i) and that derived from IC-7 contained 33% vinylamineunits (IA-7). The intrinsic viscosities of the hydrolyzed polymers were found to be 0.41dug (IA-i) and 0.38 dug (IA-7) in 50 vol % methanol-water solutions at 25°C.

Catalysis of esterolytic reactions 1393

Potentiometric Titrations: The catalytically active imidazole group in the solvolyses ofesteris kiown to be in the anionic and neutral forms(ll) and since the protonated imida-zoles and protonated amines in these copolymers would act as binding sites for negativelycharged substrates, it was necessary to determine the fraction of pendent imidazole andamine groups that existed in the protonated forms at different pH values used for kineticstudies. Potentiometric titrations were carried out in water solutions at 25°C with anionic strength of 0.02 (KC1). The titration of the imidazole and amine groups occurredover wide pH ranges, typical of polyelectrolytes, with considerable overlap. The polymersprecipitated at pH9. Modified Henderson-Hasselbach plots were constructed with pH againstlog (a1/l-z1) where c is the fraction of the total imidazole and amine that is in theneutral form. From these curves and the known composition of the polymers, the pK's of theindividual groups, imidazole and amine, were the pH values at which half of those individualgroups had been neutralized. The pK values obtained by this method and those of polyvinyl-amine(31) and poly[4(5)-vinylimidazole](ll,32) are given in Table 2.

TABLE 2. Titration of 4(5)-VIm-VAmine Copolymersd

pK

Sample Mole % VAm Imidazole Amine

IA-l 25 5.40 7.74IA-7 33 5.11 7.35

PVIm 0PVAm 100 750c

a Determined in 28.5% ethanol-water at room temperature and i = 0.015-0.02,Reference 32.

b Determined in 28.5% ethanol-water at room temperature and p 0.02,Reference 11.

c Reference 31 .d

Determined in water at 25°C and i = 0.02.

The considerably lower pK values obtained for the imidazole residues in these copolymers ascompared to those in poly[4(5)-VIm] may be ascribed to the polyelectrolyte effect from thestrongly basic amine groups in the polymers. Similar dramatic decreases in pK have beenpreviously observed in histidine graft copolymers of polyethylenimine(ln). The increasedincorporation of the interacting amine units in IA-7 over IA-i results in a lower pK valuefor the imidazole group in IA-7. The pK values of the amines obtained in these polymers(7.35-7.74) are quite similar to the pK value obtained for polyvinylamine by St. Pierreet al.(3l)

Solvolysis of Esters: The kinetic studies of the hydrolysis of activated phenyl esterscatalyzid by the copolymers of 4(5)-vinylimidazole and vinylamine, (IA-i) and (IA-7), weretypically carried out in excess catalyst, ca. 5 x l0M polymer and 5 x l05M substrate.The slopes of the lines obtained by plotting ln (A-AtJ versus time gave the pseudo-first-order rate constants (kmeas) from which the first-order rate constants (kobs) were calcu-lated by subtracting the blank rate (kblank).

kobs = kmeas_kblank

The second-order rate constant kcat was then evaluated from the equation

kcat = k05/[catalyst]Where the plots of ln(A,-At) against time deviated at higher conversions, the slopes of theinitial linear portions were taken. The critical micelle concentrations for the substrateesters have previouslybeen determined under these reaction conditions(lm) and the substrateconcentrations were always maintained well below these values

(a) Cooperative Effects--the Hydrolysis of PNPA. The interaction of imidazole groups,pendent on a polymer backbone gives rise to what is known as cooperative effects (Figure 1).The existence of this effect is examined by monitoring the effect of pH on the rate ofhydrolysis of p-nitrophenylacetate (PNPA). The catalytic activity of monomeric imidazoleincreases linearly with the fraction of neutral imidazole whereas for poly[4(5)-VIm], thereis a rapid non-linear increase in the activity at high pH values(25). This increasedeffectiveness was explained in terms of neutral-neutral and neutral-anionic cooperativeinteractions between pendent imidazole groüps(ll). The mechanism of the participation wasprobably a general base catalysis of the attack of one pendent imidazole by another neutralor anionic imidazole moiety.


Neutral -Neutral Neutral -Anionic

Fig. 1. Cooperative Interaction in Poly[4(5)-VIm].

The copoly[4(5)-VIm-VAmine] catalyzed hydrolysis of PNPA was studied over a pH range of 4-8to examine these cooperative effects. The results are shown in Table 3 and Figure 2. It is

TABLE 3. Solvolysis of pNpAa

Catalyst kcat min M' (pH)

IA-l 22(4.69) 34(6.10) 47 (7.06) 84.3(8.06) -

IA-7 21(4.65) 27(5.50) 51 (7.17)54.3(e)77 (8.09)

bImidazole - - 4(5.76) 17.5(7.20)

52.1(e)27.5(8.06) 29(9.18)

Imidazolec - - - - - - 31.8(8.03) - -Hist_PEIb - - 1(5.76) 3 (7.20) 5.5(8.06) 3(9.18)

Poly[4(5)_VIm]d - - 4(6.0) 7 (7.0) 20.5(8.0) 38(9.0)

Poly[4(5)_VIm]e - - - - - - 60.6(7.71) - -a

[catalyst] = 5 x 104M, [PNPA] = 5 x l05M, solvent = 96.7%H20-3.3% CH3CN,

p = 0.02, 26°C.

b Data taken from Reference in.

c In 10% EtOH-H20 system; taken from Reference 25.

d In 28.5% EtOH-H20 system; taken from Reference 25.

e In 20% EtOH-H20 system.

evident from this data, that the copolymers show a very strong cooperative effect in thisaqueous medium and very large catalytic rate constants are obtained. Significant rateenhancements are known to occur for these reactions with increasing water contents of thesolvent system(25) because of a greater amount of bifunctional catalysis in the polymer

through shrinkage of the polymer coil in the highly aqueous medium. Viscosity studies(25)of poly[4(5)-VIm] corroborate this explanation. However, poly[4(5)-VIm] is insoluble inthe 96.7% water systems we used for our study, and the only way of comparing our data withthose obtained previously was by extrapolating the kcat value from Overberger and Morimoto1sdata(25) to that at 96.7% water (Figure 3) and comparing this value to those obtained fromour copolymers. Such an extrapolation gives kcat = 84 min1M1 at pH8 and this comparesfavorably with the values of kcat = 84.3 (IA-l) and 77.0 (IA-7) obtained with the copoly-[4(5)-VIm-VAmine]. This agreement suggests that the vinylamine units in the copolymer donot play any important intrinsic role in these cooperative interactions, other than impart-ing water solubility to the polymers. Indeed, St. Pierre et al.(31) have shown that in thesolvolysis of PNPA in the presence of polyvinylamine, there was no evidence for aminecatalyzed aminolysis and rate constants were less than that obtained for monomeric amines

of comparable basicity.

That the cooperative effects were significantly increased in our system by the use of highlypolar aqueous media was established by using the same copolymers and substrates in 20%ethanol-water systems when the catalytic rate constants were found to be greatly lowered(Table 3).

Another important observation in our studies with the copolymers was that at the lower pHvalues studied (pH 5.5-6.1) the copolymers were found to be extremely efficient catalystscompared to monomeric imidazole: r = ka(polymer)/kr8(imidazole) = 6 to 8. The value of"r" for poly[4(5)-VIm] in this pH range in 28.5% ethanol is <1(25), i.e., the polymer isless efficient than imidazole as a catalyst. This large difference between the homopolymerand the copolymer may be attributed to two causes. First, the imidazole units in thecopolymer have lower pK values than those in the homopolymer and at this limiting pH range

OC'

80

60

CE

40

20

6

pH

Fig. 2. Second-order rate constants for the hydrolysis of PNPA by the[4(5)-VIm-VAmine] copolymers and by imidazole(A) in 96.7% water-3.3% acetonitrile (0,.) and 4n 20% ethanol-water (ri,.), = 0.02,26°C.

1395

(5.5-6.1), many more neutral imidàzoles areavailable in the copolymers than in the homo-polymers to participate in cooperative interactions. Second, in the much more polar aqueoussystems at these lower pH regions, the protonated copolymers remain less extended than dothe protonated homopolymer of [4(5)-VIm] in the less polar aqueous-ethanol systems. Themore compact copolymers can therefore take part in bifunctional interactions which may notbe possible in the extended case of. the homopolymer.

The combination of these two factors, namely, the availability of more neutral imidazoleresidues and the shrinkage of the copolymers in the very polar aqueous solvent, couldaccount for the large relative efficiency of the copolymer catalyst in the lower pH range.

(b) Electrostatic Effects--The Hydrolysis of NABA (S2-j: Overberger and coworkers(22) haveobtained rate hanceinintsof up to 50 times the rate exhibited by monomeric imidazole Inthe poly[4(5)-VIm] catalyzed hydrolysis of the negatively charged substrates 3-nitro-4-acetoxybenzoic acid (NABA, 52-) and 3-nitro4-acetoxybenzene sulfonic acid (NABS). Thelarge rate enhancements were attributed to electrostatic attraction between substrate andpolymer. Bell-shaped pH-rate profiles were obtained in these cases with a maximum ataround pH 7 where about 25% of the imidazoles remained protonated providing an attractiveforce for the negatively charged substrates

When the copolymers (IA-l) and (IA-7) were used as catalysts for the hydrolysTs of NABA in96 7' water-3 3V acetonitrile over a pH range of 4 to 8 extremely high catalytic rateconstants were obtained (Table 4) The rates were found to increase continuously with pHfor these two copolymers and bell-shaped curves were not obtained (Figure 4) The absenceof the bell-shaped pH-rate profilescan be rationalized by the factthat in the poly[4(5)-VIm-VAmine] copolymers, the imidazole groups can be completely deprotonated and yet thepolymers can maintain a charged surface from the very much less acidic primary ammonium ionsin the copolymer. The catalytic activity of all the imidazole groups are therefore restoredin the.high, pH-range while the electrostatic attraction for the substrate isstiil main-tained. Hence there is no drop in the reaction rates in this pH-range. Moreover, andprobably more importantly, It is possible that the very large coOperative effects exhibitedby these;copolymers at high pH, may be the reason for the continued rate increases. Theparticipation of the cooperative interactions in the high pH values could account for the

Catalysis of esterolytic reactions

—o— IA-I—-.—- hA-7

—A--tm

I

8

1396 C. C. OVERBERGER and S. MITRA

E

Fig. 3. Influence of water content of solvent on the rate of hydrolysisof PNPA by poly[4(5)-VIm] at pH8; from Reference 25.

TABLE 4. Solvolysis of NABAa

kcat M1 mm1 (pH)

60

vol % H20

Catalyst

ImidazoleD - - - 48(7.20) 70(8.06) 77(9.18)

IA-l 319(4.69) 360(5.35) 587(7.06) 1101(8.06) - -IA-7 320(4.65) 410(5.50) 663(7.17) 984(8.09) - -a

[catalyst] = 5 x lOM; [NABA] = 5 x lO5M; solvent - 96.7%H20-3.3%

CH3CN, p = 0.02; 26°C.

b Data taken from Reference ln.

very high absolute rate constants obtained with IA-l and IA-7. These large rate enhance-ments are largely due to the high aqueous content of the solvents used.

(ô) Hydrophobic Effects--The Hydrolysis of the Sn- Series. Hydrophobic or apolar effectshave been showntógivérisetb1àrge rate enhancements for the poly[4(5)-VIm) catalyzed


T 8

E

:b

-

4

C4• •

0pH

Fig. 4. Second-order rate constants for the hydrolysis of S2- catalyzedby [4(5)-VIm-VAmine] copolymers (o) IA-i, (.) IA-7 and by (A)imidazole in 96.7% water-3.3% acetonitrile, [52-] = 5 x lO5M,[catalyst] = 5 x lO, ii = 0.02, 26°C.

hydrolyses of long chain esters. Thus Overberger and coworkers(lf) observed that in thehydrolysis of a series of esters with increasing chain length, there was more than a 50-foldrate increase in going from an ester with a 2-carbon chain length to one with 12-carbons.The rate of monomeric imidazole catalyzed reactions, on the contrary, decreases withincrease in chain length of substrate due to increased steric hindrance. This increasedefficiency of polymeric catalysts has been ascribed to apolar or hydrophobic interactions.Overberger et al.(lf) have studied the effect of solvent composition on this hydrophobiceffect and have observed that an increase in the water content of the medium dramaticallyincreased the catalytic efficiency of the polymer, poly[4(5)-VIm], in the hydrolysis of

3-nitro-4-heptanoyloxybenzoic acid, S7- and 3-nitro-4-dodecanoyloxybenzoic acid, S12-.

The rates of hydrolyses of a series of esters, 5n (n = 2,4,7 and 12) were determined in96.7% water-3.3% CH3CN solutions at pH = 8 under conditions of ten-fold excess of catalystover substrate. The catalytic rate constants were determined from the linear portions ofthe pseudo-first—order plots and are given in Table 5 and a plot of the rate constantsagainst the length (n) of the side chain in the substrate is shown in Figure 5.

An examination of the data does indeed reveal the very large rate enhancements expectedwith long chain esters. The rate constants for the polymer catalysis increasei almost ten-fold in going to longer chain esters. Thus, with 12- the copolymers were more than 300times as efficient as imidazole in catalyzing the hydrolysis. As opposed to previousstudies(ln,lm,lo), the catalytic rate constants increased in going from to S- with thecopoly[4(5)-VIm-VAmine] indicating that hydrophobic interactions begin to play a dominantrole with these copolymers even with Sq-. This dominance is even more significant when weconsider that steric factors should be large with these copolymers in aqueous systems wherethe polymer chains are expected to be more tightly coiled.

That hydrophobicity plays an important role starting from S- is also evidenced from thefact that decelerative kinetics were observed for Sq-, S7- and S12-. This deceleration iscaused by partial precipitation of the acylated polymer which is highly hydrophobic. Suchdecelerative kinetics have previously been observed with other imidazole containing cata-lysts(ln,33).

Of the two copolymers we studied, IA-7 (with 33% VAmine units) was about 50% more reactivethan IA-i (with 25% VAmine units) for the substrate S7-. This is probably because IA-i,with less amine content, is more tightly coiled in solution, and steric effects decrease

//

V.

1398 C. C. OVERBERGER and S. MITRA

TABLE 5. Solvolyses of the S) Series in Watera

n

Catalyst 2 4 7 12

IA-lkcat(M1min1) 1,101 1,788 3,228 10,706

rb 16.2 29.8 64.6 315

IA-7 kcat(Mmin1)

rb

984

14.5

1,451

24.2

4,670

93.4

11,409

336

Poly[4(5)_VIm]C kcat(Mmin')

rd

338

5.8

- -- -

2,038

67.0

10,316

392

Imidazolee kcat(Mmin1) 68 60 50 34

a806; t = 0.02; [cat] = 5 x 104M;Solvent 96.7% H20-3.3% CH3CN; pH =

= 5xi05M.b r = kcat(pol)/kcat(imid) under conditions in (a).

c Solvent 20% EtOH-80% H20; pH = 8.0; i = 0.02; [cat] =5 x l05M; reference if.

d r = kcat(poi)/kcat(imid) under conditions in (c).

e Reference in.

Polymer solutions became turbid on standing.

IC

E

b'C

Fig. 5. Second-order rate constants for the hydrolysis of the 5n seriesby the [4(5)-VIm-VAmine] copolymers (.) IA-i, (o) IA-7 in 96.7%water-3.3% acetonitrile, pH8, [Sn] = 5 x 105M, [catalyst] =5 x l0M, ii = 0.02, 26°C.

5 x l04M;

'— IA—I: 14-7

8

4

I I I I I4

n8

I I2


the accessibility of the bulky S7- to the active imidazole sites. With 512-, however,hydrophobic effects override all other factors and IA-7 is only about 7% more reactivethan IA.-l.

In order to establish that the very large rate constants observed in our system were greatlydue to the high water content of the solution, hydrophobic interactions were studied withthe same copolymers and substrates in 20/ ethanol-water systems The results are given in

Table 6 and a plot of rate constants against the substrate chain length size, n, is shownin Figure 6. Comparison of these data with those obtained in water reveals the very large

TABLE 6. Solvolysis of the 5n Series in 20% EtOH_Watera

Catalyst

n

2 7 12

IA-i kt(M'min)rb

681

11.8

2125

69.9

4669

177.5

IA-7 kcat(Mmln)rb

659

11.4

1618

53.2

5722

217.6

Poiy[4(5)VIm]c kcat(M'min)

rb

338

5.8

2038

67.0

10,316

392.2

Imidazolec kt(M'min) 57.8 30.4 26.3

a Solvent 20% EtOH-water; pH = 8, p = 0.02; [cat] =5 x l0M.'

b r = kct(Pol)/kat(imid) under conditions in (a).

c Reference if.

=

n

Fig. 6. Second-order rate constants for the hydrolyses of the Snseries by the [4(5)-VIm.-VAmine] copolymers (') IA-i, and(o) IA-7 in 20% ethanol-80% water, pH 8, [Sn-] =5 x l05M, [catalyst] = 5 x l0M, p = 0.02, 26°C.

b

4E

b

2

•0 4I I8 12


effect of the water content of the solvent on the kc t values for the copolymers. With S2-,the value for the copolymers is much lower in 20% etanol largely due to the reduction inbifunctional catalysis in this system because of the more extended form of the polymercatalyst. With longer substrate chain lengths, the values of "r' change even more spectac-ularly until with S12-, they are about 55-60% of the values in water. This dramatic ratedecrease in 20% ethanol-water solutions is accountable only by major reductions in hydro-phobic interactions. On comparing the values of kcat or 'r' for the copolymers with thoseof poly[4(5)-VIm], it is apparent that hydrophobic effects are reduced by"dilution" withthe vinylamine units, however, where apolar interactions are not predominant, e.g., withs2-, the more highly charged copolymer is a more effective catalyst than the homopolymer atthe pH of kinetic studies, due to electrostatic effects.

(d) Solvolysis with Excess Substrate: In cases where acylated polymer intermediateaccumulate duringEhireacffon, the acylation-deacylation behavior of the polymer can befollowed by studying the solvolysis in excess substrate. Such reactions show, typically,an initial burst region followed by a slow zero-order steady state deacylation region.Assuming a Michaelis-Menten type mechanism,

k1 k2 k3E+S ES E +P1—E+P2where E is the polymer, S is the substrate, ES is the polymer-substrate complex, E is theacylated polymer, P, is the liberated phenolate anion that is estimated spectrophotometri-cally and P2 is the long-chain acid, a plot of the absorbance versus time can be described

(34) by the equation . bt[P1]=At+B.(l -e )

whereA = kt[E][S]/[S] + Km(apparent)

.

B =[E]0 (k2/k2 + k3)2/(l +

Km(apparent)/[S]02and

b =k3

+ k2/l +(K5/[S]0)

At time approaching infinity,[P1] = At + B.

We carried out the solvolysis of S12- in 96.7% water-3.3% acetonitrile with 4-10 timesexcess of the substrate over the catalyst. The absorbance versus time plots are shown inFigure 7. The slope of the zero order linear portion of the graph gives A, which is ameasure of the steady state rate of release of phenolate anion, and the intercept for t = 0obtained by extrapolation gives B, which measures the concentration of acylated residues atthe steady state. Dividing A by B gives k3 which is the rate constant for steady statedeacylation. The values of A, B and k3 as found from our study are tabulated in Table 7along with the values for poly[4(5)-VIm] as found by previous workers. It is seen that thesteady state concentration of acylated imidazoles is much less for the copolymers than forpoly[4(5)-VIm] (in 28.5% ethanol-water). This suggests that the homopolymer is much moreextended in the 28.5% ethanol-water system than the copolymer is in water.

The initial rates of these reactions, as measured from the initial slopes of the curves inthe "burst" region are often indicative of the hydrophobic nature of the polymers(lm).Under the conditions of excess substrate, the value indicates the concentration of the

polymer substrate complex [ES]. When the initial rates V are plotted against the substrateconcentrations (Figure 8), complete saturation of the copolymers was not observed. Veryhigh values of Vin were obtained, indicating efficient turnover in the copolymer system inwater.

EXPERIMENTAL

A. Sjnthesis.

(1) 4(5)-Vinylimidazole. 4(5)-Vinylimidazole was prepared by the method ofOverberger et al.(26).

(2). N-Vinyl-t-butyl Carbamate. N-Vinyl-t-butyl carbamate was prepared by themethod of Hart(27).

(3) Copoly4(5)-VIm-N-vin,yl-t-butyl Carbamate]. The appropriate amounts of thetwo monomers (Table 1) were dissolved in benzene to give total concentrations of monomersof l.5M; 1 mole % of AIBN was added and the solutions were sealed under vacuum in polymeriza-tion tubes. The samples were heated at 65°C for appropriate times (Table 1) and then pre-cipitated into acetone or ethyl ether. The copolymers were purified by repeated precipita-tions from methanol solutions into acetone (for low NVBC polymers) or ethyl ether (highNVBC-containing polymers).


Fig. 7. Absorbance vs. time plot for the steady-state rate of release of

phenolate aiThn from S12- by the [4(5)-VIm-VAmine] copolymer IA-7in 96.7% water-3.3% acetonitrile, [Sn-] = (S) 2.5 X l0-4M, (0)1.5 x l0M, (A) 1.0 x l0M.

—

Catalyst n

[S -]n

Rateof

of release

phenolate

Conc. of

Acylated

ResidueB)

k3

(A/B) — V•n1

Mx104 M minxl07 i x l0 min xl 0 M minxi05

IA-l

—12

12

12

2.51.51.0

1.671.580.36

0.806(32%)0.667(27%)0.444(18%)

20.7223.698.13

2.220.960.40

IA-7

Poly[4(5)VIm]b

12

12

12

12

2.51.51.0

2.6

1.671.530.36

6.98

0.778(31%)0.653(26%)0.417(17%)

1.65(66%)

21.4723.438.66

42.3

2.330.840.36

- -Poly[4(5)VIm]C 7 2.5 2.11 1.75(70%) 12.1 - -a pH = 7.11, p = 0.02, solvent

b Solvent 28.5% ethanol-water,

C pH = 6.85; solvent 26.7% ethanol-3.3% CH3CN-70% water, Reference lm.

004

a,

0020.08.0

I0

Time (mins)

TABLE 7. Steady State Rate of Release of Phenolate Anion from 5n by Imidazole-ContainingPolymers under Conditions of Excess Substrate(a)

96.7%-3.3% CH3CN; [cat] = 2.5 x 105M.

Reference lo.

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:O jW t:ii pu ppP 3Aoqoo.4p/cq w0 I. O w EO O Afl2XW u PALOSSP SM pue 3O9 : paesoweq e u paoeid SM eldtuPs atu,cLodo3 aq: O 6W Si noq suo.tj

. s3uaWAadxa asaq pasn SM uonjp oD4-Wa epqojaqqfl-uouu3 y •Lp/b so 5uaq pasn SUOfl[OS pae.quaDuosow aq qti 3[O•O :j: 2 :no peue 3A3M suaanseaw ,LSO3SiA aqj s3UaW9AflseW SODSA I

/tp 8 0 = HOW 709[U] (-7::-i-) 6/LP Lii 0 = HOW [U] (j)

. . :/4Lso3sLA 3suA:uI

. ••o: = N tOL8 = H 8S'O9 = 3 :punoj ZLOE = N tL8 = H t9L19 = 3 '(NsH3)L9(N9Hs3) o ssq at UOPOLPD :(7)

L66 = N tiL = H 9t9 = 3 :punoj 090E = N OLL = H t0819 = 3 'SZ(NSW3)SL(ZN9HSD) o sseq aq uo PL3 :(i-t)

:6upcp pu UOZ naUAaJe 'SS,cjUe iUawat uasq Ajaejdwo3 (1_wo olLt) eWQqAeD pue (1_luo 09EL) j/4nq- o anp sead '(q)

OOO :(1_wz) \ '(q) 8L-9t (q) (s) OYL (s) LL8 & :(p-HOeW) WN

S3q6aM uesuo o wnnoeA Aapun os 1.e P!P uaq a4aM asaq euoeo3u oueqaw qee sawn aow aaq pa dpadaA 3JM s4awic[odo paedp

-a.4d asaqj auo3aoe ou pa dpad pue paeuaouo aaM SUOfl1oS aq uaq pue 3OS e .iq j io zN apun paeaq a.4aM suonios asaqj jaADadsa% 'ppe o4Oq3OApIcq (N)oL

pue ppe DWoAqoApcq %j ssaoxa 44M pue (A/M uoe.tuaouOo %9-) joueqa u paAOssp aAaM'(L-3I)pue (1-31) 'SAaWIcjodOo [3AN-wIA-(9)ji1 aqo OMI 'SapWOq.

AO SaPL.4oLq3oAPKH .qaqi Se {[auu O UOe4daJ

OO1 = N OS9 = H t'L8S = 3 :punoj

• OO'L = N LS'L = H 6SL9 = 3 ' (ZONW3)99(zN9HS3) o sseq aq uo pe :(/.-31)

= N L89 = H SL89 '3 :puno SZ = N ZO'L =H ii9•39 3Z(ZONEtHL3)LL(ZN9HS3) o sseq aq uo poje3 :(1-3I) :sscjeue

Le2uaWaLJ O9[ 'OILL OOCE ( _wD) i (s) L (s) 9 '(s) s L (13a3) IWN

39 'ZOO = 'LLL = Hd 'W-OL X = [s/cLee3] 44M aiuoae %E'C-aeM %L96 Ut _Z1S O SLS,cLOALOS

paz/cLeeD L-VI (•) pue 1-vI (0) Aaw,cLodoo [auwVj\

aq Ao -s o uoequaDuo3 SA 3o 20

vI t,o(xtsJ •

1

v1LLIW s pu o1S1ario o • • ZOtlT


A differential titration curve was obtained by plotting pH vs. the difference in volumes ofbase at the same pH EAil](35). Since sharp inflection points were not present in thesecurves, modified Henderson-Hasselbach plots were constructed by plottinq pH aqainst loq-(a1/l-cz1), where is the fraction of neutral imidazoles.

E. Kinetic Measurements for Solvolysisof Esters.

(a) Slow Reactions in Excess Catalyst. To 2.9 ml of a buffered catalyst solution in a1.00 cm quartz c1Tàt 260, ias added 0.10 ml of an acetonitrile solution of the substrate.The final concentrations were [catalyst] = 5 x lOM, [substrate] = 5 x lO5M, = 0.02,[buffer] = 0.02M and 96.7% water-3.3% acetonitrile by volume. The buffer employed above pH6 was tris(hydroxymethyl)aminomethane-HC1. For pH 6, and below, the systems were bufferedwith sodium acetate-acetic acid. Forreactions in 20% ethanol, the substrates were preparedin ethanol and the catalysts were made in aqueous ethanol such that the final solutionswere 20% in ethanol by volume.

All data obtained under conditions of [catalyst]>>[substrate] was treated as first-orderkinetics by plotting ln(A-At) versus time. The absorbance measurements were made onBeckman DU and DU-2 Spectrophotometers. The slope of the first-order plot was taken as thepseudo-first-order rate constant (kmeas). Where significant, the blank rate constant(kblank) was obtained from earlier work(36) (for 96.7% water), and Reference 37 (for 20%ethanol). When kblank was subtracted from kas, we obtained kobs.

kobs k05/[catalyst].Infinity values were obtained after at least ten half-lives.

(b) Fast Reactions in Excess Catalyst. Fast reactions, i.e., those with half-livesless than about 2 mfn,were followed on a Durrum-Gibbs Stopped Flow Spectrophotometer. Thedata were obtained on an oscilloscope trace of %T versus time; this trace was photographed.The %T was obtained from the photographs at certain time intervals, then converted toabsorbances.

The rates of the samples notgiving first-order kinetics were obtained from the initialslope of the ln(A,,_At) time plot, where the curve was still linear.

(c) Reactions in Excess Substrate. Essentially the same procedure was followed asdescribed aboveéicept the catalyst concentration was held constant and the substrate con-centration was varied. In all cases of excess substrate, the absorbance was plottedagainst time.

Acknowle4gements: The authors would like to acknowledge financial support fromthe Department ofChenistry in the form of a Knoller Summer Fellowship (1974), a Sherwin-Williams Fellowship (1974-1977), and the National Institutes of Health under Grants 2ROl-GM15256 and lROl-CA12231.

REFERENCES

1. a) C. G. Overberger and T. J. Pacansky, 3. Poiym. Sci., Chem. Ed., 13, 931 (1975);b) ibid., J. Polyrn. Sci., Symposia, 45, 39 (l974J1cJ C. G. Overberger, T. J. Pacansky,3. Lee, T. St. Pierre, and S. Yaroslavsky, J. Polym. Sci., Symposia, 46, 209 (1974);d) C. G. Overberger and V. Okamoto, Macromolecules, 5, 363 (1972); e) C. G. Overbergerand R. C. Glowaky, J. Amer. Chem. Soc., 95, 6014J4973); f) C. G. Overberger, R. C.Glowaky, and P.-H. Vandewyer, J. Amer. Chem. Soc., 95, 6008 (1973); g) C. G. Overbergerand C. J. Podsiadly, Bioorg. Chem., 3, 1 (1974); h) ibid., 3, 35 (1974); i) L. J.Mathias and C. G. Overberger, J. Polym. Sci., Chem. Ed., in press; j) C. G. Overbergerand K. Gerberding, J. Po]ym. Sci., Polym. Lett., 11, 465 (1973); k) C. G. Overbergerand T. W. Smith, Macromolecules, 8, 401 (1975); lribid., 8, 416 (1975); m) ibid., 8,407 (1975); n) C. G. Overberger and K. W. Dixon, J. Polym.Thci., Chem. Ed., 15, l86(1977); o) C. G. Overberger and V. Kawakami, J. Polym. Sci., Chem. Ed., in pss;p) ibid., J. Polym. Sci., Chem. Ed., in press; q) C. G. Overberger and A. Meenakshi,in preparation.

2. a) I. M. Klotz and V. H. Stryker, J. Amer. Chem. Soc., 90, 2712 (1968); b) G. P. Royerand I. M. Klotz, J. Amer. Chem. Soc., 91, 5885 (1969); T I. M. Klotz, G. P. Royer,and I. S. Scarpa,. Proc. Nat. Acad. Sci., U.S., 68, 263 (1971); d) V. Birk and I. M.Klotz, Bioor . Chem., 1, 275 (1971); e) T. W. Johnson and I. M. Klotz, Macromolecules,6, 788 1973

3. ) I. N. Topchieva, A. Bashir, V. A. Kabanov, ysokomol Soedin., Ser. B, 15, 585 (1973);b) G. A. Murtazaeva, V. S. Pshezhetskii, and V. A. Kabanov, Dokl. Akad. Nauk Vzb. SSR,30, 39 (1973); c) S. L. Mkrtchyan, V. P. Torchillin, and V. A. Kabanov, Vestn. Mosk.Univ. Khim., 15, 100 (1974); d) V. S. Pshezhetskii, G. A. Murtazaeva, and V. A Kabanov,Eur. Polym. 3., 10, 571 (1974); e) S. G. Starodubtser, Vu. E. Kirsh, and V. A. Kabanov,Eur. Polym. J., 10, 739 (1974); f) S. G. Starodubtser, Vu. E. Kirsh, and V. A. Kabanov,ysokomol. Soedin, Ser. A, 16, 2260 (1974); g) V. S. Pshezhetskii, A. P. Lukyanova,

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pure appi. chem., vol. 51, pp. 1391—1404. pergamon press ltd....

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