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Synthesis and alkali cation binding properties of photodimerizable polymers with differently sized crown ethers
Vol. 24, No. 5, pp. 403-410, 1988 Printed in Great Britain. All rights reserved
0014-3057/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press pie
Eur. Polym. J.
SYNTHESIS A N D ALKALI CATION BINDING PROPERTIES OF PHOTODIMERIZABLE POLYMERS WITH D I F F E R E N T L Y SIZED CROWN ETHERS MASAMITSU SHIRAI, MITSURU KUWAHARA and MAKOTO TANAKA Department of Applied Chemistry, Faculty of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received
20 A u g u s t 1987)
Abstract--Polymers, with differently sized crown ethers as alkali cation binding sites and photodimerizable cinnamoyl moieties in their side-chains, were prepared by the cationic copolymerization of the corresponding monomers. The form of the crown-cation complexes (l : l or 2 : l) was studied by measuring quantum yields for the photodimerization of the crown-connected cinnamoyl units in the presence of alkali metal chlorides and by measuring the shift of )~ax of alkali metal picrates in THF on addition of the crown polymers. The alkali metal cation binding ability and selectivity of the photodimerizable polymers with different crown ethers (studied by a method of picrate salts extraction) were compared with those of blended samples of the photodimerizable polymers with one kind of crown ether unit. On irradiation with u.v. light, the crown-connected cinnamoyl moieties of the polymers dimerized even in dilute solutions. Studies were made of the effects on the alkali cation binding ability and selectivity of: the photoThe effects on the alkali cation binding ability and selectivity of: the photodimerization of the cinnamoyl moieties of the polymers; the degree of photodimerization of the polymers; and the kind of alkali cations used as templates during the photodimerization of the polymers; were studied.
INTRODUCTION The synthesis of many types of crown ethers and their alkali metal cation binding properties have been intensively studied [1]. An apparent relationship between cation diameter and crown ether hole-size has been widely accepted in the alkali cation binding by crown ethers. Immobilization of crown ether units on polymer supports is an interesting subject. The specificity in the cation binding properties of poly (crown ether(s)) is such that they prefer the binding of cations which are too large to fit in the cavities of crown ethers by forming 2:1 crown ether cation complexes, as a result of a co-operative action of neighbouring units[2]. The co-operative action is significantly affected by the relative positions of the neighbouring crown ether units attached to the polymer backbone [3, 4]. Synthesis and alkali cation binding properties of poly (crown ether(s)), bearing photodimerizable cinnamoyl units have been studied [5-8]. The fixing of the relative positions of two crown ether moieties by the photodimerization of the crown-connected cinnamoyl units has been shown to enhance the alkali cation binding ability of the polymers, especially for large cations. The use of alkali metal cations as a template during the photodimerization showed a significant effect on the cation binding properties of the photoreacted polymers. In a previous paper [9], the syntheses and cation binding properties of photodimerizable polymers bearing two kinds of differently sized crown ethers, e.g. 1 2 - c r o w n ~ and 15-crown-5; 15-crown-5 and 1 8 - c r o w n ~ ; 12-crown-4 and 18-crown-6, were studied. The cation binding properties of photodimerizable polymers bearing different crown ethers were found to differ significantly from those of polymers with one kind of crown ether. E P . J 24'5 - A
In the present study, synthesis and cation binding properties of photodimerizable polymers which have two kinds of crown ether moieties, i.e. 21-crown 7 units and one of the following crown ether units, 12-crown-4, 15-crown-5 and 18-crown-6; were studied. The effects of the photodimerization of the crown-connected cinnamoyl moieties and of template cations during the photodimerization on the alkali cation binding ability and selectivity were also studied. EXPERIMENTAL Materials 1 , 1 7 - D i c h l o r o - 3, 6, 9 , 1 2 , 1 5 - p e n t a o x a h e p t a d e c a n e (b.p. 180°/10 -3 mmHg) was obtained in 70% yield from SOC12 and hexaethylene glycol, which was synthesized in 26% yield from diethylene glycol, bis-2-chloroethyl ether and sodium [10]. 2,3, - (4" - F o r m y l b e n z o ) - 1,4, 7,10,13,16,19- h e p t a o x a e y c l o h e n e i c o s a - 2 - e n e (1). To a refluxing mixture of 20 g (0.15 tool)
of 3,4-dihydroxybenzaldehyde in 420 ml of n-butanol and 17 g (0.30 mol) of KOH in 16 ml of water, 46.3 g (0.15 mol) of 1,17-dicbloro-3,6,9,12,15-pentaoxaheptadecane was added dropwise and refiuxed overnight. After cooling the reaction mixture to room temperature, it was acidified with 18% HC1 and filtered. The filtrate was concentrated under reduced pressure. The oily residue was continuously extracted with hot n-heptane, which upon cooling yielded slightly yellow crystals. After recrystallization from nheptane, pure product was obtained in 37.8% yield which had the following characteristics: m.p. 35-39°; role = 384; LH-NMR (in CDC13), 6 =9.72 (s, 1H, CHO), 7.5~.8 (m, 3H, aromatic) and 4.5 3.4 ppm (m, 24H, -CH2--); i.r. (in CHC13), 1690 (C=O) and 1160-1080cm -L (C--O). Analysis calculated for CL9H2808 0.5H20: C, 57.99%; H, 7.44%. Found: C, 58.38%; H, 7.55%. 4" - (2,3- B e n z o - 1,4, 7 , 1 0 , 1 3 , 1 6 , 1 9 - h e p t a o x a c o s e n e ) acrylic a c i d (II). A mixture of 5 g
403
- 2- c y c l o h e n e i -
(0.013 mol of I,
404
MASAMITSU SHIRAI
et al.
(CH2- - c n ) ~
--(CH2--CH) 1-X
12-21P
X
o
0
CH2
CI-I2
CH2
CH2
0
0
c=o
C=O
CH
CH
CH
ell
FI
18-21P
12P
15P
18P
1
15--21P 2
3
1
2
3
21P -
0.49
0.53
0.59
0
0
0
1
Scheme 1
4.1 g (0.039 mol) of malonic acid, 19 ml of pyridine and 0.6 ml of piperidine was refluxed for 5 hr. After cooling to room temperature, the reaction mixture was poured with stirring into a mixture of 24 ml of concentrated HCI and 40 g of ice. The reaction mixture was extracted with chloroform. After removal of chloroform, the product was purified by recrystallization from ethanol/water (3:l,v/v): yield 71.3%; m.p. 124-126°; m/e = 426; tH-NMR (DMSO-----d) 6 = 7.7--6.9 (m, 3H, aromatic) and 4.4-3.3 ppm (m, 24H,---CH2--); i.r. (KBr), 1690 (C=O), 1630 (C=C) and 1170-I060cm -t (C---O). Analysis calculated for C21H3009 0.5H20: C, 57.91%; H, 7.19%. Found: C, 57.82%; H, 7.30%.
2-Vinyloxyethyl 4"-(2,3-benzo-l,4,7,10,13,16,19-heptaoxa2-cycloheneicosene)acrylate (21M). A mixture of 4.1 g (0.0091 tool) of the sodium salt of II, 25 ml of 2-chloroethyl vinyl ether, 0.058 g of dry triethylmethylammonium iodide and 0.03 g of hydroquinone was refluxed for 5 hr. After cooling to room temperature, the reaction mixture was filtered. The filtrate was concentrated under reduced pressure. The oily residue was extracted with hot mixed solvent (n-heptane/benzene, 9:1), which upon cooling yielded slightly yellow crystals. The monomer 21M was recrystallized from n-heptane/benzene (9:1), which had the following characteristics: yield 58.5%; m.p. 47-49°; m/e = 4 9 6 ; IH-NMR (CDCI3) , 6 = 7.5~5.8 (m, 3H, aromatic) and 4.5-3.6 ppm (m, 28H, --CH2--); i.r. (in CHC13). 1710 (C----O), 1630 (C----C) and 1300-1100 cm -l (C--O). Analysis calculated for C25H36010: C, 60.46%; H, 7.32%. Found: C, 59.99%; H, 7.45%.
The preparations of monomers, 2-vinyloxyethyl 4'-(2,3benzo - 1,4,7,10- tetraoxa- 2-cyclododecene)acrylate (12M), 2-vinyloxyethyl 4'-(2,3-benzo-1,4,7,10,13-pentaoxa-2-cyclopentadecene(acrylate (15M) and 2-vinyloxyethyl 4'-(2,3benzo - 1,4,7,10,13,16 - hexaoxa - 2 - cyclooctadecene)acrylate (I 8M) have been reported [9]. Picrate salts were prepared by neutralizing picric acid with alkali metal hydroxides [11]. The salts were purified by recrystallization from ethanol or ethanol-water and thoroughly dried under vacuum. THF for the measurements of 2max or picrate salts was dried on sodium and distilled carefully prior to use. Inorganic chemicals were reagent grade, used without further purification.
Polymerization Polymerizations were carried out under moisture-free conditions at - 7 8 ° in dry dichloromethane distilled from P205. Borontrifluoride etherate was used as an initiator. The copolymer composition was determined from the proton intensity ratio, 6 = 3.5-4.4 (oxyethylene protons) and 6 = 6.5-7.6 (aromatic protons) of the tH-NMR spectrum of polymers in CDC13. The polymerization conditions and characteristics of polymers are shown in Table 1. All polymers were soluble in common organic solvents such as 1,4-dioxane, benzene, THF, chloroform and dichloromethane. The preparations of 12P, 15P and 18P have been reported [9]. The molecular weights determined from VPO were 6300, 11,000 and 7300 for 12P, 15P and 18P, respectively.
405
Binding properties of photodimerizable polymers
Polymer 21P 12-21P 15-21P 18-21P
12M 0.37
Table 1. Preparation and characteristics of polymers Monomer (g) BF3OEt2 CH2CI: Polymerization time 15M 18M 21M (mmol) (ml) (hr) --0.41 ---
---0.46
1.0 0.50 0.50 0.50
0.43 0.22 0.22 0.22
8.0 7.0 7.0 7.0
Yield (%)
~,~
Xb
80 87 86 79
23,000 14,000 11,000 21,000
1.0 0.49 0.53 0.59
7.5 9.5 7.5 7.5
aFrom VPO. bRefer to Scheme l. Table 2. Preparation conditions and characteristics of photoreacted polymers' Phototransformed polymer 12-21P/Cs + Original polymer Template Concentration of template (mol/l) Dimerization degree (%)
12-21P CsC1 0.01 58
15-21P/K + 15-21P KCI 0.01 65
18-21P/K +
18-21P/Cs +
18-21P KCI
18-21P CsCI
0.01 55
0.01 64
21P/Cs + 21P CsCI 0.01 57
alrradiation with 313 nm light was carried out in a THF/methanol (1:1, v/v) mixed solvent at Y. [Cinnamoyl units] = 0.01 mol/1.
Photoreaction of polymers The photoreaction of polymers for extraction experiments was carried out using a T H F / m e t h a n o l (1:1, v/v) solution in the presence of salts as a template. A sample solution (polymer conc. < 1.0 × 10 -2 mol/l) was placed in a quartz cell (1 × I x 4 c m ) and irradiated at 5 ° with a 7 5 W highpressure Hg lamp (Toshiba SHL-100 UV) at a distance of 1 cm. Light of 313 n m wavelength was employed for the irradiation using a filter (Toshiba UV-D33S). The irradiated polymers were dialyzed through a semipermeable membrane against water for 2 days. The dialyzed solution was evaporated to dryness and the photoreacted polymers were recovered, being subjected to the extraction experiments. The degree of photodimerization of cinnamic acid ester moieties was determined by measuring the decrease in the absorbance at 1630 cm-~ due to the C--------Cgroups of the cinnamic acid esters. The irradiation conditions and the characteristics of the irradiated polymers with template salts are summarized in Table 2. The photoreaction of the polymers without templates was carried out in T H F at 5 ° with 313 n m light and the degree of photodimerization was 56 __+2% for all polymers examined. The q u a n t u m yields, ~, for the photodimerization of the cinnamoyl moieties o f the polymers were determined in air as the n u m b e r of moles of cinnamoyl moieties that disappeared per einstein of light at 366 nm, absorbed by the cinnamoyl moieties at 5 ° in T H F or in T H F / m e t h a n o l (vol ratio 1:I). The degree of photodimerization was usually kept below 20%. The intensity of irradiation light (1.6 × 10 8 einstein/cm 2 sec) was determined with a chemical actinometer (potassium ferrioxalate) [12].
Extractions The cation binding etficiencies of the polymers were evaluated by distribution equilibria of the picrate salts between an aqueous phase and an immiscible organic phase (chloroform). To 8 ml of a chloroform solution o f the polymer (3.0 x 10 -4 mol/l) was added an equal volume of an aqueous picric acid solution (6.55 x 10 -5 mol/1) containing excess metal hydroxide (1.0 x 10 -2 mol/l). After efficient agitation for 20 min at 35 °, the concentration of picrate in the aqueous phase was determined spectrophotometrically. The picrate anion in water phase has its absorption maxi m u m at 355 nm; with ¢ being 1.44 x 1041/mol cm, A Shimadzu UV-200S recording spectrophotometer was used. It was ascertained that the transference of polymers into the aqueous phase was negligibly small ( < 2%). Furthermore,
no picrate salts were transferred to the chloroform phase in the absence of crown polymers. The extractability ( f ) was determined as follows: f=
100 x ([Co]-[C])/[Co],
where [Co] and [C] are the concentrations of picrate salts in the aqueous phase before and after extraction, respectively.
RESULTS AND DISCUSSION T h e p i c r a t e s a l t s in l o w p o l a r i t y m e d i a like T H F f r e q u e n t l y e x h i b i t e d p r o n o u n c e d s h i f t s in t h e i r o p t i cal s p e c t r a in t h e p r e s e n c e o f c r o w n e t h e r s . T h e s h i f t s e s s e n t i a l l y o c c u r w h e n a t i g h t i o n - p a i r is c o n v e r t e d to a l o o s e i o n - p a i r b y b i n d i n g t h e c o u n t e r c a t i o n s to t h e c r o w n e t h e r s , l e a d i n g to a s i g n i f i c a n t i n c r e a s e in t h e i n t e r i o n i c d i s t a n c e o f t h e t i g h t i o n - p a i r . It is k n o w n that the formation of externally crown coordinated 1:1 t i g h t i o n - p a i r c o m p l e x e s in T H F s h o w s a shift o f 2maX o f a few n m . O n t h e o t h e r h a n d 2max o f alkali m e t a l p i c r a t e s in T H F s h i f t s to 380 n m w h e n t h e y f o r m 2:1 c r o w n - c o u n t e r c a t i o n c o m p l e x e s ( s a n d w i c h t y p e c o m p l e x e s ) [13]. 2max o f p i c r a t e s a l t s in T H F in the presence and absence of photodimerizable poly ( c r o w n e t h e r ( s ) ) as a d d i t i v e s is s h o w n in T a b l e 3. In t h e a b s e n c e o f p o l y m e r s , 2max w a s f o u n d at 351, 357,
Table 3. Absorption maxima of picrate salts in THF in the pressure of polymers~ ;.m~ (nm) Additive b None 12P 15P 18P 21P 12-21P 15-21P 18-21P
NaPi
KPi
RbPi
CsPi NH4Pi
351 352 ~ 360 357 351 355 358
357 358 379 368 367 365 378 367
358 c 380 374 369 370 374 373
360 ~ 367 380 372 377 375 378
~[Picrate salts] = 5.0 x 10-5 mol/l, at 25'k b[Crown units] = 2.5 x 10-4 mol/l. ¢No determination.
350 c --~ 360 358 358 359 360
MASAMITSU SHIRAI et al.
406 --
CH2--CH.
CH2~
C H 2 - - CH
CH - -
C H 2 - - CH - -
O
0
O
0
CH2
CH2
CIt2
CH2
CH2
CH:
CH2
CH2
O
0
O
0
C=O
C=O
C=O
C=O
CH
CH
CH
CH
CH
CH
CH
CH
hv
II
Scheme 2 358, 360 and 350nm for sodium picrate (NaPi), potassium picrate (KPi), rubidium picrate (RbPi), cesium picrate (CsPi), and ammonium picrate (NH4Pi), respectively, in general agreement with reported values [13]. The 2max values around 377-380nm were observed for the systems of 15P-KPi, 15P-RbPi, 18P-CsPi, 18-21P-CsPi, 15-21P-KPi and 12-21P~CsPi, suggesting the formation of sandwich type complexes. The findings generally agree with the view that alkali metal cations, which are too large to fit in the cavity of crown ethers, can be bound in the sandwich type. It is noteworthy that •rnax of CsPi in the presence of 12-21P was observed at 377 nm, suggesting sandwich type binding of Cs + by 12-21P. As expected from the cation diameter-crown hole-size relationship, 21P binds Cs ÷ in the 1 : 1 form but 12P does not bind Cs +. Thus the co-operativity of 12-crown-4 and 21-crown-7 units may play an important role in the Cs ÷ binding by 12-21P. In our earlier work [9], a similar peculiar binding type was observed in the 12-18P-KPi system, where K ÷ was bound to 12-18P in the sandwich type, although 18-crown-6 units binds K + in the 1:1 form and 12-crown--4 units does not bind K ÷. It is known that cinnamic acid ester groups attached to a polymer chain can dimerize when irradiated with u.v. light [14]. When the poly(crown ether(s)) with cinnamic acid ester moieties were irradiated in dilute solutions (polymer conc. < 10 -2 mol/l), the i.r. spectrum of the irradiated polymers showed a remarkable decrease in the absorbance of the C------Cbond at 1630cm -1 and a shift in the absorption peak due to C-------Ogroups from 1710 to 1730 cm -1. The u.v. spectra showed a remarkable decrease in the absorbance at 325 nm due to cinnamic
acid ester groups. These findings suggest that the crown-connected cinnamoyl groups attached to the polymer chain can dimerize on irradiation even in dilute solution, as reported for cinnamoyl groups in poly[2-(cinnamoyioxy)ethyi methacrylate] [15]. Under the present reaction conditions, the dimerization seemed to mainly proceed intramolecularly because insoluble materials did not appear. The intramolecular photodimerization may not always occur between adjacent cinnamoyl units. It has been reported that ~,og-dicinnamates, in which the cinnamate groups are separated by 21-35 bonds, can be cyclized by intramolecular addition [16]. The stereochemistry of the cyclobutane derivatives which are formed by the photodimerization of the pendant cinnamoyl units is interesting and it has been studied for poly(vinyl cinnamate) in the solid state by several workers [17]. In the present study, however, the structure of the photocyclized products is not elucidated. The • values for the photodimerization of cynnamoyl units of the polymers may increase when two cinnamoyi units come in closer contact by forming sandwich type complexes between cation and crown ether units as vicinal substituents of cinnamoyl units. The formation of 1:1 complexes of cation-crown ether unit repels the cinnamoyl groups by an electrostatic repulsion of the cation bound to the crown ether units, resulting in a decrease in 4. Thus • is a useful empirical parameter to estimate the type of cation-crown complexes. The • values for the polymers in the presence and absence of salts are shown in Table 4. The • values for T H F solution of polymers in the absence of salts were in the range of 0.035 to 0.047, being slightly dependent on the ringsize of crown ether units. The • values of polymers
Binding properties of photodimerizable polymers
407
Table 4. Effect o f salts on the q u a n t u m yields, O, for the photodimerization o f polymers a O Additive
12P
15P
18P
21P
12 21P
15-21P
18-21P
None LiCI NaCI KCI
0.035 b ~ 0.064 0.039
0.041 b 0.021 0.038 0.11
0.035 b 0.023 d 0.027 0.009
0.036 b 0.014 0.027 0.019
0.042 h 0.013 d 0.018 0.020
0.047 b 0.016 0.027 0.019
RbCI
0.032
0.12
0.11
0.021
CsCI
0.047
0.061
0.17
--c
~
0.026 0.046 c 0.022
0.044 0.10 c 0.047 0.11 ~ 0.029
0.045 b 0.019 0.030 0,025 0.056' 0.036 0.075 c 0.032 0.087 c 0.020
NH4CI
--~
aO was determined with 3 6 6 n m light in T H F / m e t h a n o l (1:1, v/v) at units] = 0.01 mol/l, [salts] = 5.0 × 10 3 mol/I. bln T H F at 5 ~. C[Salts] = 0.01 mol/l. do was determined at 2 5 because o f an appearance o f turbidity at 5 . CNo determination.
in the presence of alkali metal chlorides were determined in THF/methanol (1:1, v/v) because of the low solubility of salts in THF. The • values for 12P, 15P and 18P increased on addition of cations which are known to be bound to respective crown units in the sandwich form, i.e. 12P-Na ÷, 15P-K ÷, 15P-Rb ÷ 18P-Rb ÷ and 18P-Cs ÷ systems. The decrease in values was observed for 18P-K ÷ and 21P-all cations examined systems. The • value for 12-21P in the presence of Cs ÷ was higher than that in the absence of Cs ÷. A large increase in • was observed at higher concentrations of templates. Thus, in this case, a co-operative action of 12-crown-4 and 21-crown-7 units seems to take part in the Cs ÷ binding in the sandwich form, being consistent with the suggestions from the shift of '~max of CsPi in the presence of 12-21P (Table 3). The increased • value was observed for 12-21P-Rb ÷ system, although the shift of RbPi in the presence of 12-21P is not so obvious. The values of 15-21P at high concentration of KC1, RbCI and CsCI were found to be larger than those in the absence of salts. Furthermore, the • values for 18-21P were enhanced on addition of CsCI. As will be discussed later, the increase in • for 15-21P and 18-21P was not due to the co-operative action of the differently sized crown ether units of the polymers but due to the preferential binding of cations in the sandwich type by 15-crown-5 units in 15-21P or 18-crown-6 units in 18-21P. The alkali cation binding ability and selectivity of the photodimerizable poly(crown ether(s)) and their photoreacted analogues were studied by the liquid-liquid extraction of alkali metal picrates. Figure 1 shows the effect of the dimerization degree on the extraction of picrate salts by the polymers which were obtained by the irradition in the presence and absence of salts as templates. Although the extractability of CsPi with 18-21P irradiated without templates slightly increased with an increase in the dimerization degree, the extractability of CsPi with 18-21P irradiated with CsC1 template increased considerably with increasing dimerization degree. Similar results were observed for other polymers examined. Thus the effect of photodimerization on the alkali metal cation extractability was studied under almost constant degree of dimerization.
0.023 0.056 0.084 ~ 0.020
5'.
[Cinnamoyl
Figure 2 shows the alkali metal cation extractability of 21P and its photoreacted analogues. The cation extractability of 21P decreased in the order Cs ÷ > Rb + > K + >>Li + ~ Na + ~ N H ] . Although the order was the same as that for 21M, the cation extractability of 21P was higher than that of 21M. The photodimerization of 21P with and without Cs + as a template did not significantly change its alkali cation binding ability and selectivity. The cation extractability was scarcely influenced by the photodimerization of the crown-connected cinnamoyl units, when the cation-crown 1:1 complexes are predominantly formed. Figure 3 shows the alkali cation extractability of 12-21P and its photoreacted analogues. The cation selectivity order of 12-21P was the same as that of 21P. However, the cation selectivity of Cs ÷ to K + and Rb ÷ for 12-21P was improved compared with that of 21P. It has been reported that the cation binding ability of the photodimerizable poly(crown
!
5oi
/
40
30
/ ._._.....o.___---------o-----
20
"~ 0
L
I
t
I
I
20
40
60
80
100
D i m e r i z a t i o n degree (°/o)
Fig. I. Effect of the photodimerization degree of 18 21P on the extractability of CsPi at 35°. Irradiation of 18--21Pwith 313 nm light was carried out with Cs + template (0) and without templates (©). Extraction conditions: [crown units] = 3.0 x l0 -4 mol/I; [CsOH]= 0.01 tool/l; [picric acid] = 6.55 x 10-5 mol/l.
408
MASAMITSU SHIRAI et al. 10
60
5
40
v,..
20
O Li +
Na +
K+
Rb +
Cs +
NH +
Cation
Fig. 2. Extraction of alkali metal picrates at 35° by 21P (O), 21P/Cs+ ( • ) and 21P phototransformed without templates ([3). Extraction conditions: [crown units] = 3.0 x 10-4mol/1; [metal hydroxide] = 0.01 tool/l; [picric acid] = 6.55 x 10-5 mol/l.
.9 Li +
No +
K+
Rb+
Cs +
NH~
Cation
ether(s)) is considerably enhanced by the photodimerization, when the cations are bound to the polymers in the sandwich type [5-8]. Although the Cs ÷ binding of 12-21P in the sandwich type was suggested (see Tables 3 and 4), the photodimerization of 12-21P with and without Cs ÷ template gave no significant effect on its cation binding ability and selectivity. This may be due to the fact that the Cs ÷ binding by 12-21P in the sandwich type is not so stable as that previously observed for K ÷ - I 5 P and Cs÷-18P systems. The stability of a crown-cation complex in an aqueous phase governs the extractability. The more stable the crown-cation complex in an aqueous phase the more extractable it is. The cation binding ability of 18-21P and its photoreacted materials is shown in Fig. 4. The alkali cation selectivity order for 18-21P was found to be Cs ÷ > K + > Rb+>>Li ÷ ~ Na + ~ NH~-. The cation
10
Li +
No +
K+
Rb +
Cs +
NH:
Cation
Fig. 3. Extraction of alkali metal picrates at 35° by 12-21P (O), 12-21P/Cs÷ (0) and 12-21P irradiated without templates (I--1). Extraction conditions: [crown units] = 3.0 x 10-4mol/l; [metal hydroxide] =0.01 mol/l; [picric acid] = 6.55 x 10-Smol/I.
Fig. 4. Extraction of alkali metal picrates at 35° by 18-21P (O), 18-21P/K÷ (A), 18-21P/Cs÷ ( • ) and 18-21P irradiated without templates (r-q). Extraction conditions: [crown units] = 3.0 x 10 -4 mol/l; [metal hydroxide] = 0.01 mol/1; [picric acid] = 6.55 x 10-5 mol/l.
extractabilty of 18-21P was almost the same as that of a blended sample of 18P and 21P (molar ratio 0.41:0.59). This suggests that 18-crown~ and 21-crown-7 moieties in 18-21P independently contribute to the cation binding. The photodimerizations of 18-21P without templates and with K ÷ template caused no significant change in the cation binding ability and selectivity. On the other hand, use of Cs + template during the photodimerization caused a large increase in the binding ability for K ÷ and Rb ÷, and especially Cs ÷. The Cs ÷ selectivity of the photoreacted 18-21P with Cs ÷ template was much improved compared to the original material. The effect of photodimerization of 18-21P on the cation extractability may be explained as follows. The photodimerization of the pendant cinnamoyl moieties is possible between the cinnamoyl moieties with differently sized crown ether units and/or between the cinnam0yl groups with the same sized crown ether units. In the absence of templates, the photodimerization between 18-crown-6-connected cinnamoyl units and 21-crown-7-connected ones may occur predominantly because the molar ratio of 18-crown-6 units to 21--crown-7 units in 18-21P is almost unity. It was reported that 1:1 complex formation constants for 18-benzocrown-6--K+ and 21-benzocrown-7-K÷ systems were 708 and 891/mol, repectively at 25 ° in 99% DMSO [18]. Thus K + as template is predominantly bound to 18-crown-6 moieties of 18-21P in the 1: 1 type. Although the light absorption of 18--crown-6 connected cinnamoyl unit and 21-crown-7 connected units takes place simultaneously, the photodimerization between 18--crown~ connected cinnamoyl moieties may be retarded because of an electrostatic repulsion of the bound K ÷.
Binding properties of photodimerizable polymers
409
2o
60
15 4O
\.
A
10
2O
Li +
NO+
K+
Rb +
Cs +
NH~
Cotion Fig. 5. Extraction of alkali metal picrates at 35° by 18 21P (O), 18-21P/Cs ÷ ( 0 ) and 18P/Cs+-21P blended sample (I-q). 18P/Cs ÷ was obtained from the phototransformation of 18P with Cs + (dimerization degree: 91%). Extraction conditions: [crown units] = 3.0 x 10 -4 mol/1; [metal hydroxide] = 0.01 mol/1; [picric acid] = 6.55 x 10-5 mol/1.
As a results, the photodimerization between 2 1 - c r o w n - 7 connected cinnamoyl units or between 21-crown-7 connected cinnamoyl units and 18-crown~5 connected ones may occur. The latter case is the same as expected for the photodimerization without template salts. The Cs ÷ extractability of 18P was observed to be 5 times that of 21P. Furthermore for polymers bearing pendant 18-crown-6 units, the sandwich type complexes of crown ether units with Cs ÷ are much more stable than 1:1 type complexes [2]. Thus by using Cs ÷ template, the predominant photodimerization of 18-crown-6 connected cinnamoyl units of 18-21P occurred. Figure 5 shows the extractability of alkali metal picrate by 18-21P, 18-P/Cs ÷, and a blended sample of 21P and 18P irradiated with Cs ÷ template. The ratio of 1 8 - c r o w n - 6 / 2 1 - c r o w n - 7 units and dimerization degree of the blended sample are consistent with those of 18-21P/Cs ÷. The cation binding ability and selectivity of the blended sample were in good agreement with that of 18-21P/Cs ÷. This finding supports the assumption that 18-crown-6 connected cinnamoyl units predominantly react in the photodimerization of 18-21P with Cs ÷ template. Figure 6 shows the extractability of alkali metal picrates at 35 ° by 15-21P and its photoreacted products. The cation selectivity order of 15-21P was K + > Rb + > Cs + >>Li + ~ N a + ~ N H ~ and it was almost the same as that of an equimolar blended sample of 15P and 21P. This suggests independent contributions of 15-crown-5 and 21--crown-7 moieties in 15-21P to the cation binding. N o significant changes in the cation binding ability and selectivity
Li +
No +
K+
Rb +
Cs+
I'IH~
Cation
Fig. 6. Extraction of alkali metal picrates at 35° by 15-21P (O), 15-21P/K ÷ ( 0 ) and 15-21P phototransformed without templates (E]). Extraction conditions: [crown units] = 3.0 x 10 -a mol/l; [metal hydroxide] = 0.01 mol/l; [picric acid] = 6.55 × 10-5 mol/1.
were observed by the photodimerization of 15-21P without templates, where the photodimerization between 15-crown-5-connected cinnamoyl units and 21-crown-7-connected ones were predominant because the molar ratio of 15-crown-5-connected cinnamoyl units to 21-crown-7-connected ones in 15-21P is unity. When K ÷ template was used during the photodimerization, the photoreacted 15-21P (15-21P/K +) showed an increase in the binding for K ÷, Rb ÷ and Cs ÷. The K ÷ extractability of 15P was observed to be 10 times that of 21P. Furthermore, the polymers with pendant 15-crown-5 moieties can form much more stable complexes of the sandwich type between 15-crown-5 units and K + than 1:: 1 type complexes [2]. Thus the photodimerization of 15-21P with K ÷ template may mainly occur between 15-crown-5 connected cinnamoyl units. This conclusion was supported by the fact that the cation binding ability and selectivity of a blended sample of 21P and photodimerized 15P with K ÷ template agreed well with those of 15-21P/K ÷.
Acknowledgement--This research was partly supported by a Grant-in-Aid for General Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 61550683). REFERENCES
1. W. G. Gokel and S. H. Korzeniowski. Macrocyclic Polyether Syntheses. Springer, Berlin (1982). 2. S. Kopolow, T. E. Hogen Esch and J. Smid. Macromolecules 6, 133 (1973). 3. K. Yagi, J. A. Ruiz and M. C. Sanchez. Makromolek. Chem. Rapid Commun. 1, 263 (1980).
410
MASAMITSU SHIRAI et al.
4. K. Kimura, T. Maeda and T. Shono. Makromolek. Chem. 182, 1579 (1981). 5. M. Shirai, T. Orikata and M. Tanaka. Makromolek. Chem. Rapid Commun. 4, 65 (1983). 6. M. Sharai, T. Orikata and M. Tanaka. J. Polym. Sci., Polym. Chem. Edn 23, 463 (1985). 7. M. Shirai, A. Ueda and M. Tanaka. Makromolek. Chem. 186, 493 (1985). 8. M. Shirai, A. Ueda and M. Tanaka. Makromolek. Chem. 186, 2519 (1985). 9. M. Shirai, A. Ueda and M. Tanaka. J. Polym. Sci., Polym. Chem. Edn 25, 1811 (1987). 10. M. Newcomb, S. S. Moore and D. J. Cram. J. Am. chem. Soc. 99, 6405 (1977). 11. M. A. Coplan and R. M. Fuoss. J. phys. Chem. 68, 1177 (1964).
12. S. L. Murov. Handbook o f Photochemistry. Dekker, New York (1973). 13. M. Bourgoin, K. H. Wong, J. J. Hui and J. Smid. J. Am. chem. Soc. 97, 3462 (1975). 14. S. S. Labana. Ultraviolet Light Induced Reactions in Polymers. Am. Chem. Soc., Washington, D.C. (1976). 15. Y. Shindo, T. Sugimura, K. Horie and I. Mita. Nippon Kagaku Kaishi 184 (1984), Chem. Abstr. 100, 192454d (1984). 16. A. J. Ors and R. Srinivasan. J. chem. Soc. Chem. Commun. 400 (1978). 17. P. L. Egerton, E. Pitts and A. Reiser. Macromolecules 14, 95 (1981). 18. L. Mandolini and B. Masci. J. Am. chem. Soc. 106, 168 (1984).
0014-3057/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press pie
Eur. Polym. J.
SYNTHESIS A N D ALKALI CATION BINDING PROPERTIES OF PHOTODIMERIZABLE POLYMERS WITH D I F F E R E N T L Y SIZED CROWN ETHERS MASAMITSU SHIRAI, MITSURU KUWAHARA and MAKOTO TANAKA Department of Applied Chemistry, Faculty of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received
20 A u g u s t 1987)
Abstract--Polymers, with differently sized crown ethers as alkali cation binding sites and photodimerizable cinnamoyl moieties in their side-chains, were prepared by the cationic copolymerization of the corresponding monomers. The form of the crown-cation complexes (l : l or 2 : l) was studied by measuring quantum yields for the photodimerization of the crown-connected cinnamoyl units in the presence of alkali metal chlorides and by measuring the shift of )~ax of alkali metal picrates in THF on addition of the crown polymers. The alkali metal cation binding ability and selectivity of the photodimerizable polymers with different crown ethers (studied by a method of picrate salts extraction) were compared with those of blended samples of the photodimerizable polymers with one kind of crown ether unit. On irradiation with u.v. light, the crown-connected cinnamoyl moieties of the polymers dimerized even in dilute solutions. Studies were made of the effects on the alkali cation binding ability and selectivity of: the photoThe effects on the alkali cation binding ability and selectivity of: the photodimerization of the cinnamoyl moieties of the polymers; the degree of photodimerization of the polymers; and the kind of alkali cations used as templates during the photodimerization of the polymers; were studied.
INTRODUCTION The synthesis of many types of crown ethers and their alkali metal cation binding properties have been intensively studied [1]. An apparent relationship between cation diameter and crown ether hole-size has been widely accepted in the alkali cation binding by crown ethers. Immobilization of crown ether units on polymer supports is an interesting subject. The specificity in the cation binding properties of poly (crown ether(s)) is such that they prefer the binding of cations which are too large to fit in the cavities of crown ethers by forming 2:1 crown ether cation complexes, as a result of a co-operative action of neighbouring units[2]. The co-operative action is significantly affected by the relative positions of the neighbouring crown ether units attached to the polymer backbone [3, 4]. Synthesis and alkali cation binding properties of poly (crown ether(s)), bearing photodimerizable cinnamoyl units have been studied [5-8]. The fixing of the relative positions of two crown ether moieties by the photodimerization of the crown-connected cinnamoyl units has been shown to enhance the alkali cation binding ability of the polymers, especially for large cations. The use of alkali metal cations as a template during the photodimerization showed a significant effect on the cation binding properties of the photoreacted polymers. In a previous paper [9], the syntheses and cation binding properties of photodimerizable polymers bearing two kinds of differently sized crown ethers, e.g. 1 2 - c r o w n ~ and 15-crown-5; 15-crown-5 and 1 8 - c r o w n ~ ; 12-crown-4 and 18-crown-6, were studied. The cation binding properties of photodimerizable polymers bearing different crown ethers were found to differ significantly from those of polymers with one kind of crown ether. E P . J 24'5 - A
In the present study, synthesis and cation binding properties of photodimerizable polymers which have two kinds of crown ether moieties, i.e. 21-crown 7 units and one of the following crown ether units, 12-crown-4, 15-crown-5 and 18-crown-6; were studied. The effects of the photodimerization of the crown-connected cinnamoyl moieties and of template cations during the photodimerization on the alkali cation binding ability and selectivity were also studied. EXPERIMENTAL Materials 1 , 1 7 - D i c h l o r o - 3, 6, 9 , 1 2 , 1 5 - p e n t a o x a h e p t a d e c a n e (b.p. 180°/10 -3 mmHg) was obtained in 70% yield from SOC12 and hexaethylene glycol, which was synthesized in 26% yield from diethylene glycol, bis-2-chloroethyl ether and sodium [10]. 2,3, - (4" - F o r m y l b e n z o ) - 1,4, 7,10,13,16,19- h e p t a o x a e y c l o h e n e i c o s a - 2 - e n e (1). To a refluxing mixture of 20 g (0.15 tool)
of 3,4-dihydroxybenzaldehyde in 420 ml of n-butanol and 17 g (0.30 mol) of KOH in 16 ml of water, 46.3 g (0.15 mol) of 1,17-dicbloro-3,6,9,12,15-pentaoxaheptadecane was added dropwise and refiuxed overnight. After cooling the reaction mixture to room temperature, it was acidified with 18% HC1 and filtered. The filtrate was concentrated under reduced pressure. The oily residue was continuously extracted with hot n-heptane, which upon cooling yielded slightly yellow crystals. After recrystallization from nheptane, pure product was obtained in 37.8% yield which had the following characteristics: m.p. 35-39°; role = 384; LH-NMR (in CDC13), 6 =9.72 (s, 1H, CHO), 7.5~.8 (m, 3H, aromatic) and 4.5 3.4 ppm (m, 24H, -CH2--); i.r. (in CHC13), 1690 (C=O) and 1160-1080cm -L (C--O). Analysis calculated for CL9H2808 0.5H20: C, 57.99%; H, 7.44%. Found: C, 58.38%; H, 7.55%. 4" - (2,3- B e n z o - 1,4, 7 , 1 0 , 1 3 , 1 6 , 1 9 - h e p t a o x a c o s e n e ) acrylic a c i d (II). A mixture of 5 g
403
- 2- c y c l o h e n e i -
(0.013 mol of I,
404
MASAMITSU SHIRAI
et al.
(CH2- - c n ) ~
--(CH2--CH) 1-X
12-21P
X
o
0
CH2
CI-I2
CH2
CH2
0
0
c=o
C=O
CH
CH
CH
ell
FI
18-21P
12P
15P
18P
1
15--21P 2
3
1
2
3
21P -
0.49
0.53
0.59
0
0
0
1
Scheme 1
4.1 g (0.039 mol) of malonic acid, 19 ml of pyridine and 0.6 ml of piperidine was refluxed for 5 hr. After cooling to room temperature, the reaction mixture was poured with stirring into a mixture of 24 ml of concentrated HCI and 40 g of ice. The reaction mixture was extracted with chloroform. After removal of chloroform, the product was purified by recrystallization from ethanol/water (3:l,v/v): yield 71.3%; m.p. 124-126°; m/e = 426; tH-NMR (DMSO-----d) 6 = 7.7--6.9 (m, 3H, aromatic) and 4.4-3.3 ppm (m, 24H,---CH2--); i.r. (KBr), 1690 (C=O), 1630 (C=C) and 1170-I060cm -t (C---O). Analysis calculated for C21H3009 0.5H20: C, 57.91%; H, 7.19%. Found: C, 57.82%; H, 7.30%.
2-Vinyloxyethyl 4"-(2,3-benzo-l,4,7,10,13,16,19-heptaoxa2-cycloheneicosene)acrylate (21M). A mixture of 4.1 g (0.0091 tool) of the sodium salt of II, 25 ml of 2-chloroethyl vinyl ether, 0.058 g of dry triethylmethylammonium iodide and 0.03 g of hydroquinone was refluxed for 5 hr. After cooling to room temperature, the reaction mixture was filtered. The filtrate was concentrated under reduced pressure. The oily residue was extracted with hot mixed solvent (n-heptane/benzene, 9:1), which upon cooling yielded slightly yellow crystals. The monomer 21M was recrystallized from n-heptane/benzene (9:1), which had the following characteristics: yield 58.5%; m.p. 47-49°; m/e = 4 9 6 ; IH-NMR (CDCI3) , 6 = 7.5~5.8 (m, 3H, aromatic) and 4.5-3.6 ppm (m, 28H, --CH2--); i.r. (in CHC13). 1710 (C----O), 1630 (C----C) and 1300-1100 cm -l (C--O). Analysis calculated for C25H36010: C, 60.46%; H, 7.32%. Found: C, 59.99%; H, 7.45%.
The preparations of monomers, 2-vinyloxyethyl 4'-(2,3benzo - 1,4,7,10- tetraoxa- 2-cyclododecene)acrylate (12M), 2-vinyloxyethyl 4'-(2,3-benzo-1,4,7,10,13-pentaoxa-2-cyclopentadecene(acrylate (15M) and 2-vinyloxyethyl 4'-(2,3benzo - 1,4,7,10,13,16 - hexaoxa - 2 - cyclooctadecene)acrylate (I 8M) have been reported [9]. Picrate salts were prepared by neutralizing picric acid with alkali metal hydroxides [11]. The salts were purified by recrystallization from ethanol or ethanol-water and thoroughly dried under vacuum. THF for the measurements of 2max or picrate salts was dried on sodium and distilled carefully prior to use. Inorganic chemicals were reagent grade, used without further purification.
Polymerization Polymerizations were carried out under moisture-free conditions at - 7 8 ° in dry dichloromethane distilled from P205. Borontrifluoride etherate was used as an initiator. The copolymer composition was determined from the proton intensity ratio, 6 = 3.5-4.4 (oxyethylene protons) and 6 = 6.5-7.6 (aromatic protons) of the tH-NMR spectrum of polymers in CDC13. The polymerization conditions and characteristics of polymers are shown in Table 1. All polymers were soluble in common organic solvents such as 1,4-dioxane, benzene, THF, chloroform and dichloromethane. The preparations of 12P, 15P and 18P have been reported [9]. The molecular weights determined from VPO were 6300, 11,000 and 7300 for 12P, 15P and 18P, respectively.
405
Binding properties of photodimerizable polymers
Polymer 21P 12-21P 15-21P 18-21P
12M 0.37
Table 1. Preparation and characteristics of polymers Monomer (g) BF3OEt2 CH2CI: Polymerization time 15M 18M 21M (mmol) (ml) (hr) --0.41 ---
---0.46
1.0 0.50 0.50 0.50
0.43 0.22 0.22 0.22
8.0 7.0 7.0 7.0
Yield (%)
~,~
Xb
80 87 86 79
23,000 14,000 11,000 21,000
1.0 0.49 0.53 0.59
7.5 9.5 7.5 7.5
aFrom VPO. bRefer to Scheme l. Table 2. Preparation conditions and characteristics of photoreacted polymers' Phototransformed polymer 12-21P/Cs + Original polymer Template Concentration of template (mol/l) Dimerization degree (%)
12-21P CsC1 0.01 58
15-21P/K + 15-21P KCI 0.01 65
18-21P/K +
18-21P/Cs +
18-21P KCI
18-21P CsCI
0.01 55
0.01 64
21P/Cs + 21P CsCI 0.01 57
alrradiation with 313 nm light was carried out in a THF/methanol (1:1, v/v) mixed solvent at Y. [Cinnamoyl units] = 0.01 mol/1.
Photoreaction of polymers The photoreaction of polymers for extraction experiments was carried out using a T H F / m e t h a n o l (1:1, v/v) solution in the presence of salts as a template. A sample solution (polymer conc. < 1.0 × 10 -2 mol/l) was placed in a quartz cell (1 × I x 4 c m ) and irradiated at 5 ° with a 7 5 W highpressure Hg lamp (Toshiba SHL-100 UV) at a distance of 1 cm. Light of 313 n m wavelength was employed for the irradiation using a filter (Toshiba UV-D33S). The irradiated polymers were dialyzed through a semipermeable membrane against water for 2 days. The dialyzed solution was evaporated to dryness and the photoreacted polymers were recovered, being subjected to the extraction experiments. The degree of photodimerization of cinnamic acid ester moieties was determined by measuring the decrease in the absorbance at 1630 cm-~ due to the C--------Cgroups of the cinnamic acid esters. The irradiation conditions and the characteristics of the irradiated polymers with template salts are summarized in Table 2. The photoreaction of the polymers without templates was carried out in T H F at 5 ° with 313 n m light and the degree of photodimerization was 56 __+2% for all polymers examined. The q u a n t u m yields, ~, for the photodimerization of the cinnamoyl moieties o f the polymers were determined in air as the n u m b e r of moles of cinnamoyl moieties that disappeared per einstein of light at 366 nm, absorbed by the cinnamoyl moieties at 5 ° in T H F or in T H F / m e t h a n o l (vol ratio 1:I). The degree of photodimerization was usually kept below 20%. The intensity of irradiation light (1.6 × 10 8 einstein/cm 2 sec) was determined with a chemical actinometer (potassium ferrioxalate) [12].
Extractions The cation binding etficiencies of the polymers were evaluated by distribution equilibria of the picrate salts between an aqueous phase and an immiscible organic phase (chloroform). To 8 ml of a chloroform solution o f the polymer (3.0 x 10 -4 mol/l) was added an equal volume of an aqueous picric acid solution (6.55 x 10 -5 mol/1) containing excess metal hydroxide (1.0 x 10 -2 mol/l). After efficient agitation for 20 min at 35 °, the concentration of picrate in the aqueous phase was determined spectrophotometrically. The picrate anion in water phase has its absorption maxi m u m at 355 nm; with ¢ being 1.44 x 1041/mol cm, A Shimadzu UV-200S recording spectrophotometer was used. It was ascertained that the transference of polymers into the aqueous phase was negligibly small ( < 2%). Furthermore,
no picrate salts were transferred to the chloroform phase in the absence of crown polymers. The extractability ( f ) was determined as follows: f=
100 x ([Co]-[C])/[Co],
where [Co] and [C] are the concentrations of picrate salts in the aqueous phase before and after extraction, respectively.
RESULTS AND DISCUSSION T h e p i c r a t e s a l t s in l o w p o l a r i t y m e d i a like T H F f r e q u e n t l y e x h i b i t e d p r o n o u n c e d s h i f t s in t h e i r o p t i cal s p e c t r a in t h e p r e s e n c e o f c r o w n e t h e r s . T h e s h i f t s e s s e n t i a l l y o c c u r w h e n a t i g h t i o n - p a i r is c o n v e r t e d to a l o o s e i o n - p a i r b y b i n d i n g t h e c o u n t e r c a t i o n s to t h e c r o w n e t h e r s , l e a d i n g to a s i g n i f i c a n t i n c r e a s e in t h e i n t e r i o n i c d i s t a n c e o f t h e t i g h t i o n - p a i r . It is k n o w n that the formation of externally crown coordinated 1:1 t i g h t i o n - p a i r c o m p l e x e s in T H F s h o w s a shift o f 2maX o f a few n m . O n t h e o t h e r h a n d 2max o f alkali m e t a l p i c r a t e s in T H F s h i f t s to 380 n m w h e n t h e y f o r m 2:1 c r o w n - c o u n t e r c a t i o n c o m p l e x e s ( s a n d w i c h t y p e c o m p l e x e s ) [13]. 2max o f p i c r a t e s a l t s in T H F in the presence and absence of photodimerizable poly ( c r o w n e t h e r ( s ) ) as a d d i t i v e s is s h o w n in T a b l e 3. In t h e a b s e n c e o f p o l y m e r s , 2max w a s f o u n d at 351, 357,
Table 3. Absorption maxima of picrate salts in THF in the pressure of polymers~ ;.m~ (nm) Additive b None 12P 15P 18P 21P 12-21P 15-21P 18-21P
NaPi
KPi
RbPi
CsPi NH4Pi
351 352 ~ 360 357 351 355 358
357 358 379 368 367 365 378 367
358 c 380 374 369 370 374 373
360 ~ 367 380 372 377 375 378
~[Picrate salts] = 5.0 x 10-5 mol/l, at 25'k b[Crown units] = 2.5 x 10-4 mol/l. ¢No determination.
350 c --~ 360 358 358 359 360
MASAMITSU SHIRAI et al.
406 --
CH2--CH.
CH2~
C H 2 - - CH
CH - -
C H 2 - - CH - -
O
0
O
0
CH2
CH2
CIt2
CH2
CH2
CH:
CH2
CH2
O
0
O
0
C=O
C=O
C=O
C=O
CH
CH
CH
CH
CH
CH
CH
CH
hv
II
Scheme 2 358, 360 and 350nm for sodium picrate (NaPi), potassium picrate (KPi), rubidium picrate (RbPi), cesium picrate (CsPi), and ammonium picrate (NH4Pi), respectively, in general agreement with reported values [13]. The 2max values around 377-380nm were observed for the systems of 15P-KPi, 15P-RbPi, 18P-CsPi, 18-21P-CsPi, 15-21P-KPi and 12-21P~CsPi, suggesting the formation of sandwich type complexes. The findings generally agree with the view that alkali metal cations, which are too large to fit in the cavity of crown ethers, can be bound in the sandwich type. It is noteworthy that •rnax of CsPi in the presence of 12-21P was observed at 377 nm, suggesting sandwich type binding of Cs + by 12-21P. As expected from the cation diameter-crown hole-size relationship, 21P binds Cs ÷ in the 1 : 1 form but 12P does not bind Cs +. Thus the co-operativity of 12-crown-4 and 21-crown-7 units may play an important role in the Cs ÷ binding by 12-21P. In our earlier work [9], a similar peculiar binding type was observed in the 12-18P-KPi system, where K ÷ was bound to 12-18P in the sandwich type, although 18-crown-6 units binds K + in the 1:1 form and 12-crown--4 units does not bind K ÷. It is known that cinnamic acid ester groups attached to a polymer chain can dimerize when irradiated with u.v. light [14]. When the poly(crown ether(s)) with cinnamic acid ester moieties were irradiated in dilute solutions (polymer conc. < 10 -2 mol/l), the i.r. spectrum of the irradiated polymers showed a remarkable decrease in the absorbance of the C------Cbond at 1630cm -1 and a shift in the absorption peak due to C-------Ogroups from 1710 to 1730 cm -1. The u.v. spectra showed a remarkable decrease in the absorbance at 325 nm due to cinnamic
acid ester groups. These findings suggest that the crown-connected cinnamoyl groups attached to the polymer chain can dimerize on irradiation even in dilute solution, as reported for cinnamoyl groups in poly[2-(cinnamoyioxy)ethyi methacrylate] [15]. Under the present reaction conditions, the dimerization seemed to mainly proceed intramolecularly because insoluble materials did not appear. The intramolecular photodimerization may not always occur between adjacent cinnamoyl units. It has been reported that ~,og-dicinnamates, in which the cinnamate groups are separated by 21-35 bonds, can be cyclized by intramolecular addition [16]. The stereochemistry of the cyclobutane derivatives which are formed by the photodimerization of the pendant cinnamoyl units is interesting and it has been studied for poly(vinyl cinnamate) in the solid state by several workers [17]. In the present study, however, the structure of the photocyclized products is not elucidated. The • values for the photodimerization of cynnamoyl units of the polymers may increase when two cinnamoyi units come in closer contact by forming sandwich type complexes between cation and crown ether units as vicinal substituents of cinnamoyl units. The formation of 1:1 complexes of cation-crown ether unit repels the cinnamoyl groups by an electrostatic repulsion of the cation bound to the crown ether units, resulting in a decrease in 4. Thus • is a useful empirical parameter to estimate the type of cation-crown complexes. The • values for the polymers in the presence and absence of salts are shown in Table 4. The • values for T H F solution of polymers in the absence of salts were in the range of 0.035 to 0.047, being slightly dependent on the ringsize of crown ether units. The • values of polymers
Binding properties of photodimerizable polymers
407
Table 4. Effect o f salts on the q u a n t u m yields, O, for the photodimerization o f polymers a O Additive
12P
15P
18P
21P
12 21P
15-21P
18-21P
None LiCI NaCI KCI
0.035 b ~ 0.064 0.039
0.041 b 0.021 0.038 0.11
0.035 b 0.023 d 0.027 0.009
0.036 b 0.014 0.027 0.019
0.042 h 0.013 d 0.018 0.020
0.047 b 0.016 0.027 0.019
RbCI
0.032
0.12
0.11
0.021
CsCI
0.047
0.061
0.17
--c
~
0.026 0.046 c 0.022
0.044 0.10 c 0.047 0.11 ~ 0.029
0.045 b 0.019 0.030 0,025 0.056' 0.036 0.075 c 0.032 0.087 c 0.020
NH4CI
--~
aO was determined with 3 6 6 n m light in T H F / m e t h a n o l (1:1, v/v) at units] = 0.01 mol/l, [salts] = 5.0 × 10 3 mol/I. bln T H F at 5 ~. C[Salts] = 0.01 mol/l. do was determined at 2 5 because o f an appearance o f turbidity at 5 . CNo determination.
in the presence of alkali metal chlorides were determined in THF/methanol (1:1, v/v) because of the low solubility of salts in THF. The • values for 12P, 15P and 18P increased on addition of cations which are known to be bound to respective crown units in the sandwich form, i.e. 12P-Na ÷, 15P-K ÷, 15P-Rb ÷ 18P-Rb ÷ and 18P-Cs ÷ systems. The decrease in values was observed for 18P-K ÷ and 21P-all cations examined systems. The • value for 12-21P in the presence of Cs ÷ was higher than that in the absence of Cs ÷. A large increase in • was observed at higher concentrations of templates. Thus, in this case, a co-operative action of 12-crown-4 and 21-crown-7 units seems to take part in the Cs ÷ binding in the sandwich form, being consistent with the suggestions from the shift of '~max of CsPi in the presence of 12-21P (Table 3). The increased • value was observed for 12-21P-Rb ÷ system, although the shift of RbPi in the presence of 12-21P is not so obvious. The values of 15-21P at high concentration of KC1, RbCI and CsCI were found to be larger than those in the absence of salts. Furthermore, the • values for 18-21P were enhanced on addition of CsCI. As will be discussed later, the increase in • for 15-21P and 18-21P was not due to the co-operative action of the differently sized crown ether units of the polymers but due to the preferential binding of cations in the sandwich type by 15-crown-5 units in 15-21P or 18-crown-6 units in 18-21P. The alkali cation binding ability and selectivity of the photodimerizable poly(crown ether(s)) and their photoreacted analogues were studied by the liquid-liquid extraction of alkali metal picrates. Figure 1 shows the effect of the dimerization degree on the extraction of picrate salts by the polymers which were obtained by the irradition in the presence and absence of salts as templates. Although the extractability of CsPi with 18-21P irradiated without templates slightly increased with an increase in the dimerization degree, the extractability of CsPi with 18-21P irradiated with CsC1 template increased considerably with increasing dimerization degree. Similar results were observed for other polymers examined. Thus the effect of photodimerization on the alkali metal cation extractability was studied under almost constant degree of dimerization.
0.023 0.056 0.084 ~ 0.020
5'.
[Cinnamoyl
Figure 2 shows the alkali metal cation extractability of 21P and its photoreacted analogues. The cation extractability of 21P decreased in the order Cs ÷ > Rb + > K + >>Li + ~ Na + ~ N H ] . Although the order was the same as that for 21M, the cation extractability of 21P was higher than that of 21M. The photodimerization of 21P with and without Cs + as a template did not significantly change its alkali cation binding ability and selectivity. The cation extractability was scarcely influenced by the photodimerization of the crown-connected cinnamoyl units, when the cation-crown 1:1 complexes are predominantly formed. Figure 3 shows the alkali cation extractability of 12-21P and its photoreacted analogues. The cation selectivity order of 12-21P was the same as that of 21P. However, the cation selectivity of Cs ÷ to K + and Rb ÷ for 12-21P was improved compared with that of 21P. It has been reported that the cation binding ability of the photodimerizable poly(crown
!
5oi
/
40
30
/ ._._.....o.___---------o-----
20
"~ 0
L
I
t
I
I
20
40
60
80
100
D i m e r i z a t i o n degree (°/o)
Fig. I. Effect of the photodimerization degree of 18 21P on the extractability of CsPi at 35°. Irradiation of 18--21Pwith 313 nm light was carried out with Cs + template (0) and without templates (©). Extraction conditions: [crown units] = 3.0 x l0 -4 mol/I; [CsOH]= 0.01 tool/l; [picric acid] = 6.55 x 10-5 mol/l.
408
MASAMITSU SHIRAI et al. 10
60
5
40
v,..
20
O Li +
Na +
K+
Rb +
Cs +
NH +
Cation
Fig. 2. Extraction of alkali metal picrates at 35° by 21P (O), 21P/Cs+ ( • ) and 21P phototransformed without templates ([3). Extraction conditions: [crown units] = 3.0 x 10-4mol/1; [metal hydroxide] = 0.01 tool/l; [picric acid] = 6.55 x 10-5 mol/l.
.9 Li +
No +
K+
Rb+
Cs +
NH~
Cation
ether(s)) is considerably enhanced by the photodimerization, when the cations are bound to the polymers in the sandwich type [5-8]. Although the Cs ÷ binding of 12-21P in the sandwich type was suggested (see Tables 3 and 4), the photodimerization of 12-21P with and without Cs ÷ template gave no significant effect on its cation binding ability and selectivity. This may be due to the fact that the Cs ÷ binding by 12-21P in the sandwich type is not so stable as that previously observed for K ÷ - I 5 P and Cs÷-18P systems. The stability of a crown-cation complex in an aqueous phase governs the extractability. The more stable the crown-cation complex in an aqueous phase the more extractable it is. The cation binding ability of 18-21P and its photoreacted materials is shown in Fig. 4. The alkali cation selectivity order for 18-21P was found to be Cs ÷ > K + > Rb+>>Li ÷ ~ Na + ~ NH~-. The cation
10
Li +
No +
K+
Rb +
Cs +
NH:
Cation
Fig. 3. Extraction of alkali metal picrates at 35° by 12-21P (O), 12-21P/Cs÷ (0) and 12-21P irradiated without templates (I--1). Extraction conditions: [crown units] = 3.0 x 10-4mol/l; [metal hydroxide] =0.01 mol/l; [picric acid] = 6.55 x 10-Smol/I.
Fig. 4. Extraction of alkali metal picrates at 35° by 18-21P (O), 18-21P/K÷ (A), 18-21P/Cs÷ ( • ) and 18-21P irradiated without templates (r-q). Extraction conditions: [crown units] = 3.0 x 10 -4 mol/l; [metal hydroxide] = 0.01 mol/1; [picric acid] = 6.55 x 10-5 mol/l.
extractabilty of 18-21P was almost the same as that of a blended sample of 18P and 21P (molar ratio 0.41:0.59). This suggests that 18-crown~ and 21-crown-7 moieties in 18-21P independently contribute to the cation binding. The photodimerizations of 18-21P without templates and with K ÷ template caused no significant change in the cation binding ability and selectivity. On the other hand, use of Cs + template during the photodimerization caused a large increase in the binding ability for K ÷ and Rb ÷, and especially Cs ÷. The Cs ÷ selectivity of the photoreacted 18-21P with Cs ÷ template was much improved compared to the original material. The effect of photodimerization of 18-21P on the cation extractability may be explained as follows. The photodimerization of the pendant cinnamoyl moieties is possible between the cinnamoyl moieties with differently sized crown ether units and/or between the cinnam0yl groups with the same sized crown ether units. In the absence of templates, the photodimerization between 18-crown-6-connected cinnamoyl units and 21-crown-7-connected ones may occur predominantly because the molar ratio of 18-crown-6 units to 21--crown-7 units in 18-21P is almost unity. It was reported that 1:1 complex formation constants for 18-benzocrown-6--K+ and 21-benzocrown-7-K÷ systems were 708 and 891/mol, repectively at 25 ° in 99% DMSO [18]. Thus K + as template is predominantly bound to 18-crown-6 moieties of 18-21P in the 1: 1 type. Although the light absorption of 18--crown-6 connected cinnamoyl unit and 21-crown-7 connected units takes place simultaneously, the photodimerization between 18--crown~ connected cinnamoyl moieties may be retarded because of an electrostatic repulsion of the bound K ÷.
Binding properties of photodimerizable polymers
409
2o
60
15 4O
\.
A
10
2O
Li +
NO+
K+
Rb +
Cs +
NH~
Cotion Fig. 5. Extraction of alkali metal picrates at 35° by 18 21P (O), 18-21P/Cs ÷ ( 0 ) and 18P/Cs+-21P blended sample (I-q). 18P/Cs ÷ was obtained from the phototransformation of 18P with Cs + (dimerization degree: 91%). Extraction conditions: [crown units] = 3.0 x 10 -4 mol/1; [metal hydroxide] = 0.01 mol/1; [picric acid] = 6.55 x 10-5 mol/1.
As a results, the photodimerization between 2 1 - c r o w n - 7 connected cinnamoyl units or between 21-crown-7 connected cinnamoyl units and 18-crown~5 connected ones may occur. The latter case is the same as expected for the photodimerization without template salts. The Cs ÷ extractability of 18P was observed to be 5 times that of 21P. Furthermore for polymers bearing pendant 18-crown-6 units, the sandwich type complexes of crown ether units with Cs ÷ are much more stable than 1:1 type complexes [2]. Thus by using Cs ÷ template, the predominant photodimerization of 18-crown-6 connected cinnamoyl units of 18-21P occurred. Figure 5 shows the extractability of alkali metal picrate by 18-21P, 18-P/Cs ÷, and a blended sample of 21P and 18P irradiated with Cs ÷ template. The ratio of 1 8 - c r o w n - 6 / 2 1 - c r o w n - 7 units and dimerization degree of the blended sample are consistent with those of 18-21P/Cs ÷. The cation binding ability and selectivity of the blended sample were in good agreement with that of 18-21P/Cs ÷. This finding supports the assumption that 18-crown-6 connected cinnamoyl units predominantly react in the photodimerization of 18-21P with Cs ÷ template. Figure 6 shows the extractability of alkali metal picrates at 35 ° by 15-21P and its photoreacted products. The cation selectivity order of 15-21P was K + > Rb + > Cs + >>Li + ~ N a + ~ N H ~ and it was almost the same as that of an equimolar blended sample of 15P and 21P. This suggests independent contributions of 15-crown-5 and 21--crown-7 moieties in 15-21P to the cation binding. N o significant changes in the cation binding ability and selectivity
Li +
No +
K+
Rb +
Cs+
I'IH~
Cation
Fig. 6. Extraction of alkali metal picrates at 35° by 15-21P (O), 15-21P/K ÷ ( 0 ) and 15-21P phototransformed without templates (E]). Extraction conditions: [crown units] = 3.0 x 10 -a mol/l; [metal hydroxide] = 0.01 mol/l; [picric acid] = 6.55 × 10-5 mol/1.
were observed by the photodimerization of 15-21P without templates, where the photodimerization between 15-crown-5-connected cinnamoyl units and 21-crown-7-connected ones were predominant because the molar ratio of 15-crown-5-connected cinnamoyl units to 21-crown-7-connected ones in 15-21P is unity. When K ÷ template was used during the photodimerization, the photoreacted 15-21P (15-21P/K +) showed an increase in the binding for K ÷, Rb ÷ and Cs ÷. The K ÷ extractability of 15P was observed to be 10 times that of 21P. Furthermore, the polymers with pendant 15-crown-5 moieties can form much more stable complexes of the sandwich type between 15-crown-5 units and K + than 1:: 1 type complexes [2]. Thus the photodimerization of 15-21P with K ÷ template may mainly occur between 15-crown-5 connected cinnamoyl units. This conclusion was supported by the fact that the cation binding ability and selectivity of a blended sample of 21P and photodimerized 15P with K ÷ template agreed well with those of 15-21P/K ÷.
Acknowledgement--This research was partly supported by a Grant-in-Aid for General Scientific Research from the Ministry of Education, Science and Culture of Japan (No. 61550683). REFERENCES
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