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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.51 n.1 Concepción mar. 2006 

J. Chil. Chem. Soc., 51, Nº 1 (2006)



1Departamento de Polímeros, Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile.

2Dirección de Ciencias Básicas, Departamento de Química, Universidad Iberoamericana de Ciencias y Tecnología, Casilla 13901, Santiago, Chile.

3Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.



Copolymers containing 4-vinylpyridine as common monomer with tert-butyl acrylate and acrylic acid of different comonomer compositions were synthesized and characterized. Copolymer composition was determined by elemental analysis, from which monomer reactivity ratios (MRR, r) were estimated using straight line intersection procedures such as Fineman-Ross and Kelen-Tüdõs methods and a nonlinear one, the Reactivity Ratios Error in Variables Model. In this case, the values of MRR are r4VPy = 0.046 and rtBA = 0.054 (for 4VPy-co-tBA), and r4VPy = 0.0 and rAA = 0.610 (for 4VPy-co-AA). A copolymer tending to the alternation and another containing isolated units of one of the monomers between small blocks of the another one were obtained, depending on the chemical nature of the comonomers. The different proposed comonomer distributions are analyzed in terms of the obtained MRR values and compared with related systems. The MRR are connected with structural properties of the monomers such as polarity, aromaticity and electron delocalization.

Keywords: functionalized vinyl copolymers; monomer reactivity ratios; comonomer sequence and distribution; 4-vinylpyridine.



The reactivity of a free radical depends on the nature of the groups close to the radical carbon [1]. When in a vinyl monomers CH2=CHR, the side group R can induce the delocalization of the radical electron, the radical stability increases. According to this concept some of the more common substituents of the vinyl group can be arranged in the following decreasing order of electron attractor power: C6H5 > CH=CH2 > COCH3 > CN > COOR > Cl > alkyl > OCOCH3. Thus, styrene (R = C6H5) has a radical which stabilization by resonance of 84 kJ/mol, a high value in comparison to vinyl acetate (R = OCOCH3), which has a very unstable radical [2]. However, resonance is not the only factor contributing to the monomer reactivity and polar effects beside steric hindrance must be considered .

Vinylpyridines are other monomers that bear aromatic groups as side chain, which gives to the corresponding monomer units, a high stabilization by resonance. By this reason, the reactivity of this kind of monomers is rather high and usually blocks of vinylpyridines are formed when they copolymerize with other monomers [3,4,5]. Moreover, they present other interesting properties from both fundamental and applied point of view due to the presence of pyridinic nitrogen atoms with basic character. These ones can interact with other polymeric materials by means of different kind of specific interactions such as hydrogen bonding to form compatible polymer blends with interesting properties [6-11].

Taking into account the high reactivity of monomers like vinylpyridines, it is interesting to study their copolymerization with other functionalized vinyl monomers and to analyze the kind of the comonomeric distribution in the copolymer obtained as a consequence of the different chemical functions.

The aim of this work is the synthesis, characterization and estimation of the monomer reactivity ratios (MRR, r) of copolymers of 4-vinylpyridine (4VPy) with tert-butyl acrylate (tBA) [poly(4-vinylpyridine-co-tert-butyl acrylate) (4VPy-co-tBA)] and with acrylic acid (AA) [poly(4-vinylpyridine-co-acrylic acid) (4VPy-co-AA)] (Scheme 1). Like this it would be possible to study the reactivity of a monomer containing an aromatic side chain and monomers bearing carbonyl groups contiguous to the backbone of the macromolecule and to analyze the effect of the chemical structure on the comonomer reactivity and on the comonomer distribution in the copolymer chains.


Monomer and copolymer preparation

Commercial samples of 4VPy, tBA and AA from Aldrich were distilled under vacuum before copolymer synthesis. Copolymers were obtained by radical polymerization in bulk at 323 K under nitrogen and a,a'-azobisisobutyronitrile (AIBN) as initiator. The monomer feed ratio was varied in a series of copolymerizations of both monomers. Copolymerization time was controlled to obtain low conversions of monomer to copolymer. Purification of the copolymers was achieved by dissolution in methanol and precipitation with mixtures petroleum benzine/ethyl ether and acetone/water for 4VPy-co-tBA and 4VPy-co-AA respectively. Copolymer samples were dried in vacuum at 298 K until constant weight.

Copolymer characterization

Copolymer were characterized by 1H-NMR in a Bruker AC 250P spectrometer using tetramethylsilane (TMS) as an internal standard. FTIR spectra in KBr were recorded using a Nicolet Magna-IR 550 instrument. Viscosity measurements were carried out with a Ostwald viscometer with negligible kinetic energy corrections. Intrinsic viscosity ([h]) was determined according to the Solomon-Gotessman relationship [12]. Compositions of the copolymers were determined by elementary analysis, following the variation of nitrogen content arising from the 4VPy comonomer units.


Intrinsic viscosities ([h]) in methanol at 298 K for unfractionated samples of 4VPy-co-tBA and 4VPy-co-AA are summarized in Table 1. [h] values should be useful in estimating qualitatively the degree of polymerization. 1H-NMR and FTIR spectra are in agreement with the copolymer structure expected. When the monomer and copolymer spectra are compared in the 1H-NMR study, the signal corresponding to the vinyl protons disappears from the spectral region close to 6.0 ppm. The comparative study of the FTIR spectra of the copolymers with regard to the corresponding monomers allows confirmation of the results obtained by 1H-NMR. The following characteristics absorption bands (in cm-1) are observed: 2929 (alkane C-H stretching), 1721 (ester carbonyl stretching), 1599/1558 (heteroaromatic ring stretching), 1460 (alkane -CH2- bending), 1418 (alkane -CH3 bending), 1375 (alkane C-H bending), 827 (heteroaromatic C-H out of plane bending) [for 4VPy-co-tBA] and 3434 (carboxylic acid O-H stretching), 2940 (alkane C-H stretching), 1714 (carboxylic acid carbonyl stretching), 1585 (heteroaromatic ring stretching), 1462 (alkane -CH2- bending), 1402 (alkane C-H bending), 836 (heteroaromatic C-H out of plane bending) [for 4VPy-co-AA]. The solubility of three different compositions of each copolymer was checked in a variety of solvents. The copolymer are soluble in a wide variety of them.

Compositions of the feed (f) and the resulting copolymers (F) are also compiled in Table 1. According to this results, the resulting copolymers are formed by comparable quantities of monomer units, as Figure 1 demonstrates (the discontinuous line represents the case of an ideal copolymerization, i. e., f = F). No excess of any comonomer is observed, which can be attributed to a similar reactivity of both monomers.

Table 1. Intrinsic viscosity ([h]) and feed (f4VPy) and copolymer
(F4VPy) compositions for the studied copolymers.

Table 2. Copolymerization data for 4VPy-co-tBA and 4VPy-co-AA: feed compositions ratio x, copolymer compositions ratio y, Fineman-Ross parameters G and F [10] and Kelen-Tüdõs parameters h, x and a [11].


Figure 2 shows the FR and KT plots for 4VPy-co-tBA as a representative example. From this kind of representations, it was possible to evaluate the MRR of the comonomers of 4VPy-co-tBA and 4VPy-co-AA. Table 3 compiles these values.

Table 3. Monomer reactivity ratios (r) obtained by FR, KT and RREVM methods
for 4VPy-co-tBA and 4VPy-co-AA.

In order to estimate the monomer reactivity ratios, the classical straight line procedures proposed by Fineman and Ross (FR) [13] and by Kelen and Tüdõs (KT) [14] were used. A computer procedure based on a nonlinear minimization algorithm statistically valid, which is known as Reactivity Ratios Error in Variable Method (RREVM) [15], was also used, starting from the MRR values obtained by the KT straight line method. Table 2 collects the copolymerization parameters according to the FR and KT methods, at different compositions. Data are presented considering 4VPy as monomer 1.

In spite of some differences between the MRR values, good agreement between both procedures is found. The variation in MRR values depending on the method used, are commonly obtained when linear procedures are used because the main weakness of these methods, i. e., the use of statistically invalid assumptions [15]. However, they can be considered as good initial values for running the non-linear computer methods [15], which allow one to take properly into account all the sources of experimental error. No groups of variables are considered to be independent and free of error, or dependent with constant error [15]. For this reason, MRR values were also determined by means of RREVM. In this method, the MRR values obtained by the KT method were used as starting values, using errors of 2 % for the monomer compositions in feed and of 5 % for the copolymer composition. Table 3 also shows MRR obtained by the RREVM method. Figure 3 is a representative example of the 95 % posterior probability contour for estimated r4VPy and rtBA for 4VPy-co-tBA. This kind of procedures gives elliptical probability contours as a reliability parameter.

According to the definition of MRR for 4VPy-co-tBA:

r4VPy = k(4VPy-4VPy) / k(4VPy-tBA) and rtBA = k(tBA-tBA) / k(tBA-4VPy)

with kii and kij, the self-propagating and cross-propagation rate constants respectively, this copolymer presents a tendency to the alternation (r4VPy = 0.046 and rtBA = 0.054, according to the RREVM method). With both reactivity ratios zero neither type of chain end can add its own monomer, so a perfectly alternating copolymer results, F = 50 mol % regardless of f, until one of the monomers is used up, at which point, polymerization stops [13]. In this case both MRR are close to zero. For that, a tendency to alternation is postulated and a F4VPy varying around 50 mol % can be observed (Figure 1-a). The presence of an aromatic group in the side chain of 4VPy conditions the relative monomer reactivity by means of the stabilization by resonance of the radicals generated during the copolymerization process. This stabilization process increases as the electronic delocalization increases, as would be expected in copolymers containing 4VPy and other aromatic units [3,17,18]. Other structural factor contributing significantly to increase the stability of a growing radical and then the reactivity of the corresponding monomer is the presence of some electron-attracting groups contiguous to the backbone of the macromolecule. This behaviour has been reported for some copolymers containing acrylates and methacrylates [19-22] and can be attributed to the carbonyl group on its corresponding growing radical in the propagation step. This group is polar because the electronegativity difference between the carbon and oxygen atoms. Thus, the positive charge density generated on the carbonyl carbon atom can favour a significant electron attraction in the tBA radicals of 4VPy-co-tBA. In this way, the presence of both the pyridinic group in 4VPy and the carbonyl group in tBA, can explain the kind of copolymer obtained. No preference of any comonomer to enter into the macromolecule is detected. A similar behaviour is observed in 4VPy-co-AA for 4VPy, which shows a trend to the cross propagation (r4VPy = 0.0). The relative reactivity of AA in relation to 4VPy (rAA = 0.610, r4VPy = 0.0) is slightly higher than the relative reactivity of tBA in relation to 4VPy (rtBA = 0.054, r4VPy = 0.046). This behaviour reveals the bigger steric hindrance of the tBA units to copolymerize as a consequence of its higher molecular volume. The same carbonyl carbon atom effect mentioned above allows explanation to the higher relative reactivity of AA. In this way, a copolymer formed by isolated units of 4VPy between short sequences (small blocks) of AA is obtained.


MRR values were obtained for the studied copolymers using both linear methods and the RREVM non-linear procedure. A good agreement is observed between the different methods. From the last one, the following values were estimated: r4VPy = 0.046 and rtBA = 0.054 (for 4VPy-co-tBA), and r4VPy = 0.0 and rAA = 0.610 (for 4VPy-co-AA).

The particular values of MRR allow postulate the formation of an alternating copolymer in the case of 4VPy-co-tBA, which was interpreted in terms of the stabilization by resonance and by electron attraction of the corresponding growing radicals. In the case of 4VPy-co-AA a cross propagation of 4VPy and a formation of small blocks of AA are proposed. Thus, the influence of the side chain nature and volume of the vinyl monomer over its relative reactivity and over the comonomer distribution was revealed.


We express our thanks to Dirección de Investigación Universidad de Concepción, project 201.025.022-1.0 for financial support.


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