Ring-opening reaction of 5-membered cyclic ammonium salt groups at the end of poly(tetrahydrofuran)

ELSEVIER

REACTIVE & FUNCTIONAL POLYMERS Reactive & Functional Polymers 37 (1998) 57-63

Ring-opening reaction of 5-membered cyclic ammonium salt groups at the end of poly(tetrabydrofuran) Hideaki Oike, Hiroshi Ha&no, Yasuyuki Tezuka * Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 0-okayamu, Meguro-ku, Tokyo 152, Japan

Received 15 June 1997; revised version received 24 July 1997; accepted 25 July 1997

Abstract The ring-opening reaction of 1-methylpyrrolidinium salt groups located at the end of poly(tetrahydrofuran) (poly(THF)) segment was studied with a series of p-substituted benzoate, having either methoxy, methyl, chloro or nitro substituent as a counter-anion. At lOO”C,such p-substituted benzoate anions as having an electron-donating methoxy or methyl substituent caused a ring-opening reaction of pyrrolidinium salt end groups, while others having an electron-withdrawing chloro or nitro substituent were poorly reactive to undergo the ring-opening of the end groups. Accordingly, the pathway of this nucleophilic reaction is primarily governed by the basic& i.e., pK, of the corresponding p-substituted benzoic acid. At 12OT, on the other hand, the nucleophilic substitution reaction at the N-methyl position of the pyrrolidinium salt end groups occurred predominantly. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:

l-Methyl pyrrolidinium salt; Ring-opening reaction; Substituted benzoate anion; pK,; Telechelics

1. Introduction Recently, considerable attention has been focused on efficient construction of novel and unconventional macromolecular architectures. Although much work has been devoted to the achievement of this goal, it is still crucial to develop a superior synthetic system utilizing uniform-size polymers with appropriate functional end groups [ 11. We have so far synthesized a series of uniform-size telechelic polymers possessing moderately strained 4or 5-membered cyclic or 6-membered bicyclic ammonium salt end groups, as well as 5membered cyclic sulfonium salt groups [2-51. We have also demon*Corresponding author. Tel: +81 (3) 5734-2498, Fax: +81 (3) 5734-2876.

strated a novel synthetic methodology for preparing a variety of branched (star polymers [6] and polymacromonomers [7]) and network (model network) homopolymers [6] as well as graft and network copolymers [4,5,8] by making use of efficient counter-ion exchange and subsequent ring-opening reactions of cyclic onium salt groups located at one or both chain ends of a hydrophobic polymer segment. In particular, hydrophobic polymers having 1-methylpyrrolidinium salt end groups were found to be attractive, since they were capable of forming the self-assembled higher-order macromolecular structures through the noncovalent (ionic) interaction, and their preorganized structures might be permanently fixed through the ring-opening reaction at the elevated temperature. In order to obtain deeper insight for this novel polymeric ion-coupling reaction system, we investi-

1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII S1381-5148(97)00119-3

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& Functional Polymers 37 (1998) 57-63

gated in this paper on the ring-opening reaction of 1-methylpyrrolidinium salt end groups of poly(THP) with a series ofp-substituted benzoate counter anions.

Table 1 Ion-exchange reaction of poly(THF) having pyrrolidinium salt end groups (1) with sodium salts of substituted benzoate a Run

2

R

Yield (w)

1

2a 2b 2c 2d 2e 2f

4-Me0 4-Me H 4-Cl 3,5-(MeO)z 4-NO2

111 183 190 192 195 176

2. Experimental 2.1. Materials Poly(THP) (ikf, = 5000) having l-methylpyrrolidinium salt end groups having triflate counter anions (1) was synthesized through the reaction of a bifunctional living poly(THP) with I-methylpyrrolidine, as reported before [4]. Sodium benzoate (Koso Chemical Co., Ltd.) was used as received. Sodium salts of benzoate derivatives, having either 4-methoxy, 4-methyl, 4-chloro, 4-nitro or 3,Uimethoxy substituent, were prepared quantitatively from the commercially available corresponding free acid (Nacalai Tesque Inc. and Tokyo Chemical Industry Co., Ltd.) with equimolar quantity of sodium hydroxide in water. 2.2. Zon-exchange reaction of 1 with a sodium salt of a benzoate derivative A 2-ml THP solution containing 0.200 g of 1 was added dropwise with vigorous stirring into 80 ml of an aqueous solution containing a sodium salt of a benzoate derivative (10 equiv with respect to pyrrolidinium salt end groups) cooled below 5°C. The suspended solution was continuously stirred for 1 h. The precipitate was filtered, washed with cold water for several times and finally dried in vacua. This precipitation procedure was then repeated twice to complete the ion-exchange reaction. The product 2 (0.176-o. 195 g; see Table 1) was obtained as white powder after freeze drying from benzene solution. 2.3. Ring-opening reaction of 2 A poly(THP) having 1-methylpyrrolidinium salt end groups having a series of substituted benzoate counter-anions (2) (20 mg) was placed in a test tube equipped with three-way stopcock, then heated to melt in an oil bath thermostated at 80-120°C under nitrogen atmosphere, and keeping the temperature for the period given in Table 2. The recovered product was directly subjected to spectroscopic and chromatographic analyses.

2

3 4 5 6

a Charged amount of 1; 200 mg.

Table 2 Reaction of poly(THF) (2) having pyrrolidinium salt end groups by a series of substituted benzoate counter-anions Run

2

pKaa

Temperature W)

Time (h)

Yteldb,C (%)

3: 4c

2a

4.41

120 100 100 80

5 24 30 24

91 82 90 43

60:40 85 : 15 87:13 99:l

5 6 7

2b

4.31

120 100 80

5 24 24

98 93 45

16124 86: 14 92:8

8 9 10

2e

4.20

120 100 80

5 24 24

94 85 45

62:38 79:21 91:9

11 12 13

2d

3.98

120 100 80

5 24 24

95 52 8

23~17 61:39 n.d. *

14 15 16

2e

3.97e

120 100 80

5 24 24

86 12 25

40:60 50:50 86:14

17 18

2f

3.42

120 100

5 24

65 0

16:84

a Value of the corresponding benzoic acid derivative [ 101. bTotal substitution yield. CDeterminedby ‘HNMR. * nd. = Not determined. eFromRef. [ll].

2.4. Measurement GPC measurements were performed using a Tosoh Model CCPS equipped with a refractive index detector Model RI 8020, a W detector Model W 8020 at 254 nm, and a conductivity detector Model CM 8010.

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& Functional Polymers 37 (1998) 57-63

59

A TSK G3OOOHXL column was used with THF as eluent at a flow rate of 1.0 ml/min. IR spectra were recorded on a JASCO FT/IR410 infrared spectrometer by casting the sample on a NaCl plate. ‘H-NMR spectra were recorded with a JEOL JNM-AL300 apparatus in CDCls at 40°C. Proton chemical shifts were referenced to the signal of tetramethylsilane.

ucts after the first precipitation showed that the ionexchange yield remained at most 60%. By repeating the precipitation treatment, however, the completely ion-exchanged product 2 was obtained with high recovery yield (Table 1). Thus, the ion-exchange reaction was confirmed to proceed regardless of the type of substituents on the benzoate derivatives.

3. Results and discussion

3.2. Ring-opening reaction

3.1. Ion-exchange reaction

The molten state reaction of l-methylpyrrolidinium salt groups in 2 with a series of substituted benzoate counter-anions was carried out under nitrogen atmosphere in bulk at 80-120°C (Table 2). The nucleophilic or basic reaction on the end-standing I-methylpyrrolidinium salt groups in 2 might take place, in principle, in five different ways, namely: (1) a ring-opening substitution by attack on endo-methyZene to produce an amino ester group (Scheme 2), (2) a nucleophilic substitution by attack on exo-

A THF solution of poly(THF) having l-methylpyrrolidinium salt end groups having Mate counter anions (1) was simply added into ice-cooled aqueous solution containing an excess amount of a sodium salt of a series of benzoate derivatives, having either 4-methoxy, 4-methyl, 4-chloro, 4-nitro or 35dimethoxy substituent (Scheme 1). The IR and ‘H-NMR spectroscopic study of the recovered prod-

CF,SO;

-OSO,CF,

1

2a (R=4-MeO) b (R=4-Me)’ c (R=H) d ( R = 4-Q ) e ( R = 3,5-(Me0)2) f (R=4-N02) Scheme 1. Ion-exchange reaction of 1.

A

H3C

‘N +3

-PTHF-

)

-PTHF-:$H2h02CGfR

02cGR 2a-f

3a-f Scheme 2. Ring-opening substitution of 2.

60

H. Oike et al. /Reactive & Functional Polymers 37 (1998) 5743

4000

3000

2000

1500

1000

500

Wavenumber[cm-1] Fig. 1. IR spectra of 2a before (A) and after (B) the heating treatment at 100°C (run 3 in Table 2).

A -

-PTHF-Na

+ ReC0,CH3

4

2a-f

Scheme 3. Exo-cyclic demethylation of 2.

methylene to produce an ester group with elimination of 1-methylpyrrolidine, (3) a nucleophilic substitution by attack on exo-methyl (N-methyl) to produce an N-alkylpyrrolidine group with elimination of a methyl ester of a substituted benzoate (Scheme 3), and a Hofmann elimination by proton abstraction at the B-position, either (4) in an

en&cyclic pyrrolidinium ring unit, or (5) in an exe-cyclic tetramethylene unit. Illuminati and Lillocci observed both endo- and exe-cyclic substitution as well as Hofmann-type elimination pathways in the reaction of iV,N-dimethylpyrrolidinium iodide with sodium methoxide in methanol at 130°C [9]. In the present polymer-chain end reaction, on the

61

H. Oike et al./Reactive & Functional Polymers 37 (1998) 57-63

I

0

I

I

I

I’

2

I

I

I

II

0

4

11

18

11

8

6

1

a

’

10

a

11

1

12

Elution volume (ml) Fig. 2. GPC traces of 1 (A) and 2a (B) after the heating treatment at 100°C (run 3 in Table 2) (RI, refractive index; UV, ultraviolet; 52, conductivity traces).

contrary, only two of them were observed to occur by anyone of substituted benzoates. One is the path (1) to produce poly(THF) having amino ester groups (3) by the ring-opening reaction (Scheme 2), and another is the path (3) to produce poly(THF) having cyclic amine end groups (4) (Scheme 3). The results of the IR and ‘H-NMR spectroscopic as well as GPC analysis on the products are collected in Figs. 1-3, respectively. In Fig. 1, IR spectra of 2a (run 3 in Table 2) before and after the heating treatment are compared. The spectrum after heating (B) contains the absorption at 1715 cm-’ along with the elimination of the absorption at 1545 cm-’ visible before heating (A), indicating ester formation through the nucleophilic substitution reaction by 4-methoxybenzoate counter-anion. The rest of

the spectrum is apparently intact during the heating treatment. The GPC chromatogram after the heating treatment is compared with that of the starting 1 (Fig. 2). The peak tailing observed for 1 is presumably ascribed to the interaction of ionic end groups of poly(THF) with gel surfaces of GPC column as reported before [4]. The elimination of conductivity trace, along with the formation of UV trace during the precipitation and heating processes agrees with the counter-ion exchange, followed by the ring-opening reaction by counter-anion. In addition, the constantly narrow molecular weight distribution of the product indicates the absence of noticeable degradation of poly(THF) main chain during the heating treatment.

H. Oike et al/Reactive

& Functional Polymers 37 (1998) 57-63

e H&O

b

o~‘Vt.J-~d 0

&H,

a

7H3

l/LN-o “O c d b

e

b

a

,CQ-OCHB

h

g

I

l-J e

f

C

I,,,,,,,

N+“+OYLN n

,,,,,,,,, 8

d

b C

IIIllIIII 7

3

IIIIIIIII 6

b

a

IIIIIIIII 5

IIIIIIIII 4

IIIIIIIII 3

IIIIIIIII 2

IIIIIIllll

1

PPM 0

Fig. 3. 300-MHz ‘H-NMR spectra of 4 (bottom) and 2a before (middle) and after (top) the heating treatment at 1OO“C(run 3 in Table 2) (CDC13, WC).

In Fig. 3, ’H-NMR spectra of the reaction product from 2a before (middle) and after (top) the heating treatment are listed, Also that of 4 (bottom) is shown in the figure as for comparison. In the spectrum after heating, signals due to 1-methylpyrrolidinium salt groups visible before heating are eliminated, and replaced by new peaks due to amino ester groups. In particular, the signal due to the ester methylene

as a triplet at 4.30 ppm (f) and the signal due to the N-methyl as a singlet at 2.21 ppm (e) appeared after the heating treatment strongly support the formation of the ring-opened product 3a. Minor signals are detectable additionally in the region at 2.40-2.60 ppm, and they can be assigned to those due to the cyclic amine end groups in 4, produced by a substitution reaction at the N-methyl position of

H. Oike et al. /Reactive & Functional Polymers 37 (1998) 5743

1-methylpyrrolidinium salt end group. Thus, the total extent in the substitution reaction as well as the reaction selectivity can be estimated by comparing the integral area of the relevant signals, namely, those due to aromatic protons (g) in a substituted benzoate moiety either from ionic or covalent form versus (f), and (f) versus those at 2.40-2.60 ppm, respectively. The obtained results are summarized in Table 2. Consequently, it is obvious that the scope of the present nucleophilic substitution reaction is primarily governed by the nature of the substituent on the aromatic ring of substituted benzoate counter-anions. Among a series of 2 possessing various p-substituted benzoate counter anions, 2a and 2b having electrondonating methoxy and methyl substituent produced the corresponding ring-opened product, 3a and 3b, respectively, in high yields and with the high ringopening selectivity at 100°C (runs 2, 3 and 6), while 2d (R = 4-Cl) showed moderate reactivity (the total substitution yield of 52%; run 12). On the contrary, 2f (R = 4-N02) failed to cause any reaction at 100°C (run 18). Thus, this reactivity order apparently follows the order of the pK, value of corresponding substituted benzoic acids [ 10,l l] as listed in Table 2. Hence, the reactivity of a given benzoate derivative as a counter-anion will be predictable. Thus, for an example, 3,klimethoxybenzoate having pK, of 3.97 exhibited, indeed, similar reactivity to 4-chlorobenzoate having p& of 3.98 as shown in Table 2. At higher reaction temperature up to 120°C it was found that the total substitution yield increases accordingly through either the ring-opening or the methylation reaction on the pyrrolidinium salt end groups. Nevertheless, the reaction at higher temperature resulted in the lowering of the selectivity of the ring-opening reaction over the methylation. Thus, in order to obtain the high reaction selectivity in this reaction process, the reaction must be carried out at low temperature. And indeed, the reaction of 2a at 80°C exclusively gave the ring-opened product 3a in 43% yield (run 4).

63

4. Conclusions This paper described the ring-opening reactivity of 1-methylpyrrolidinium salt group located at the end of poly(THF) segment having a series of substituted benzoate counter-anions. It was found that the ring-opening reactivity was governed by the nature of the substituent on an aromatic ring, and the electron-donating p-methoxy or p-methyl substituent could enhance the rate of the nucleophilic reaction to give the corresponding ring-opened product in high yield. Acknowledgements We are grateful to Professors M. Kakimoto (Tokyo Institute of Technology) and Y. Yoshida (Toy0 University) for our access to the NMR apparatus. This study was supported in part by a Grant-inAid for Scientific Research (No. 08455436) from Ministry of Education, Science, and Culture, Japan. References [l] Y. Tezuka, Prog. Polym. Sci. 17 (1992) 471. [2] Y. Tezuka, E.J. Goethals, Makromol. Chem. 188 (1987) 783. [3] F. D’haese, E.J. GoethaIs, Br. Polym. J. 20 (1988) 103. [4] Y. Tezuka, T. Shida, T. Shiomi, K. Imai, E.J. Goethals, Macromolecules 26 (1993) 575. [5] Y. Tezuka, H. Imai, T. Shiomi, Macromol. Chem. Phys. 198 (1997) 627. [6] Y. Tezuka, E.J. Goethals, Makromol. Chem. 188 (1987) 791. [7] Y. Tezuka, S. Hayashi, Macromolecules 28 (1995) 3038. [8] T. Shiomi, K. Okada, Y. Tezuka, H. Kazama, K. Imai, Makromol. Chem. 194 (1993) 3405. [9] G. Illuminati, C. Lillocci, J. Org. Chem. 42 (1977) 2201. [lo] J.F.J. Dippy, Chem. Rev. 25 (1939) 151. [ll] K.C. Srivastava, J. Prakt. Chem. 35 (1967) 118.