139

CHAPTER 6

The suspension polymerization of vinyl chloride in the presence of a solvent for PVC

Abstract

An attempt was made to modify the course of the polymerization reaction by changing the composition of the polymer-rich phase. The addition of o-dichlorobenzene, which is a solvent for both PVC and VCM, results in a more swollen and thus less dense polymer-rich phase. As a result, the dynamics of the growing polymer chains increases and the appearance of the hot spot diminishes with an increasing amount of o-dichlorobenzene, until it is completely disappeared with the addition of at least 10 wt % of o-dichlorobenzene. Simultaneously the porosity of the PVC grains decreases significantly. The presence of o-dichlorobenzene clearly affects the formation of 2,4-dichloro-n-butyl and chloromethyl branches. Especially at very high monomer conversions, exceeding 85%, the number of these defects does not increase as dramatically as in case of the regular suspension polymerization of VCM. In addition to this, the thermal stability for PVC produced with very high monomer conversions in the presence of o-dichlorobenzene is also improved. Apparently, the intramolecular side reactions, which cause the formation of these types of defects, are hampered probably as a result of CH-- interactions between the growing macroradical and VCM or o-dichlorobenzene. At these very high monomer conversions, o-dichlorobenzene probably replaces VCM whose concentration has become very low, resulting in an almost constant number of 2,4-dichloro-n-butyl and chloromethyl branches formed during the overall polymerization process.

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6.1 Introduction

As opposed to the addition of the nonsolvent n-octane to the polymerization process of VCM, as described in the previous chapter, in this chapter the addition of a solvent for PVC and VCM is described. Many authors studied the solution polymerization of VCM in different solvents in comparison with bulk polymerizations 1-6. Only some publications have appeared, which describe the study of the precipitation polymerization of VCM in the presence of a solvent, such as diethyl oxalate and 2,4-dichloropentane or tetrahydrofuran 7,8, albeit never in case of a suspension polymerization. The addition of a solvent for both PVC and VCM to the polymerization system, in this case o-dichlorobenzene, is expected to increase the degree of swelling of the PVC in the polymer-rich phase. Although the concentration of VCM in the polymer-rich phase decreases because of dilution by o-dichlorobenzene, the total amount of VCM will not change as the polymer becomes more swollen 9. Especially at very high monomer conversions, when normally the polymer-rich phase almost reaches its Tg, this phase is expected to remain more swollen as o-dichlorobenzene is still present when VCM is almost depleted. A schematic ternary phase diagram of the PVC / VCM / o-dichlorobenzene system is constructed once more after the examples presented by Tompa 10,11 as is shown in Figure 6.1.

The suspension polymerization of VCM in the presence of a solvent for PVC

141

o-DCB

VCM PVC

Figure 6.1 Schematic ternary phase diagram for the PVC / VCM / o-dichlorobenzene (o-dcb) system: binodial (solid line); proposed tie lines (dotted line); constant weight fraction of o-dichlorobenzene in overall polymerization system (broken line)

VCM is a poor solvent for PVC while o-dichlorobenzene is a good solvent for both PVC and VCM. Similar to the addition of n-octane to the polymerization system (Chapter 5), the weight fraction of o-dichlorobenzene is constant during the entire polymerization with respect to the overall composition. Polymerization occurs along the dashed line from the VCM / o-dichlorobenzene side to the PVC / o-dichlorobenzene side, which corresponds to a certain amount of o-dichlorobenzene that has been added to the reaction mixture and could be regarded as a kind of conversion scale. When more o-dichlorobenzene is added to the polymerization system, this dashed line will move upwards in the ternary phase diagram in the direction of the o-dichlorobenzene vertex. As long as this dashed line still crosses the binodial, the polymerization process will occur in a two-phase system for a certain period of time. When so much o-dichlorobenzene is added that this dashed line will not cross the binodial anymore, the polymerization process will occur completely in a

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one-phase system and can be considered as if it were a solution polymerization. The polymerization always starts homogeneously in a pure liquid polymer-lean phase and if the dashed line eventually crosses the binodials, the polymerization mixture will phase separate and polymerization continues in both the pure liquid polymer-lean phase and a more gel-like polymer-rich phase. After a certain period of time, the polymer-lean phase will be completely consumed and polymerization only continuous in the polymer-rich phase. As already described in the previous chapter for the case of a polymerization in a two-phase system, the intersection of the dashed line with the tie lines, of which a few have been proposed in Figure 6.1, corresponds to the overall composition of the polymerization mixture at a certain moment during polymerization. The intersections of these tie lines with the binodial hereby give the corresponding compositions of the two separate phases. In the presence of o-dichlorobenzene, just as in case of n-octane, both the polymer-lean and the polymer-rich phase contain all three components, whereby the ratio between these components is unequal for both phases 11,12. In the presence of larger amounts of o-dichlorobenzene more VCM will be available in the swollen polymer-rich phase, although its concentration is expected to decrease. Due to the presence of this larger amount of VCM in the polymer-rich phase, the polymer-lean phase will contain less VCM than in the regular case. Therefore, this phase will be depleted earlier during the polymerization and as a result, the critical conversion Xf is expected to shift to lower values. With the consumption of VCM during the polymerization, the ratio between VCM and o-dichlorobenzene changes in the polymerization mixture during the entire process, whereby the solvent quality of the VCM / o-dichlorobenzene mixture continuously enhances. As a consequence, PVC will become more and more swollen, and the composition of the polymer-rich phase will therefore change continuously. Especially at high monomer conversions, when polymerization only occurs in the polymer-rich phase, the presence of an increasing amount of o-dichlorobenzene in relation to VCM becomes more important. At these high monomer conversions the polymer-rich phase will remain more swollen than in case of the regular polymerization process, resulting in a larger mobility of the growing polymer chains.

The suspension polymerization of VCM in the presence of a solvent for PVC

143

Therefore, the influence of the presence of o-dichlorobenzene on the course of the suspension polymerization of VCM has been examined, even up to very high monomer conversions, together with the resulting particle morphology, the formation of defect structures in the polymer chain and the allied thermal stability.

6.2 Experimental

The polymerization process as well as the different characterization methods have been described elaborately in the experimental sections of the previous chapters. The only variation in the polymerization process is the addition of various amounts of o-dichlorobenzene, viz. 1.6, 2.5, 3.0, 5.0 and 10.3 wt % in proportion to the initial amount of VCM. o-Dichlorobenzene, purchased from Merck (99%), was used as received, and added to the aqueous medium consisting of PVA and the buffer salt, before evacuation of the reactor to remove oxygen.

6.3 Results and Discussion

6.3.1 Polymerization trend

The influence of the presence of o-dichlorobenzene in the reaction mixture on the polymerization process of VCM was studied by the addition of the different amounts of o-dichlorobenzene. The courses of the pressure inside the reactor and the temperature of the heating system during these experiments are depicted in the Figures 6.2 and 6.3, respectively.

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144

0 200 400 600 800 1000 1200 14004.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

pres

sure

re

acto

r (ba

r)

polymerization time (min)

Figure 6.2 The course of the pressure during polymerization for various amounts of o-dichlorobenzene: 0 wt % (dashed line); 1.6 wt % (dotted line); 3.0 wt % (dash / dotted line); 10.3 wt % (solid line)

Just as for n-octane, the vapor pressure of o-dichlorobenzene is very low compared to that of VCM, namely < 0.01 versus 8.6 atm at 57.5 C, respectively. Therefore, a similar kind of lowering in the vapor pressure of the VCM / o-dichlorobenzene mixture is expected to be found. For the small amounts of o-dichlorobenzene no significant change in pressure is observed. However, with the addition of 10 wt % of o-dichlorobenzene a continuous decreasing pressure during the polymerization is indeed determined. This decrease is caused by the continuous increasing concentration of o-dichlorobenzene in this mixture. With the addition of o-dichlorobenzene a significant increase in the polymerization time that is needed to reach the desired monomer conversion of 87% is observed. This increasing polymerization time is probably a result of the deceleration of the reaction. Due to the increasing presence of o-dichlorobenzene the monomer concentration decreases and, as a consequence, the polymerization rate decreases

The suspension polymerization of VCM in the presence of a solvent for PVC

145

(see also equation 4.1 in Chapter 4). Besides this, a less pronounced gel effect, due to the presence of o-dichlorobenzene, can also cause an elongation of the polymerization time, as is shown in Figure 6.3.

0 100 200 300 400 500 60056

57

58

59

60

61

62

tem

pera

ture

hea

ter

(C)

polymerization time (min)

Figure 6.3 The course of the temperature of the heating system during polymerization to maintain the polymerization temperature at 57.5 C for various amounts of o-dichlorobenzene: 0 wt % (dashed line); 1.6 wt % (dotted line); 3.0 wt % (dash / dotted line); 10.3 wt % (solid line)

The hot spot gradually fades away with an increasing amount of o-dichlorobenzene. Due to the dilution effect of o-dichlorobenzene the monomer concentration is lowered, which inevitably results in a lowering of the polymerization rate and heat produced during polymerization. The gel effect probably also diminishes as the polymer-rich phase becomes more swollen in the presence of o-dichlorobenzene resulting in a higher mobility of the active polymer chains. This increased mobility enlarges the possibility of bimolecular termination, which will in turn result in a less pronounced gel effect. As can be observed in Figure 6.3, the hot spot has completely disappeared with the addition of 10 wt % of o-dichlorobenzene.

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146

6.3.2 Molecular weight

The molecular weight of PVC was studied in relation to the various amounts of o-dichlorobenzene (Figure 6.4).

30000

40000

50000

60000

70000

80000

90000

100000

0 2 4 6 8 10o-dichlorobenzene (wt %)

MW

(g/m

ol)

0.0

1.0

2.0

3.0

4.0

Mw

/Mn

Figure 6.4 Molecular weight development with increasing amount of o-dichlorobenzene in the

reaction mixture: nM (); wM (); nw MMD = ()

Both the number and weight average molecular weights of the final PVC products show a clear decreasing trend with the increasing amount of o-dichlorobenzene. Within the presence of a solvent the monomer concentration decreases, which in turn affects the decrease in the kinetic chain length (equations 4.1 and 4.1 Chapter 4). Secondly, every solvent will participate to some extent in the process of chain transfer of growing macroradicals, which will also result in a lower average molecular weight. Finally, the diminishing gel effect, which is the result of a less hindered bimolecular termination due to the increased mobility of the growing polymer chains, could in this case also have a small contribution to the decrease in molecular weight.

The suspension polymerization of VCM in the presence of a solvent for PVC

147

6.3.3 Particle morphology

The presence of o-dichlorobenzene during the polymerization of VCM also influences the final porosity of the PVC grains as can be deduced from Figure 6.5, in which the results from the Hg-intrusion test of these samples are shown.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.010.1110pore diameter (m)

Hg

intru

sion

volu

me

(ml/g

)

Figure 6.5 Mercury intrusion test of PVC powders for various amounts of o-dichlorobenzene: 0 wt % (); 2.5 wt % (); 5.0 wt % (); 10.3 wt % ()

The mean pore diameter increases with an increasing amount of o-dichlorobenzene. This increase in the size of the pores is probably caused by less dense packing of the primary particles, which appear to be larger in size and more fused together than in case of regular PVC grains. In this case it is better to talk about the packing of irregularly shaped agglomerates of primary particles, instead of separate particles, which results in larger pores or cavities between these agglomerates. The differences in the appearance of the primary particles are clearly visible when comparing SEM pictures of the inner part of PVC grains prepared in the presence of different amounts of o-dichlorobenzene as shown in Figure 6.6.

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148

A B

C

Figure 6.6 SEM pictures of PVC particles prepared with different amounts of o-dichlorobenzene: (A) 0 wt %; (B) 5.0 wt %; (C) 10.0 wt %

The interior of the PVC grains, produced in the presence of o-dichlorobenzene, has changed in the opposite direction as in case of the addition of n-octane. The addition of n-octane resulted in a decrease in size and a better separation of the primary particles, which altogether resulted in a smaller average pore diameter and a higher porosity of the PVC grains (Chapter 5). The lower porosity of the PVC grains formed in the presence of o-dichlorobenzene is supposed to be caused by two factors. Due to the formation of the agglomerates, consisting of primary particles that are fused together, pores which are situated inside these agglomerates are probably less accessible, if at all. Another reason for the lowered porosity is probably the remaining of a significant amount of o-dichlorobenzene inside the polymer grains, as its removal from the grain is very difficult even at 410-6 bar. This difficult removal of o-dichlorobenzene is caused by the

The suspension polymerization of VCM in the presence of a solvent for PVC

149

fact that o-dichlorobenzene is encapsulated within the polymer matrix, which is still below its Tg. The amounts of o-dichlorobenzene, which were still left in the PVC samples after this evacuation, have been determined by means of thermogravimetric analysis (TGA) and are presented in Table 6.1.

Table 6.1 Amount of o-dichlorobenzene left in the PVC grains after evacuation Initial amount of o-dcb (wt %) o-dcb left after evacuation (wt %) 10.3 5.0 5.1 2.9 3.0 2.2 2.5 1.6 1.6 0.9

Due to the presence of o-dichlorobenzene inside the polymer grains the primary particles are still partly swollen and, consequently, will occupy a large part of the grain, which would normally just be empty.

6.3.4 Defect structures

Of main interest is the influence of the presence of o-dichlorobenzene on the occurrence of side-reactions and, consequently, the formation of defect structures. By NMR-study only a significant change in the number of branches was found, whereas the changes in the number of other defects were all within the experimental error. In Figure 6.7 the relation is shown between the amount of o-dichlorobenzene, which was added to the reaction mixture, and both the total number of branches and the number of the different types of branches, separately.

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150

3.0

4.0

5.0

6.0

7.0

8.0

0.0 2.0 4.0 6.0 8.0 10.0

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10o-dichlorobenzene (wt %)

nu

mbe

r bra

nche

s / 1

000

VCM

Figure 6.7 Development of different types of branching in the polymer main chain per 1000 monomeric units for various amounts of o-dichlorobenzene: total branches (x); 1,2-dichloroethyl branches (); long chain branches (); 2,4-dichloro-n-butyl branches () and chloromethyl branches ()

From these results can be concluded that only the number of 2,4-dichloro-n-butyl and chloromethyl branches is significantly decreasing within the presence of o-dichlorobenzene during the suspension polymerization of VCM when compared to the regular PVC produced with a comparable monomer conversion of about 87%.. A possible explanation for the fact that only part of the branches appear to be formed to a lesser extent could be that only the occurence of the intramolecular side reactions is decreasing, although this is in contrast to the expectations. When the system

The suspension polymerization of VCM in the presence of a solvent for PVC

151

becomes more diluted by the addition of a solvent the intramolecular reactions are expected to become dominant over the intermolecular reactions as the polymer repeat unit concentration is decreasing, resulting in less overlap of the individual polymer coils 13. Furthermore, this decrease is only displayed up to 3 wt% of o-dichlorobenzene. A larger amount of o-dichlorobenzene does not seem to give any additional effect. For the moment no clear explanation can be found for both phenomena, but the conversion study of the polymerization in the presence of 3 wt % of o-dichlorobenzene as decribed in Section 3.3.6 will probably elucidate these uncertainties.

6.3.5 Thermal stability

The thermal stability of the PVC samples prepared in the presence of an increasing amount of o-dichlorobenzene was tested by means of the dehydrochlorination test as described in Chapter 3. The relation between the induction time (ti) and stability time (tst), and the amount of o-dichlorobenzene is depicted in Figure 6.8.

4

6

8

10

0 2 4 6 8 10o-dichlorobenzene (wt %)

t i (m

in)

16

18

20

22

t st

(m

in)

Figure 6.8 Dehydrochlorination test: ti, induction time () and tst, stability time () versus amount of o-dichlorobenzene

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152

From these results can be concluded that the start of the dehydrochlorination process is delayed in comparison to that of the regular polymer as ti increases linearly with the amount of o-dichlorobenzene. As already mentioned, o-dichlorobenzene could not be removed completely from these samples (Table 6.1). For that reason the resulting values of ti and tst were all corrected for the amount of o-dichlorobenzene left inside the polymer grains. It might be anticipated that o-dichlorobenzene itself could have some influence on the process of dehydrochlorination. However, Bengough and Sharpe 6,14,15 found that o-dichlorobenzene, when used as a solvent for PVC during the dehydrochlorination test performed in solution, did not play an active role in the degradation process. For this reason we assume that any participation of o-dichlorobenzene during the dehydrochlorination of PVC can be excluded. It seems plausible to assign this increase in induction time to the decreased number of the 2,4-dichloro-n-butyl branches, which are believed to be important initiating sites of the dehydrochlorination process due to the presence of a tertiary chlorine at the branch point carbon. It appears from the constant difference between ti and tst for all samples that in spite of the larger resistance against the first eliminations of HCl, the material degrades as fast as regular PVC once the degradation has started.

6.3.6 Conversion study VCM polymerization in the presence of 3 wt % of o-dichlorobenzene

To examine the influence of the presence of o-dichlorobenzene on the overall polymerization process of VCM, experiments were carried out up to five different monomer conversions, ranging from 12 up to 96%, for the amount of 3 wt % of o-dichlorobenzene.

Both the number and weight average molecular weights only show a minor decrease with the addition of 3 wt % of o-dichlorobenzene, which is probably the result of the dilution effect of o-dichlorobenzene as already mentioned in Section 6.3.2 (Figure 6.9).

The suspension polymerization of VCM in the presence of a solvent for PVC

153

25000

40000

55000

70000

85000

100000

0 10 20 30 40 50 60 70 80 90 100

monomer conversion (%)

mo

lecu

lar w

eigh

t (g/m

ol)

Figure 6.9 The course of number and weight average molecular weights with increasing

monomer conversion for regular polymerizations: nM (); wM () and with the addition of 3 wt % of o-dichlorobenzene: nM (); wM ()

The course of the number of the different types of branching with increasing monomer conversion in the presence of 3 wt % o-dichlorobenzene, in comparison with the results of the conversion series (Chapter 3) is shown in Figure 6.10.

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154

3.5

3.9

4.3

4.7

5.1

5.5

0 10 20 30 40 50 60 70 80 90 100monomer conversion (%)

nu

mbe

r MB

/ 100

0 VC

M

0.0

0.4

0.8

1.2

1.6

2.0

0 10 20 30 40 50 60 70 80 90 100monomer conversion (%)

nu

mbe

r LCB

/ 10

00 V

CM

0.2

0.6

1.0

1.4

1.8

2.2

0 10 20 30 40 50 60 70 80 90 100monomer conversion (%)

nu

mbe

r BB

& EB

/ 10

00 V

CM

Figure 6.10 Development of the number of different types of branching with increasing monomer conversion with the addition of 3 wt % of o-dichlorobenzene: 2,4-dichloro-n-butyl branches BB (); 1,2-dichloroethyl branches EB (), chloromethyl branches MB () and long chain branching LCB (); in relation to the growth of the number of branches for the regular polymerizations (open symbols)

The suspension polymerization of VCM in the presence of a solvent for PVC

155

From these results can be deduced that the growth in number of the 2,4-dichloro-n-butyl and chloromethyl branches is diminished significantly with increasing monomer conversion in the presence of o-dichlorobenzene, whereas the long chain branches and 1,2-dichloroethyl branches do not present any appreciably difference.

The rapid growth in the formation of 2,4-dichloro-n-butyl and chloromethyl branches, for monomer conversions beyond 85%, is clearly suppressed in the presence of o-dichlorobenzene. A plausible explanation for this effect could be a similar kind of interaction between the growing polymer chain and VCM or o-dichlorobenzene. It might be anticipated that both VCM and o-dichlorobenzene are able to protect the growing macroradical by means of donor-acceptor interactions with the polymer chain, with VCM and o-dichlorobenzene acting as -bases while PVC is a strong Lewis acid. As these interactions only seem to hinder the occurrence of intramolecular side-reactions, the radical chain end appears to be protected mostly, although no clear explanation for this can be given at the moment. At very high monomer conversions, beyond 85%, the effect of o-dichlorobenzene becomes more pronounced. It will act as a substitute for VCM whose concentration and, consequently, its protective action has become very low. Therefore, the growth in number of the 2,4-dichloro-n-butyl and chloromethyl branches is expected to be smaller with increasing monomer conversions, beyond 85%, as the intramolecular side-reactions will still be hindered due to the presence of o-dichlorobenzene.

When examining the thermal stability by means of the dehydrochlorination test, an obvious improvement is observed also, especially at very high monomer conversions near the limiting conversion of 96% (Figure 6.12).

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156

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90 100monomer conversion (%)

dhc-

rate

x 1

000

(%/m

in)

Figure 6.12 Dehydrochlorination (dhc) rate in relation to increasing monomer conversion for a regular polymerization () or after addition of 3 wt % o-dichlorobenzene ()

6.4 Conclusions

The suspension polymerization of VCM in the presence of o-dichlorobenzene was studied. The hot spot diminishes with the addition of o-dichlorobenzene and after the addition of at least 10 wt % the hot spot was completely vanished. Due to the addition of o-dichlorobenzene the monomer concentration in both the polymer-lean and polymer-rich phase decreases. As a result, the polymerization rate decreases and, as a consequence, less heat is created. Besides this, the polymer-rich phase will also be more swollen in the presence of o-dichlorobenzene, especially at higher monomer conversions. Therefore, bimolecular termination will probably be enhanced and, consequently, the gel effect will be less pronounced as the radical concentration will slightly decrease. This decreasing radical concentration also results in a lowering of the reaction rate and heat development. Due to the decrease in reaction rate, an increasing amount of o-dichlorobenzene requires a longer period of time to reach a monomer conversion of 87%. The polymerization of VCM in the presence of 3 wt % of o-dichlorobenzene with increasing monomer conversion up to 96% was also investigated. The numbers of 2,4-dichloro-n-butyl and chloromethyl branches are significantly lower for very high

The suspension polymerization of VCM in the presence of a solvent for PVC

157

monomer conversions when compared to those for regular PVC, while the thermal stability has significantly increased. The presence of o-dichlorobenzene especially seems to hinder the occurrence of the intramolecular side-reactions such as backbiting and H-shifts. This hindrance might be the result of the formation of a kind of protection shield around the polymer chain by means of CH- interactions. The intermolecular processes do not seem to be hampered at all by the presence of VCM or o-dichlorobenzene.

6.5 References

1. Blout, E. R.; Hohenstein, W. P.; Mark, H. Vinyl chloride; In Monomers. A collection of data and procedures on the basic materials for the synthesis of fibres, plastics and rubbers; Blout, E. R., Hohenstein, W. P., Mark, H., eds. Interscience Publishers, Inc.: New York, 1949; pp 1-32.

2. Talamini, G. J.Polym.Sci. 1966, 4, 535-537. 3. Crosato-Arnaldi, A.; Gasparini, P.; Talamini, G. Makromol.Chem. 1968, 117, 140-152. 4. Vidotto, G.; Crosato-Arnaldi, A.; Talamini, G. Makromol.Chem. 1968, 114, 217-225. 5. Crosato-Arnaldi, A.; Talamini, G.; Vidotto, G. Makromol.Chem. 1968, 111, 123-136. 6. Breitenbach, J. W.; Olaj, O. F.; Schindler, A. Makromol.Chem. 1969, 122, 51-64. 7. Ryska, M.; Kolinsk, M.; Lim, D. J.Polym.Sci., Part C 1967, 16, 621-631. 8. Mickley, H. S.; Michaels, A. S.; Moore, A. L. J.Polym.Sci. 1962, 60, 121-140. 9. Olaj, O. F.; Breitenbach, J. W.; Reif, H.; Parth, K. J. Angew.Chem. 1971, 83, 370. 10. Tompa, H. Transactions from the Faraday Society 1949, 45, 1142-1152. 11. Tompa, H. Phase relationships; In Polymer solutions; Butterworths Scientific Publications:

London, 1956; pp 174-232. 12. Scott, R. L. The Journal of Chemical Physics 1949, 17, 268-279. 13. Ahmad, N. M.; Heatley, F; Lovell, P. A. Macromolecules 1998, 31, 2822-2827. 14. Bengough, W. I.; Sharpe, H. M. Makromol.Chem. 1963, 66, 31-44. 15. Bengough, W. I.; Varma, I. K. Eur.Polym.J. 1966, 2, 49-59.