Effects of 2,4-ionenes of different molar masses on the oxidative coupling of thiol catalyzed by cobaltphthalocyanine

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Effects of 2,4-ionenes of different molar masses on the

oxidative coupling of thiol catalyzed by cobaltphthalocyanine

Citation for published version (APA):

Streun, van, K. H., Piet, P., & German, A. L. (1987). Effects of 2,4-ionenes of different molar masses on the

oxidative coupling of thiol catalyzed by cobaltphthalocyanine. European Polymer Journal, 23(12), 941-946.

https://doi.org/10.1016/0014-3057(87)90037-1

DOI:

10.1016/0014-3057(87)90037-1

Document status and date:

Published: 01/01/1987

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EFFECTS OF 2,4-IONENES OF D I F F E R E N T MOLAR

MASSES ON THE OXIDATIVE COUPLING OF THIOL

CATALYZED BY COBALTPHTHALOCYANINE

K. H. VAN STREUN, P. PIET and A. L. GERMAN

Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

(Received 3 March 1987)

Almtraet--A photometric method is developed for the determination of the molar mass of 2,4-ionene by attaching a chromophoric moiety to its end-groups. Molar mass determination of these polyelectrolytes by acid-base titration of the amine end-groups is also possible, and confirms the photometric method. The effects of molar mass of 2,4-ionenes on the oxidative coupling of 2-mercaptoethanol (ME) was studied in the presence of cobalt(II)phthalocyanine-tetra-sodium-sulphonate (CoTSPc). Monomeric analogues of 2,4-ionene showed no increase of reaction rate as compared with the polymer-free system. However, relatively low molar mass ionene (M, = 1740) showed a dramatic increase of reaction rate. The reactivity showed an optimum around pH = 8. Saturation kinetics (Michaelis-Menten kinetics) were observed. The calculated turnover number was 3200 sec- t and3600sec 1 for)kl"n= 6600and h,St, = 1740, respectively. The stoichiometry of the reaction was disturbed at low thiol concentration.

INTRODUCTION

We have studied the effect of cationically charged polymers on the so-called Merox-process i.e. the oxidative coupling of thiols to disulphides in the presence of the catalyst, cobalt(II)phthalocyanine-

tetra-sodium sulphonate (CoTSPc, 1, Scheme 1) S09No

CoTSPc

4 R S H + 02

, 2 R S S R . 2 H20

oo+s-L -

OH

0

N ~ N

SO,No

I

Scheme I. Conventional (polymer free) thiol oxidation.

2 B r e CH 3 CH 3

t*

t®

(CH3)2N(CH2)2N(CH3)

2 ÷ BF(CH2}

4 Br

>

-ty(CH2½-~(CH2)4]

F

3 4 CH 3 CH 3 2

activity [2, 3]. Two major effects are believed to cause this p h e n o m e n o n : (1) coulombic interaction between the negative charge of the sulphonate groups of CoTSPc and the positive charge on the polymer

Scheme 2. Synthesis of 2,4-ionene. [1,2]. Especially poly(vinylamine) (PVAm) and

2,4-ionene, a poly(quaternary a m m o n i u m ) salt (2, Scheme 2) were found to give a surprisingly high increase in reaction rate. Experiments showed that a n increase o f the positive charge density on the polymer chain resulted in an enhancement of the catalytic

backbone a n d (2) an increase of the degree of dis- sociation of the thioi in the cationically charged polymer domain, resulting in a locally enhanced thiolate a n i o n concentration. In the case of P V A m also coordinative binding between the metal complex 941

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942 K. H, VAN STREUN et al.

and the polymer is assumed to occur [4, 5] and M i c h a e l i s - M e n t e n kinetics in thiol and oxygen have been observed [6]. The reaction rates with both P V A m and 2,4-ionene showed an o p t i m u m around p H = 8 [2, 7], caused by two opposing effects on increasing the pH, viz. an increase of thiolate anion concentration and an increase of ionic strength. In the case o f P V A m the collapse of the catalytic activity above the p H - o p t i m u m is also due to loss o f polymer charge (deprotonation). M o s t interestingly, a molar mass dependence o f the catalytic activity was observed in the case of P V A m [8]. Therefore, we describe in this paper our investigations on m o l a r mass dependence of the catalytic activity and kinetics o f ionene containing systems. The synthesis o f 2,4-ionene involves a step reaction o f N , N , N ' , N ' -

tetramethylethylenediamine ( T M E D A , 3) with 1,4- d i b r o m o b u t a n e (4) (Scheme 2). In the literature, molar masses o f these types o f polymers have been determined by viscometry, light scattering and ultra- centrifugation [9-11]. However, these methods are often complicated and laborious, due to the poly- electrolytic character of these polymers. Therefore, in this paper we present two relatively simple methods o f determining molar masses o f 2,4-ionenes, both based on the determination of amine end-groups of aminated 2,4-ionenes. The first m e t h o d measures the

o f the activity and kinetics of catalytic systems based on 2,4-ionene.

EXPERIMENTAL

All chemicals;and solvents were used as purchased unless mentioned otherwise. TMEDA, 1,4-dibromobutane and 2-mercaptoethanol were obtained from Fluka.

Synthesis of 2,4-ionene

2,4-Ionenes were synthesized according to the method of Rembaum et al. [9] with a few modifications. The reaction conditions were adapted in such a way as to accomplish the complete solution of the resulting polymer throughout the reaction.

A typical experiment: 4.063 g TMEDA (3) and 7.560 g 1.4-dibromobutane (4) (Scheme 2) were dissolved in 70 g water~iimethylformamide (DMF) (1/1, w/w) and placed in a waterbath at 40°C under N 2. After two or more days an excess of TMEDA was added to the reaction mixture in order to aminate the product. After two more days, the reaction mixture was precipitated in acetone. After filtration and washing with acetone, the product (5, Scheme 3) was dried under reduced pressure (0.1 kPa) at 50°C. Yields, determined by mass were between 90 and 100%. In order to obtain higher molar masses, reaction times (2-14 days), temperature (20-60°C) and solvent combinations [e.g. DMF/methanol (1/l,w/w) and DMF/water (4/1,w/w)] were varied. However, the use of these solvent combinations appeared to carry the risk of precipitation of polymer as polymerization proceeds.

(CH3)2N-[2,4-IONENE]-Br * xs TMEDA

,* (CH3)2N-[2,4-IONENE]-N~H3)2

2

3

5 * xs ( ~ C H 2 B r

6 CH,

CH.

/ ~

I® °

I o ~

, ~ /

~T-/<()>CH2N-[2,4-IONENE]-.NCH2(O)

/

2 Br e

CH 3 ~ H 3 7

Scheme 3. Amination and benzylation of 2,4-ionene.

CH 3

CH~

• ~ • I e I ~ e

( C H 3 ) 3 N C H ~ Br e

~/X~__~CH2N(CH2),NC

~ H 5 2Br

CH 3

8

9 CH

CH

CH~

/~(, ))CH2N(CH2)2NCH2~O)

2Br

{,( ))CH.N(CH_)_N(CH_).N(CH_)_NCH_(())

z I

zz I

z41

zz I

CH 3

CH 3 CH 3

10

11

Scheme 4. Chemical structure of compounds used in Table l.

4 Br e

u.v.-absorbance of a c h r o m o p h o r e (a benzylic group), chemically bonded to the amine end-groups. The second m e t h o d determines the a m o u n t o f amine end-groups by direct acid-base titrations. These methods will be shown to be sufficiently accurate to determine the relatively small molar mass dependence

Determination of molar mass

Photometric method. The aminated 2,4-ionene (5) was reacted with an excess of benzylbromide (6) (Scheme 3) in DMF/water (1/1, w/w) at 40°C under N2 for two days. The product work-up was similar to that used for the aminated 2,4-ionenes, The u.v.-spectra were measured with a Hewlett-Packard 8451A diode array spectrophotometer at

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Oxidative coupling of thiol 943 25°C in a 0.4 M NaC1 solution at ). = 264 nm. The slope of

the plot of absorbance vs concentration of ionenes (in g/dm 3) gives the absorptivity (a, in I0 dm 2-g- ~ ).

Titration method. Titration of the aminated 2,4-ionenes was carried out in a 0.4 M NaC1 solution under N 2 . A 0. I M HC1 (Merck, Titrisol) solution was added until pH = 3 was reached. Then this solution was titrated with a 0.01 M NaOH solution (Merck, Titrisol). The pH was monitored with a combi-electrode (Radiometer, GK 2401B) connected to a digital pH meter (Radiometer PHM 82). From the equivalence points and the total mass of 2,4-ionene, the molar mass can be calculated.

Catalytic experiments. CoTSPc was synthesized by the

method described by Weber and Bush [12]. The substrate, 2-mercaptoethanol (Scheme 1. R = CH2CH2OH, abbrevi- ated as ME), was distilled before use. Activity measurements were carried out at constant 02 pressure in an all-glass double-walled Warburg apparatus, equipped with a power- ful mechanical glass stirrer. The stirring speed was maintained at 2500rpm during the reaction. The Warburg apparatus was further equipped with a pH-electrode (Radi- ometer GK 2401B). The catalyst was prepared by adding an aqueous solution of CoTSPc to the reactor containing an aqueous ionene solution. Adjustments of pH were accom- plished by adding minor amounts of a 0.01 M NaOH solution (Merck, Titrisol). The mixture was degassed twice and saturated with 02 . During the reaction the temperature was maintained at 25°C and the O 2 pressure at 100 kPa. After injecting ME into the reactor, a drop of pH was observed due to the acidity of the thiol. The oxygen uptake rate was monitored with a digital flowmeter (Inacom). The initial reaction rates were determined as the maximum oxygen consumption. The total reaction volume was 0.1 dm 3. The peroxide content of the reaction mixture was determined by iodometry as described by Vogel [14].

R E S U L T S A N D D I S C U S S I O N

Determination o f the m o l a r mass of 2,4-ionene by measuring the u.v.-absorbance o f a chemically bonded c h r o m o p h o r e (benzylbromide, 6, Scheme 3) requires the value of the molar absorptivity (E, in 1 0 d m 2 . m o l ~) o f the chemically bonded benzylbromide. In addition, it should be proved experimentally that E is independent o f the length o f the attached 2,4-ionene chain. In Table 1, where some c h r o m o - phoric analogues of 2,4-ionene are listed, it is shown that E is relatively insensitive to the chain length. We have chosen c o m p o u n d 11 as reference compound. F r o m L a m b e r t - B e e r ' s law is follows that

g p o I = Eref/apo 1

where Mpo~ is the molar mass of aminated 2,4-ionene (including its c h r o m o p h o r i c end-groups) and apo~ is the absorptivity of the u n k n o w n ionene, calculated from the slope of the absorbance vs corrcentration curve (c in g. dm 3). The latter measurement requires that 2,4-ionene is first aminated with T M E D A (3) leading to (5) and then reacted with the c h r o m o p h o r e

1 . 2 1

~.,

. 2 0 i J i i J i i 0 1 2 3 4 5 6 7 2 , 4 - I o n e n e ( g . d m - 3 )

Fig. 1. Absorbance (2 =264nm) vs concentration of the modified 2,4-ionene (gdm 3) at 25°C, [NaC1]=0.4mol.dm 3. I::]: Reference compound; /X: Mn = 1800; ©: ~n = 2750; &: Mn = 8800; II: AT¢,= 16,600.

benzylbromide (6) (Scheme 3). The calculated molar mass minus the molar mass o f the benzylbromide groups gives the molar mass o f the aminated 2,4-ionene. As in fact the total n u m b e r o f end-groups is measured, the calculated m o l a r mass is the number- average molar mass (/Qn). Figure 1 shows absorbance vs concentration curves for several modified 2,4-ionenes. The linearity of the curves proves an important prerequisite, i.e. that L a m b e r t - B e e r ' s law is obeyed in the present case. An alternative m e t h o d o f determining the a m o u n t of end-groups of aminated 2,4-ionenes is acid-base titration. The m o l a r mass calculated in this m a n n e r is again the number- average quantity and can therefore be compared with the results of the photometric m e t h o d (Table 2). The two methods are in good agreement although the relative discrepancy tends to increase toward higher m o l a r mass. The calculated molar masses of the ionenes, though relatively low, lie within the range o f data reported in literature [9, 11].

Catalytic experiments. Figure 2 shows reactivity vs

p H for 2,4-ionenes of different number-average m o l a r mass (At" n = 1740 and At n = 6600). Both curves show a broad m a x i m u m around p H = 8. The curves indicate an all-over lower activity for Mn = 6600. The p H o p t i m u m suggests a dependence o f the reactivity on the local thiolate anion concentration (pK~ (ME) = 9.6 [13]). The m o d e r a t e decrease of reactivity above p H = 8 is thought to be caused by an increase of ionic strength of the reaction mixture, including competitive ion effects ( R S - and O H ). This effect is

Table 1. M o l a r absorptivity (E) o f v a r i o u s ionene analogues at 2 = 264 n m (Scheme 4) C o m p o u n d M E (10 d m 2" mol t ) 8 230 352 9 396 344 10 458 778* 11 790 761" *Two c h r o m o p h o r i c g r o u p s p e r m o l e c u l e .

Table 2. C o m p a r i s o n between molar m a s s ( ~ , ) of a m i n a t e d 2,4-ionenes determined by the p h o t o m e t r i c and

titration m e t h o d s P h o t o m e t r i c T i t r a t i o n 1810 1740 2650 2750 8800 10,100 16,600 18,300

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944 K. H. VAN STREUN et al. 10 10 8 7 E 6 - o LU 6 4 E n,- 2 Fig. 2. i 6 7 8 9 10 11 pH

Dependence of catalytic activity on pH [CoTSPc]=2 x 10-Tmol'dm 3; [ME]=0.14mol.dm-3; [ME] = 0.14mol.dm -3, [N+]= 10-3tool.din -3, 25°C. VI:

/~. = 1740; A: ~ . = 6600. -'-'. 8 a.., 2 = 0 5 10 15 20 25 102[ME'I (moL. dm -3)

Fig. 3. Catalytic activity as a function of the substrate (2-mercaptoethanol) concentration. [CoTSPc] = 2 x 10-Tmol.dm -3, p H = 8 . 2 [N+]=10-3mol.dm-3; 25°C,

[]: Jk~ n = 1740; A: -~n = 6600. less pronounced than in the case of P V A m [2]. The

latter can be explained by the fact that P V A m in contrast to the ionenes suffers f r o m coil de- p r o t o n a t i o n as p H increases [8]. The reactivity of the m o n o m e r i c analogues o f 2,4-ionene and the conventional system (without polymer) are listed in Table 3. C o m p a r i n g reaction rates in Table 3 and Fig. 2, it can be seen that a m i n i m u m cationic charge density on the polymer backbone is required to increase catalytic activity. Similar m o l a r mass dependence was observed for P V A m [8]. Only a few quaternary a m m o n i u m groups are involved in binding the cobalt complex to the polymer backbone. The residual quaternary a m m o n i u m groups of the backbone are available to create a cationic polymer d o m a i n thus increasing the local thiolate anion concentration. F o r PVAm, the dependence o f the reaction rate on thiol concentration can be described by M i c h a e l i s - M e n t e n kinetics [6]:

k l k 2

E + S , - - 2 ES , E + P

k - I

in which E stands for catalyst (CoTSPc), S for substrate (mercaptoethanol) and P for products (disulfide). It can be derived that

k2[E0][S] R =

KM + [S] which can be rewritten as

R -1 = (k2' [ E o ] ) - I +

KM(k:"

E o ) - 1 . [S]-'

Table 3. Reactivity of some monomeric analogues of 2,4-ionene, and of the conventional system. Conditions are as follows: pH = 8.2; [CoTSPc] = 2 x l0 -7 mol.dm-3; [ME] = 0.14 mol.dm-3; IN+]= l0 -3 tool.din -3 (except for the conventional system); 25°C

10 4 Reaction rate

System (tool ME dm-~'sec -I) Tetra methylammonium-hydroxide 0.29

N,N'-diethyI.N,N,N',N'-tetramethyl

ethylene diammonium dibromide 0.29 OH (conventional) 0.27

where R stands for the reaction rate (expressed in mol M E . d m - 3 . s e c - J ) , K u for the Michaelis constant [(k_ i + k2)/k~] and [E0] for the initial catalyst concentration. K s is the substrate binding constant

(kt/k_l)" k: is also called the turnover n u m b e r (di- mension sec -1) In Fig. 3 the reactivity is plotted vs the thiol concentration. It appears that a plateau is reached at relatively low thiol concentration, whereas deviations from saturation behaviour are observed for high thiol concentrations ( > 0 . 1 5 m o l ' d m - 3 ) . A n y possible deviation from Michaelis-Menten kinetics should manifest itself in the non-linearity o f the so-called L i n e w e a v e r - B u r k plot, i.e. a double reciprocal plot o f reactivity vs thiol concentration as shown in Fig. 4. In spite o f some non-linearity, it seems justified to draw some major conclusions from these curves. F r o m the slope, the turnover number and Michaelis constant can be estimated, both are listed in Table 4. Assuming that k2~k_~ i.e.

2400 2000 ~E 1600

?

i 1200 o E -.- BOO 400 = i '0 40 60 [ M E ] -1 (tool -1 drn 3)

Fig. 4. Lineweaver-Burk plot. Reaction conditions as for Fig. 3.

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Oxidative coupling of thiol 945 Table 4. Comparison between the turnover numbers (k2) and

Michaelis constants (KM) of 2,4-ionene and PVAm containing systems. Reaction conditions are as in Table 3

10 s. KM k2 (sec ') (mol.dm -3) 2,4-ionene ~ t = 1740 3700 + 200 l 1.3 + 2.3

hT. = 6600 3300 + 200 6.8 + 1.4 PVAm h4. = 3 x 10 a 2800 + 200 90.0 + 20

K M ~ - K s ~, the smaller Michaelis c o n s t a n t s for the

ionenes indicate a s t r o n g e r s u b s t r a t e b i n d i n g as com- p a r e d w i t h P V A m . T h e reactivity vs the c o b a l t complex c o n c e n t r a t i o n is s h o w n in Fig. 5. The o b s e r v e d linearity indicates the absence o f mass t r a n s p o r t l i m i t a t i o n s (identical r e a c t i o n c o n d i t i o n s were used). T h e slopes o f the curves, i.e. the t u r n o v e r n u m b e r s , are 3600sec -~ a n d 3 2 0 0 s e c - l for h,~to= 1740 a n d ~,7'. = 6600, respectively, a n d c o n f i r m the k2 d a t a from the L i n e w e a v e r - B u r k plots. T h e effect o f i o n e n e c o n c e n t r a t i o n o n the catalytic activity is s h o w n in Fig. 6. T h e i o n e n e c o n c e n t r a t i o n is expressed as [N + ], calculated f r o m the n i t r o g e n c o n t e n t o f 2,4-ionene d e t e r m i n e d by e l e m e n t a l analysis (i.e. 8.37%). A p p a r - ently, a m i n i m u m i o n e n e c o n c e n t r a t i o n is required to e n h a n c e the r e a c t i o n rate. As expected, f u r t h e r increase o f the i o n e n e c o n c e n t r a t i o n will d i m i n i s h the thiolate a n i o n c o n c e n t r a t i o n locally, i.e. a r o u n d the active centre. In Scheme 1 the overall r e a c t i o n e q u a - t i o n is presented, which a c c o r d i n g to earlier w o r k in o u r l a b o r a t o r y [6] results from:

2 R S H + 0 2 -~ R S S R + H:O2 (a) 2 R S H + H202 -~ R S S R + 2 H 2 0 (b)

4 R S H + 02 ~ 2 R S S R + 2 H 2 0

T h e s t o i c h i o m e t r y o f the r e a c t i o n can be checked by m e a s u r i n g the peroxide c o n t e n t after the r e a c t i o n by i o d o m e t r y [14]. T h e results are listed in T a b l e 5. The h i g h residual p e r o x i d e c o n t e n t (relative to the initial thiol c o n t e n t ) at low thiol c o n c e n t r a t i o n is very striking. A p p a r e n t l y , u n d e r these c o n d i t i o n s , the thiol

14 12 I U ~ 8 ~ 8 E n~ 4 2 0 1 2 3 4 5 10 7 [ C o T S P c ] (mot'dm -3)

Fig. 5. ° Catalytic activity vs CoTSPc concentration. [ N + ] = 1 0 3mol.dm 3; p H = 8 . 3 , [ M E ] = 0 . 1 4 m o l . d m - S ; 25°C, I-q: /1~ n = 1740; /k: ~'. = 6600. 10 8 E 6 h i E 4 c : 2 f i i f -7 -6 -5 -4 -3 -2 -1 Log [N+'I (mot.dm -3)

Fig. 6. Catalytic activity vs ionene concentration (expressed in [N ÷ ], see text), pH = 8.2; [CoTSPc] = 2 × I0 7mol.dm 3, [ME] = 0.14mol.dm 3, 25oc, Vq:

M, = 1740; A: z~ n = 6600.

a n d peroxide c o n c e n t r a t i o n s in the b u l k are t o o low for the second o x i d a t i o n step (b) to occur. This u n f o r t u n a t e l y effects the s t o i c h i o m e t r y o f the reaction. T h e o b s e r v e d ( a p p a r e n t ) r e a c t i o n rate, as calculated f r o m the m e a s u r e d oxygen u p t a k e a n d the a s s u m e d s t o i c h i o m e t r y o f the overall reaction, will be higher t h a n the " t r u e " reaction rate based o n thiol conversion, in p a r t i c u l a r at low thiol c o n c e n t r a t i o n s . This will influence the L i n e w e a v e r - B u r k plot in such a way t h a t the " t r u e " c o n s t a n t s , k2 a n d K M, will be smaller a n d greater, respectively, t h a n the calculated ( a p p a r e n t ) ones. These d e v i a t i o n s are negligible a t thiol c o n c e n t r a t i o n s used t h r Q u g h o u t the p r e s e n t kinetic investigation, i.e. 0.14 m o l . d m - 3.

CONCLUSIONS

T h e p h o t o m e t r i c a n d t i t r a t i o n t e c h n i q u e s corre- s p o n d well a n d are satisfactory m e t h o d s o f deter- m i n i n g the m o l a r m a s s o f 2,4-ionene. T h e latter polyelectrolyte shows a b e h a v i o u r similar to P V A m in the catalytic o x i d a t i o n o f thiols in the presence o f CoTSPc. A d v a n t a g e s o f the i o n e n e c o n t a i n i n g system over the P V A m c o n t a i n i n g system are: (1) larger t u r n o v e r n u m b e r s ( 3 2 0 0 - 3 6 0 0 s e c - ] ) , (2) smaller Michaelis c o n s t a n t s [(6.8-11.3) × 1 0 - 3 m o l . d m -s] indicative o f a s t r o n g e r s u b s t r a t e - b i n d i n g , a n d (3) a p H i n d e p e n d e n t cationic c h a r g e o n the p o l y m e r b a c k b o n e , p r o v i d i n g h i g h activity o v e r a wider p H range. M o n o m e r i c a n a l o g u e s o f 2,4-ionene s h o w e d n o increase o f the catalytic activity as c o m p a r e d with Table 5. Residual peroxide content of the reaction mixture. Reaction

conditions as described in Table 3 M, = 1740 /14, = 6600 RSH (tool) H202 (mol) % a* H:O 2 (mol) % a*

1.4x 10 s 5.63x 10 4 79.5 5.12× 10 4 73.2 1.4x 10 2 5.35 x 10 4 7.6 4.41 × 10 4 6.3 2.1 × 10 -2 2.56 × 10 -4 2.4 2.60 x 10 -4 2.5 *Percentage of thiol reacting with oxygen only, i.e. thiol con-

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946 K . H . VAN STREUN et al. the conventional (polymer-free) system. However,

relatively low m o l a r mass ionene (,~7 n = 1740) appeared to enhance the reaction rate dramatically. Evidently, a m i n i m u m cationic charge density on the polymer backbone is necessary to induce the observed rate accelerations. The residual peroxide content at low thiol concentration, as determined by iodometry, is relatively high, indicating that the assumed overall stoichiometry o f the reaction is disturbed at low thiol concentration.

Acknowledgements--This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).

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