Spontaneous copolymerization of acrylic acid with 2-methyl-2-thiazoline

Eur. Polym. J. Vol. 29, No. 8, pp. 1089-1093, 1993 Printed in Great Britain

0014-3057/93 $6.00 + 0.00 Pergamon Press Ltd

SPONTANEOUS COPOLYMERIZATION OF ACRYLIC ACID WITH 2-METHYL-2-THIAZOLINE B. L. RIVAS,* G. S. CANESSA and S. A. POOLEY Polimeros, Departmento de Quimica, Universidad de Concepci6n, Casilla 3-C, Concepci6n, Chile (Received 28 September 1992) Abstract--Acrylic acid as an electrophilic monomer was copolymerized in the absence of initiator under various experimental conditions with 2-methyl-2-thiazoline as a nucleophilic monomer. The copolymers were characterized by elemental analysis, FT-i.r., ~H-NMR and )3C-NMR spectroscopy. The copolymer compositions were determined by elemental analyses. All the copolymers are statistical; they are rich in acrylic acid units. From the copolymer compositions, FT-i.r., tH-NMR and ~3C-NMR spectra, it was possible to suggest a copolymer structure.

INTRODUCTION

EXPERIMENTAL PROCEDURES

Materials

The process of copolymerization usually requires an initiator, catalyst or high energy radiation but the "spontaneous copolymerization via zwitterion" or "no-catalyst copolymerization via zwitterion intermediates" occurs in the absence of any added initiator [1-5]. This polymerization involves a nucleophilic m o n o m e r ( M N ) with an electrophilic m o n o m e r (ME). The interaction of the monomers generates a "genetic zwitterion" + M N M E - which functions as the key intermediate responsible for initiation as well as for propagation. The following general scheme shows the growth of the "genetic zwitterion" into oligo- and macrozwitterion.

2-Methyl-2-thiazoline (MTIA) (Aldrich) and AA (Merck) were purified by distillation, p-Methoxyphenol (Aldrich) was dried under vacuum for 7 days. CH3CN was distilled from phosphorus pentoxide. Acetone and benzene were dried by the usual methods [18]. Copolymerizations A mixture of AA and MTIA (total quantity 0.06 mol), in the presence of p-methoxyphenol (0.008 mol) as a radical polymerization inhibitor, in 5 ml of solvent or in bulk was placed in a polymerization flask under N 2. The vessel was kept at 70° for 48 or 96 hr. The copolymerization mixture was added to diethyl ether and the copolymer was separated by centrifugation, purified by reprecipitation and dried in vacuum.

MN + ME-, +MNME--, +MN(MEMN).ME -

Characterization

In general: +MN--(MEMN--).ME-

+ +MNME---,

+ M N ( M E - - M N - - ) . + IM E If the growth reaction involves only "polyaddition" and "polycondensation" reactions, an alternating copolymer is obtained. It is also possible that a lateral reaction may occur by interaction of the zwitterionic species with monomer, M N or ME, leading to production of statistical copolymers. Continuing our studies on non-catalysed copolymerization through zwitterion intermediates [6-17], we now report the study of the copolymerization of acrylic acid (AA) as M E with 2-methyl-2-thiazoline ( M T I A ) as M N :

N

CH2ffiCH O00H

S

GH3

MTIA (MN)

AA(ME)

*To whom all correspondence should be addressed.

The i.r. spectra were recorded on a Perkin-Elmer 1600 FT-i.r. spectrometer with a Hewlett Packard colour plotter. The ~H-NMR spectra were recorded on Varian XL 100, XL 200 GEM and Bruker AC 200 spectrometers. The t3C-NMR spectra were recorded on a Bruker AC-200 (50 MHz). The chemical shifts are reported in ppm relative to internal TMS. RESULTS AND DISCUSSION The copolymerizations A A with M T I A without initiator were carried out with various proportions of the monomers but keeping constant the total amount (0.06 tool). The A A / M T I A copolymers are semisolid coloured hygroscopic materials. The copolymerization conditions and copolymer compositions are summarized in Table l which shows that copolymerization yield increases on increasing the concentration of A A in the feed (see copolymers 1-5). An increase in the copolymerization temperature produces a higher copolymer yield (see copolymer 3 and 7; 6 and 8). The more polar solvent C H a C N gave the highest yield. By means of elemental analyses, the copolymer compositions were determined from the N / C ratios which are independent of occluded water (see Table 1). Because A A is more reactive than M T I A ,

1089

B. L. RWAS et al.

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Table 1. Copolymerization of AA with MTIA at 70 Feed ratio. Copol. time Solvent Yieldt N/C AA/MTIA (hr) (5 ml) ~* (%) ratio:i: 1 0.33:1.00 48 CH3CN 37.5 39 0.1466 2 0.48 : 1.00 48 CH3CN 37.5 49 0.1525 3 1.00:1.00 48 CH3CN 37.5 76 0.1446 4 2.00:1.00 48 CH3CN 37.5 88 0.1363 5 3.00:1.00 48 CH 3CN 37.5 91 0. I 178 6 1.00: 1.00 48 Bulk -87 0.1353 7 1.00:1.00 96 CH3CN 37.5 78 0.1330 8 1.00:1.00 96 Bulk -90 0.1423 9 1.00:1.00 48 CH3COCH 3 20.7 60 0.1383 10 1.00:1.00 48 C6H6 2.3 54 0.1486 *Dielectric constant of the solvent at room temperature. tHygroscopic copolymers. :[:Determined from elemental analysis. §Determined from N/C ratio. Copolymer No.

t h e r e is p r e f e r e n t i a l i n c o r p o r a t i o n o f A A c o p o l y m e r d e p e n d i n g o n [AA] in t h e feed,

in t h e

Probable structures for AA /MTIA copolymers As MTIA has two nucleophilic centres (N and S a t o m s ) , it c a n be s u p p o s e d t h a t t h e r e a r e t w o p o s s i b l e r e a c t i o n c e n t r e s f r o m this m o n o m e r r e a c t i n g o n t h e m e t h y l e n e c a r b o n o f A A (see S c h e m e 1).

P a t h w a y A yields t h i o a m i d i c c o p o l y m e r s w i t h t h e )~S g r o u p in t h e s i d e - c h a i n . P a t h w a y B yields copolymers with the

--N=~--S-g r o u p in t h e m a i n c h a i n .

i ~ ~ ~ " ~ +

pathwav A

r'--" N-CH2-.CH~

Copol. composition AA/MTIA 1.32:1.00 1.22:1.00 1.36:1.00 1.52:1.00 1.97 : 1.00 1.54: 1.00 1.59:1.00 1.40:1.00 1.48:1.00 1.28:1.00

CH2=CH.COOH

/ ~ N-CH2CH2-COOe

~. ",,'~ - C H 3

("genetic

..

lit

,",,-.--C H2CH2-N-CH2-(pH"v" ,',,.,-CH2CH2-N-CH2CH2-COO'~," (polymers) ,C=S COOH C=S, CH3 CH3 (At) (A2)

CH3

CH3

~,/S-CH2-CH ~ COOH

L~S-CH2CH2COO

Ltl

("genetic zwitterions')

CH3 c.3 •~ - C H2CH2-N=C-S-CH2-CIH"v" N'CH2C H2-N=C-S-CH2CH2-COO-"~(polymers) COOH (81)

(82)

Scheme 1

Spontaneous copolymerization of AA with MTIA

1091

67.50

3233.8

v E2487.8

2922.6 1007.7

15.16

I 4000

i

I

i

,

3500

3000

2500

2000

17137 1537.3-'

~

v

1324.9

1500

i

It

1000

500

¢m-I

Fig. 1. FT-i.r. spectrum of copolymer 6 (NaC1, acquisitions: 4.0). By chemical and spectroscopic methods, it is possible to determine the more probable copolymer structure. Thus, the 13C-NMR spectra of all copolymers show only four signals corresponding to sp 2 carbons, at 6 = 177.412; 176.674; 171.953 and 168.004ppm (copolymer 6 in D20). Because the ~3C-NMR spectrum does not show the sp 2 carbon signal corresponding to > ~ S (thioamide) which should absorb around 200 ppm, (cf the following low molecular compounds [19]) S

CH 3

CH3__ I~__N//

S

CH2CH

3

CH3 CH2__~__N~

~CH3 199.7 ppm

~CH2 CH 3 204.4 ppm

yields a copolymer with --N~-----~--S-units in the main chain. This unit is also present in the monomer structure of MTIA. The sp 2 carbons of MTIA absorb at 166.0 ppm (13C-NMR, 62 MHz, CDC13, 29°). A similar chemical shift would be expected for this carbon in the copolymer structure. The spectra for all copolymers showed a signal at 168 ppm. The copolymers are coloured, explained by conjugation of the electrons from the sulphur atom with the double bond of the Schiff base: CH3 - - C H 2 - - .N.= ~ - - S - - - C H 2 ~ CH3

it is concluded that the ring-opening of MTIA does not produce a thioamide and that the nucleophilic attack on AA is through the sulphur atom (pathway B) which is more nucleophilic than the nitrogen atom. The FT-i.r. spectra confirm this copolymer structure. Figure I shows the i.r. spectrum of copolymer 6. It contains the stretching bands at 3244cm -~ (Yon), 1713 cm -l (Vc_-o acid and ester) and 1632cm -j

On the other hand, the tH-NMR spectra of all copolymers are the same, differing only in the intensities of signals. Figure 2 shows the tH-NMR spectrum of copolymer 6. Detailed analysis of the signal areas (AI), (A2), (B1) and (B2) indicates that the AA units in the backbone are ---CH2CH2COO--- and --CH:--~H--.

(--S--~=--N--). There is no sign of the stretching band corresponding to the thioamides (6c=s) which absorb between 1090 and I 140 cm- l [20]. These results support a copolymer structure formed from pathway B, where the "genetic zwitterion"

COOH Accordingly, an average copolymer structure rich in AA would be better represented by Scheme 2. In this diagram, the chemical shifts are related (6 in ppm) from Fig. 2 with the different types of proton included in the structure. The ~3C-NMR assignations

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B.L. Rwas et al.

f

J I

J

J

I

I

I

/

f

J I 5

I

lO

J I

I

I

8 (ppm) Fig. 2. ~H-NMR spectrum of copolymer 6 (200 MHz, room temperature, DMSO-d6). to sp 2 carbons are also included. The values of the chemical shifts for 'H- and '3C-NMR given in Scheme 2 are based on the chemical shift data for the monomer (MTIA) and on the chemical shifts for the low molecular weight model compounds [19, 20]. The excess of AA in the copolymer structure (see Scheme 2) might be explained by the ion-dipole reaction between a zwitterionic species with an AA molecule:

CONCLUSIONS These new copolymers which contain sulphur in the backbone showed that the MTIA ring was opened in a way different from that for the isoelectronic cyclic iminoetber 2-methyl-2-oxazoline. For this reason, copolymer structures with units ---CH2--N~---C(CH3)---S--CH2-- were obtained. On the other hand, the acrylic acid reacted to form ester (--CH2---CO(O--CH2--) and acid (--CH2---~H--) COOH

CH3 N+~_]S,~"~"~.C H 2 ~ I H--C H2 - - C H " COOH COOH

1H-NMR

2.8

3.8

2.4

2.8

structures. Because of the greater reactivity of the acrylic acid being greater than that of 2-methyl-2thiazoline, all the copolymers were statistically rich in acrylic acid units.

4.1

2.4

2,8

4.1

2.4

C,H3 -CH2CH2-COO-CH2CH2-N = C-S.CH2*CH-CH2-CH2.N=C-S-CH2.CH-CH2-CH.CH2CH2.N=C-S.CH2I

13C-NMR

,

171.953

I

I

168.004

(~OOH

I

t

I COOH

'

I~ T

168.004

177.412 176.678

~hme2

I COOH

177,412 176.678

!

/

168.004

Spontaneous copolymerization of AA with MTIA

Acknowledgement--The authors thank Direeci6n de Investigaei6n, Universidad de Concepci6n (Grant No 20.13.39-A and 20.12.49). REFERENCES

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EPJ 29/8--E

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10. B. L. Rivas, G. S. Canessa, S. A. Pooley, H. A. Maturana and U. Angne. Eur. Polym. J. 21,939 (1985). 11. B. L. Rivas, G. S. Canessa and S. A. Pooley. Polym. Bull. 14, 239 (1985). 12. B. L. Rivas, G. S. Canessa and S. A. Pooley. Makromolek. Chem. 187, 71 (1986). 13. B. L. Rivas, G. S. Canessa and S. A. Pooley. Makromolek. Chem. 188, 149 (1987). 14. B. L. Rivas, G. S. Canessa and A. Pooley. Makromolek. Chem. Rapid. Commun. 8, 365 (1987). 15. B. L. Rivas, G. S. Canessa and S. A. Pooley. Eur. Polym. J. 25, 225 (1989). 16. B. L. Rivas, G. S. Canessa and S. A. Pooley. Makromolek. Chem. 190, 2665 (1989). 17. B. L. Rivas, G. S. Canessa and S. A. Pooley. Polym. Bull. 23, 171 (1990). 18. Organikum. VEB, Deutscher Verlag der Wissenschaften, Berlin (1972). 19. H. O. Kalinowski, S. Berger and S. Braun. I~C-NMRSpektroskopie, p. 191. Georg Thierne Verlag, Stuttgart (1984). 20. E. Pretsch, T. Clerc, J. Seibl and W. Simon. Tables of Spectral Data for Structure Determination of Organic Compounds. Springer Verlag, Berlin (1983).