Boletín de la Sociedad Chilena de Química
versão impressa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.45 n.2 Concepción jun. 2000
SYNTHESIS AND STRUCTURE OF
S.A.POOLEY, G.S. CANESSA AND B.L. RIVAS
Departamento de Polímeros, Facultad de Ciencias Químicas
Universidad de Concepción, Casilla 160-C, Concepción, Chile
(Received: January 20, 2000 - Accepted: April 12, 2000)
In memoriam of Doctor Guido S. Canessa C.
The spontaneous copolymerization reaction in acetonitrile at 35C by 48 h between N-(2-hydroxyethyl)ethyleneimine with 3,4,5,6-tetrahydrophthalic anhydride under different experimental conditions was studied. The yield and molecular weight ranged between 57 - 78 % and 8.5 - 9.1·103 respectively. The copolymer composition determined from 1H NMR spectra demostrated that as decreases the concentration of HEEI units in the feed, decreases the incorporation of HEEI units in the copolymer. According to the all spectroscopic data a copolymer structure was suggested. It considers the 3,4,5,6-tetrahydrophthalic anhydride units forming part either the backbone and the side chain , and the anhydride bonds are placed only in the side chains.
KEYWORDS: Zwitterion, spontaneous copolymerization, anhydrides-heterocyclic amines copolymerization.
Se estudió la polimerización en solución de anhidrido 3,4,5,6-tetrahidroftálico (monómero nucleofílico) con N-(2-hidroxietil)etilenimina (monómero electrofílico), en ausencia de iniciador, a diferentes concentraciones comonoméricas iniciales. Los copolímeros fueron caracterizados por espectroscopía FT-IR y 1H-RMN. Se propone una estructura para los copolímeros en base a los datos espectroscópicos y a la composición copolimérica.
PALABRAS CLAVES: Polimerización espontánea, copolimerización de anhidridos-aminas heterocíclicas.
There is an important number of heterocycles with nucleophilic (MN) and electrophilic (ME) reactivity. These organic compounds can react as monomers in spontaneous copolymerizations, which occur with ring opening in absence of any added initiator or catalyst
When the monomers combine, they form a zwitterion intermediate (+MN-ME-, 1) which is the so-called genetic zwitterion and responsible for both initiation and propagation. Two molecules of 1 produce a dimeric zwitterion (2) and subsequent reactions of 2 with 1 give rise to the formation of a macrozwitterion 3, which is an alternating copolymer of MN and ME-. In the later stages of copolymerization, when the concentration of macrozwitterions becomes higher, reactions between two macrozwitterions take place as shown in equation , which brings about a sharp increase of molecular weight.
It is possible that a zwitterion induces an intramolecular reaction to give a cyclic compound, e.g., 4 from a zwitterion 3. This is one way by which the copolymerization may be terminated:
However, the amount of 4 is very small, and hence, the reaction  can usually be neglected.
The alternating copolymerization is achieved according to the reactions -. In addition to these reactions, homo-propagations are possible as given in reactions (6) and (7):
These homo-propagations bring about a biased copolymer composition. The reaction of genetic or macrozwitterions is in competition with other [ion-ion reactions as given by reactions -] and with monomers [ion-dipole reactions shown as reaction  and ]. The course of these depends on various factors derived from the combination of MN and ME, i. e., the reactivities of MN and ME and those of zwitterions produced from MN and ME.
Different authors have been reported results including lactones, oxazolines, aziridines, anhydrides, etc.(1-5) We have also studied these copolymerization reactions particularly among lactones, cycles anhydrides, oxazolines and aziridines.(6-19)
The present paper reports the spontaneous copolymerization of N-(2-hydroxyethyl)ethyleneimine, HEEI as nucleophilic monomer with 3,4,5,6-tetrahydrophthalic anhydride , THPhA1,2. as electrophilic monomer. The reaction was studied in a polar solvent as acetonitrile by using different feed monomer ratios.
N-(2-hydroxyethyl)ethyleneimine (Aldrich) was purified by distillation. 3,4,5,6-tetrahydrophthalic anhydride (Merck. 98%) was used without further purification. The solvents were purified according to the published methods(20).
A set of 5 copolymerizations keeping constant the total amount (0.04 mole) of both comonomers was carried out. General procedure: In a polymerization glass, a mixture of comonomers with different mole ratios was prepared. 10 mL of acetonitrile were added under N2(g). The copolymerization was maintained at room temperature for 48 h at room temperature (30 C). After that, it was precipitated in diethyl ether, the copolymers separated by centrifugation, purified by reprecipitation in DMSO/diethyl ether and dried in oven under vacuum up to constant weigth.
The FTIR spectra were recorded on a Magna Nicolet 550 spectrophotometer. The 1H-NMR spectra were recorded on a Bruker AC-250-P spectrometer at 29 C using TMS as internal standard. Thermogravimetric analysis were carried out on a Polymer Laboratories STA-625 Thermal Analyser System. The measurements were performed under N2(g) atmosphere at a heating rate of 20 / min. The molecular weights were determinated in DMSO at 85C by a Knauer pressure osmometer.
RESULTS AND DISCUSSION
Table I shows the copolymer composition, yields, and molecular weights of HEEI/THPhA1,2 copolymers which were synthesized varying the comonomer ratio.
|HEEI/THPhA1,2||HEEI/THPhA1,2 a)||x 10-3b)|
|1||3.00 : 1.00||2.21 : 1.00||57||8.8|
|2||2.00 : 1.00||1.30 : 1.00||72||8.6|
|3||1.00 : 1.00||0.74 : 1.00||78||9.1|
|4||0.50 : 1.00||0.65 : 1.00||80||8.5|
|5||0.33 : 1.00||0.526 : 1.00||68||8.9|
It is observed a strong dependency of the copolymer composition with the feed comonomer ratio. As decreases the concentration of HEEI units in the feed, decreases the incorporation of HEEI in the copolymer. The molecular weights of the HEEI/THPhA1,2 copolymers ranged between 8.5 x103 and 9.1 x103 . The yield increases slightly as increases the concentration of THPhA1,2 in the feed, except for the copolymer 5.
The FTIR spectra of the HEEI/THPhA1,2 copolymer samples 1, 3 and 5 are shown in Figure 1. FTIR spectrum of copolymer 5 shows among others, absorption bands at 3403 cm-1 corresponding to n(O-H) of hydroxylic protons; at 2937 cm-1 n(Csp3-H); at 1852 cm-1 and 1772 cm-1 n(C=O) of anhydride; at 1717 cm-1 n(C=O) of ester and at 1645 cm-1 n(C=O) of amide. FTIR spectrum of copolymer 1 did show not bands of anhydride n(C=O) . The presence of stretching bands C=O of an amide and the absence of bands of anhydride allow to conclude that the copolymerization would occur by opening of aziridine and anhydride rings, yielding amido ester copolymer structures:
where "a" and "b" correspond to the mole ratios in which are both comonomer units and not blocks of them. The opening of the cyclic anhydride and aziridine rings has been widely studied by the present authors.( 15-19 ) The presence of these two copolymer units permits suggest four possible diads, some of them would constitute the backbone of HEEI/THPhA1,2 copolymers.
|Fig. 1. FTIR spectra of HEEI/THPhA1,2 |
copolymers a)1, b)3 and c)5 (KBr,4
By comparision of the FTIR spectra (see Figure 1) is shown that the intensity of the band (O-H), placed at 3400 cm-1, decreases as the copolymer is richest in THPhA1,2 units. The band (C=O) ester at 1717 cm-1 is present in all copolymer spectra. The copolymers 3 and 5 present besides the stretching bands C=O of anhydride, which were confirmed by FTIR Fourier deconvolution technique of the carbonyl absorption region.
The 1H-NMR spectra of HEEI/THPhA1,2 copolymers 1, 3 and 5 are shown in Figure 2. Bands at low field (2.5-4.3 ppm) are attributed to methylene protons linked to heteroatom: d= 4.12 ppm to COOCH2; d= 3.50 ppm to CH2OH and CH2NCO; d= 2.84 ppm to CH2-N and two signals at higher field assigned, in principle, to methylene protons of cyclohexene anhydride units. The shieldest signal (d=1.53 ppm) is produced by the four protons linked to carbon atoms 4 and 5 (C4,5H2) of THPhA1,2 unit and the signal deshieldest at d= 2.27 ppm, is assigned to the four methylene protons of carbons 3 and 6 coming from cyclohexene ring (C3,6H2), neighborn to sp2 carbon. These two signals must be of same intensity (area), however, the signal at = 2.27 ppm (C3,6H2) shows a lower intensity, which means that a part of methylene protons neighborn to double bond of THPhA1,2 units must be diasterotopics and absorb at lower field (d= 2.84 ppm). The amount of these proton types correspond to the difference between the areas of the signals at d= 1.53 ppm (C4,5H2) and d= 2.27 ppm (C3,6H2), therefore, the signal at d= 2.84 ppm is enhanced in this amount. The chemical shift of these protons is related with different magnetic environment around the C3,6H2 protons of the cyclohexene units by formation of diads through amide, ester, and anhydride bonds. The chemical shift of the signals can be attributed to the anisotropic effect of the carbonyl group which is in conjugation with the cyclohexene ring, forming a rigid system, probably with an amide group that contain a sharp double bond character.
Fig.2.1H-NMR spectra of HEEI/THPhA1,2 copolymer samples a)1, b)3 and c)5 (250MHz,DMSO-ds,29C)
As the total area of protons of THPhA1,2 (eight protons), is given by the double of the area of signal at d=1.53 ppm (C4,5H2), the copolymer composition was determined from this area and from the total area of the present protons in the copolymer (see Table I).
The experimental area for the non-equivalent protons and the difference observed to the area for the signals of the methylene protons of the cyclohexene ring which absorb at high field are shown in Table II.
|Table II. Proton experimental areas obtained from 1H-NMR spectra of HEEI/THPhA1,2 copolymers and the difference of areas between the protons (C4,5H2) and (C3,6H2)of THPhA1,2 units.|
|Copol.||Signal area at||Signal area||Signal area||Signal area||Signal area||D(area)|
|No||d=4.12||at d=3.50||at d= 2.84||at d = 2.27||at d = 157||Dd = 1.53-|
Table II shows a decrease of the signal at d= 3.50 ppm, which is attributed to the decrease of CH2OH groups as increase the incorporation of THPhA1,2 units to the copolymer. It is attributed to the partial esterification of the hydroxyl groups by THPhA1,2, which is in agree with the observed by FTIR spectroscopy (see Figure 1). The side esterification of HEEI units with different cyclic anhydrides has been previously reported by the authors. (15-19)
Based only in the copolymer composition information it is possible postulate the following structure A for the copolymer 5 [(HEEI) 0.526 (THPhA1,2 )1.00] (see Table I), which considers that the anhydride units are placed only at the backbone:
However, the structure A is not in agree with the spectroscopic data from 1H-NMR, due to it presents a high concentration of CH2-OH groups. According to this structure, the signal area of CH2OH (d= 3.50 ppm) must be equal to the signal area of the protons CH2-COO (d= 4.12 ppm), which is not in agree with experimental values (see Table II copolymer 5). Therefore, the THPhA units are not forming diads THPhA-THPhA through the anhydride bond at backbone, but would be taking part of side chains.
According to the copolymer composition, experimental proton areas, FTIR data and a careful analysis of all signals from 1H-NMR spectra , it is postulated a structure B as general structure model to include the copolymer structure of all five copolymers HEEI/THPhA1,2. The structure B is formed by subunits whose relative mole concentrations were calculated for each one of the copolymers (see Table III). This constitutional structure model considers to THPhA1,2 units forming part either the backbone and the side chain, and the anhydride bonds are placed only in the side chains.
Table III shows the calculated values of the variables Xn ( X0, X1, X2, X3), Y and Zm for the structure B of all five copolymers HEEI/THPhA1,2. These variables represent to the relative concentration of the constitutional subunits present in the copolymer but not blocks of these subunits. From these values it is possible to calculate the copolymer composition according to:
W = [X0 + X1 + X2 + X3 + 2Y + Z0 + Z1 + Z2] / [X1 + 2X2 +3X3 + Y + Z0 + 2Z1 + 3Z2]
Table III shows also an excellent concordance between the experimental and calculated copolymer composition values for the all five copolymers HEEI/THPhA1,2.
|Table III. Structural composition of HEEI/THPhA1,2 copolymers|
| || ||HEEI/THPhA1,2|
|1||1.42||0.57||0.50||0.518||2.21 : 1.00||2.21 : 1.00|
|2||0.554||1.57||0.23||0.813||1.30 : 1.00||1.30 : 1.00|
|3||0.602||0.355||0.65||0.80||0.74 : 1.00||0.74 : 1.00|
|4||0.14||0.30||1.00||0.45||0.50||0.20||0.65 : 1.00||0,645 : 1.00|
|5||0.05||0.25||1.35||0.10||0.70||0.526 : 1.00||0,527 : 1.00|
Table III shows also that in 1 and 2 copolymer there is an incorporation of THPhA1,2 units in the backbone (subunit Y). As increases the concentration of THPhA1,2 units in the copolymer, these are placed preferably at the side chain, esterifying the CH2OH groups. When the concentration of THPhA1,2 units is higher than that the HEEI units (copolymers 3 to 5 ) appear subunits X2 , X3 and/or Z2 , which present anhydride bonds. From mole concentrations of these subunits (seeTable III) it the theoretical area for each signal of the 1H-NMR spectra of the copolymers HEEI/THPhA1,2 was calculated. In Table IV are compared the experimental and calculated values of the signal areas for the different protons. To the experimental value of the signal at d= 2.84 ppm, was substracted the D area, which is given in the last column in Table III, and to the area of signal at d= 2.27 ppm, was added the same value, due to the signals at d= 2.27 ppm and at d= 1.53 ppm, present the same number of protons and have the same area.
|No||d=4.12 ppm||d=3.5 ppm||d=2.84 ppm|
|d=2.27 ppm||d= 1.53 ppm|
Table IV shows a very good correlation between the experimental and calculated area values. Therefore, it is concluded that the model of proposed structure is adequate to describe the synthesized HEEI/THPhA1,2 copolymers.
The authors thank Dirección de Investigación, Universidad de Concepción, Chile (Grant No 97024014-1.3).
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