Makromol. Chem. 187,2649 -2653 (1986) 2649

On the preferential extraction of the more isotactic parts of PVC with acetone. A verification by I3C NMR analysis

Gerard0 Martinez, Carmen Mijangos, Jose Luis Millcfn *

C.S.I.C., Instituto de Plbticos y Caucho, Juan de la Cierva 3, 28006 Madrid, Spain

Antonio Alemany

C.S.I.C., Instituto de Quimica Orghica General, Juan de la Cierva 3, 28006 Madrid, Spain

(Date of receipt: December 23, 1985)*)

SUMMARY: Acetone-soluble and -insoluble fractions were obtained from two different PVC samples: a

polymer prepared in our laboratory at 60 "C using 2,2 '-azoisobutyronitrile (AIBN) as initiator (sample A) and a commercial polymer prepared at 70 "C (sample B). The content of isotactic, syndiotactic and heterotactic triads was accurately measured by high resolution 13C NMR. From the obtained data it follows that the isotactic triads accumulate preferably in the soluble fraction, the amount of which depends on the type of polymer. In agreement with earlier results, the soluble fraction is about 40% less stable than the insoluble fraction. The reasons for this behaviour are discussed in the light of the higher content of both isotactic triads and some defect structures, as found in the low molecular weight soluble extracts of PVC.

Introduction

It has been shown recently that the instability of PVC, whether thermal or photo- chemical, is closely related to the fraction of polymer which is soluble in acetone's2), and that the concentration of this fraction depends on the polymerization conditions, particularly the temperature when other conditions are similar3s4). This instability might be explained by a higher isotactic content or by a higher concentration of defect structures in the soluble fraction. In fact, it has been reported recently that some of the known defect structures in PVC are preferentially extracted with acetone5.@. Because these results provide new information as to some structural features of PVC, it appeared of prime necessity to compare more accurately the 13C NMR data for the acetone-insoluble and -soluble fractions in order to make quite sure of the higher isotactic content of the latter fractions. This paper deals with the results obtained for two PVC samples prepared under different experimental conditions.

Experimental part

Sample A was prepared by the bulk polymerization process at 60 OC using 2,2'-azoisobutyro- nitrile (AIBN) as initiator. Sample B was a commercial polymer obtained by bulk polymeriza-

a) Revised manuscript of June 18, 1986.

0025-1 16)</86/$03.00

2650 G. Martinez, C. Mijangos, J. L. Millhn, A. Alemany

tion at 7OoC stopped at 62% conversion. The extraction conditions with acetone have been described previously'. 3.

Number-average molecular weights were determined by osmometric measurements at 34 "C on solutions in cyclohexanone using a Knauer membrane osmometer (Tab. 1).

The tacticities were estimated from IR absorbance ratio at 1428 and 1434 cm-1 (A14,8/A1434). The spectra of films, cast from tetrahydrofuran solutions, were recorded using a Perkin Elmer model 783 spectrometer (Tab. 1).

I3C NMR spectra were recorded on an XL-300 Varian instrument, operating at 75,5 MHz. Previously, an estimation by the inversion recovery method of the T, relaxation times for the triad signals of the methine carbon was carried out. All triad signals relax with approximately the same of 0,3 s. To ensure integral accuracy, a flip angle of 80 O , a pulse repetition rate of 4 s, and broad-band proton decoupling was used. In the measurements the spectral width was 2000 Hz and 16 K data points were used. The samples were examined as 10 wt.-Vo solutions in mixtures of deuterated dimethyl sulfoxide/l,2-dichlorobenzene (vol. ratio 1 : 4), usually at 80 "C. 10 mm sample tubes were used and 10000 scans gave a very satisfactory signal-to-noise ratio.

These conditions proved to be quite appropriate for evaluating the content of triads without any degradation taking place during the spectra recording. Temperatures higher than 80°C could have degraded the isotactic labile conformations whose concentration is higher in the soluble fractions'). The calculations were carried out by measuring the areas with a compensat- ing polar planimeter. The estimated limit of error for these measurements was ca. 0,2%. The obtained values are given in Tab. 2, where P,,, P,, or P,, refer to the probability of iso, hetero and syndiotactic triads, respectively.

Soluble and insoluble fractions were degraded in powder state under nitrogen atmosphere at 180 "C. The results were analyzed in an earlier work1). They are indicated in Tab. 1.

Results and discussion

As shown by data in Tab. 1, within the limits of experimental errors, both acetone- soluble fractions (As and Bs) are quite similar in molecular weight. On the other hand, the molecular weights found for the whole polymers are in good agreement with those calculated by means of the equation Mnc = l / (Zq/&), which accounts for the accuracy of both the concentration of the soluble fraction and the molecular weight measurements.

As shown by Fig. 1, the good resolution of the obtained 13C NMR spectra allows for the content of iso, hetero and syndiotactic triads to be determined accurately by measuring the areas of the respective peaks. From the values obtained (Tab. 2) it follows that the content of isotactic triads is about 1,5% higher for the soluble fraction as compared with the insoluble fraction. Tab. 2 also includes the values of the probabilities for the whole polymers calculated by Z q P&)/Z q, where y refers either to the insoluble or to the soluble fraction and P ( 4 ) is the fraction of triads, iso, hetero or syndiotactic, that have been determined from the areas in the spectrum of each fraction. The fact that for all the triads the values obtained agree completely with those obtained from measurements on the spectrum of the whole polymers, confirms the reliability of the measurements. Unfortunately, similar calculations for isotactic pentads from the signals over the range of 4 3 3 to 46,O ppm (not shown in Fig. 1) proved much less accurate, because of difficulties in measuring the peak areas, deriving from the overlapping of bands.

On the other hand, Tab. 1 shows the values of the A1428/&34 absorbance ratio which is known to be a relative measure of syndio dyaddiso dyads ratio. In that these

On the preferential extraction of the more isotactic parts of PVC. . . 2651

Tab. 1. Characteristics of PVC samples A and B and their acetone-soluble and -insoluble fractions

Sample (wt.-Yo after M, . 1 0 - 3 M”c” IR extraction with acetone) 4 2 a cm-1

cm-I

As Soluble 24,8 - 1,06

Aw Unfractionated 38,9 39,3 1 ,I2

Ai Insoluble 50,7 - 1,15

Bs Soluble 25,8 - 1,07

Bw Unfractionated 36,O 36,O 1,15

Bi Insoluble 41,4 - 1,16

fraction (27,8)b)

polymer

fraction (72,2)b)

fraction (25,O)

polymer

polymer (75,O)

- 1 a) Calculated fromM = - (wi = wt.-Yo of soluble or insoluble fraction, M, =

“c z Wi/Mi osmometric molecular weigth of fractions).

uAi = 3 , 4 . 1 0 - 3 mol-qo . min - 1 1). b, Degradation rate at 180°C under nitrogen atmosphere: uAs = 5,7 . mol-Yo . min

r m I r r

Fig. 1. 13CNMR spectra of the acetone- soluble fraction of PVC sample A in the methine carbon region

5 8 5 1

6 in ppm

values appear to be in good agreement with those of 13C NMR the higher isotactic content of the soluble fraction relative to the insoluble one is quite assessed.

As indicated above, the content of isotactic triads of the soluble fractions only exceeds by I - 2 % that of the insoluble fractions (Tab. 2). These differences are consistent with earlier results on selective substitution by sodium benzenethiolate a t low temperatures, in which between 1 and 2% of isotactic triads, depending on the type of polymer, were demonstrated t o be extremely reactive relative to the remaining

2652 G . Martinez, C. Mijangos, J . L. Millan, A. Alemany

Tab. 2. Probabilities of isotactic (P-), heterotactic (P,), and syndiotactic placements (P,,) of PVC samples

Sample pm pmr PIT

As 0,203 0,508 0,289

Aw

Ai 0,189 0,508 0,303 Bs 0,207 0,512 0,281

0,198 0,504 0,298 Bw { 0,504a) { 0,3Ola) Bi 0,192 0,501 0,307

0,509 { 0,508a)

= probability of k e respective placement

= wt.-% of soluble or insoluble fraction;

(P-, P, or P,) of the fractions).

triads’). This led to the conclusion that the isotactic reactive triads are a definite type of conformation which was proposed to be the gttg* conformation‘). Thus, such conformation should be regarded as the one which is more abundant in the soluble fraction, but, unfortunately, its direct determination is not possible at the current stage of research work.

That the low molecular weight extracts are of higher isotacticity has already been suggested by the characterization through ’ H and 13C NMR of thirteen PVC samples investigated in a IUPAC Working Party on PVC defects5.@. Despite the strong differences in polymerization conditions, all the samples proved to be rather similar in overall tacticity, but the extracted fractions of two of them were appreciably more isotactic than the whole samples5.@.

The Working Party also succeeded in determining, by means of chemical methods, the content of defect structures, i.e. total unsaturation, internal double bonds, allylic and tertiary chlorines, chloromethyl branches, etc., which are usually claimed to be responsible for PVC instability. With respect to the low molecular weight soluble fractions of three of the thirteen samples investigated, the content of chloromethyl groups and of chain end unsaturations‘was found to be clearly higher than those of the whole polymers5.@.

Similar results have been obtained by H and 13C NMR studies on the same PVC samples and their soluble fractions”. These studies also demonstrated that the soluble fractions contain less long branches and internal double bonds per chain than the whole samples.

In conclusion, the acetone soluble fractions of PVC prove to be more isotactic and to contain more chloromethyl groups and terminal unsaturations and less long branches and internal double bonds per chain.

On the preferential extraction of the more isotactic parts of PVC. . . 2653

As illustrated by Tab. 1, the soluble fraction of sample A degrades markedly more rapidly than the insoluble fraction. The ratio of degradation rates, as determined from the slopes of degradation curves, indicates that the soluble fraction is 40% less stable.

On the basis of the above quoted results, claims could be reasonably made that both some isotactic triads and chloromethyl groups and chain end unsaturations are responsible for the low stability of the soluble fraction and, so, of PVC. This would be in good agreement with previous attempts to correlate degradation rate either with defect structures” or with a definite type of isotactic triads’). The individual contribution of each factor cannot be stated currently, at least in a quite reliable way. So far, the best correlation between degradation rate and defects is the one which gives main consideration to both the internal double bonds and branches longer than chlor~methyls~). As to the influence of isotactic content, a linear correlation was found recently, but important deviations from this behaviour were observed for polymers with considerable amount of defect structures3).

G. Martinez, C. Mijangos, J. Millb, J. Appl. Polym. Sci. 28, 33 (1983)

W, 211 (1985) 2, F. Castillo, G. Martinez, R. Sastre, J. Millan, V. Bellenger, J. Verdu, Polym. Degrad. Stab.

3, G . Martinez, C. Mijangos, J. Millh, Eur. Polym. J. 21, 387 (1985) 4, J. MillPn, G. Martinez, C. Mijangos, unpublishing results

T. Hjertberg, E. SOrvik, J. Vinyl. Technol. 7 , 53 (1985) 6) G. S. Park, J. Vinyl Technol. 7 , 60 (1985) 3 G. Martinez, C. Mijangos, J. MillPn, J. Appl. Polym. Sci. 29, 1735 (1984) *) M. F. Darricades-Llauro, A. Michel, A. Guyot, H. Waton, R. Petiaud, Q. T. Pham, J.

9, A. Guyot, M. Bert, P. Burille, M. F. Llauro, A. Michel, PureAppl. Chem. 53, 401 (1981) Macromol. Sci., Chem. 23, 221 (1986)