University of Groningen
New aspects of the suspension polymerization of vinyl chloride in relation to the low
thermal stability of poly(vinyl chloride)
Pauwels, Kim Francesca Daniëla
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to
cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2004
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Pauwels, K. F. D. (2004). New aspects of the suspension polymerization of vinyl chloride in relation to the
low thermal stability of poly(vinyl chloride) s.n.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 18-06-2017
CHAPTER 4
The Trommsdorff effect during
the suspension polymerization of VCM
Abstract
The occurrence of a heat effect during the suspension polymerization of VCM, which appears just
after the start of the pressure drop, was studied. This so-called hot spot is a result of the gel or
Trommsdorff effect, which already starts from the very beginning of the polymerization process,
after the formation of a two-phase system inside the polymerizing monomer droplets. However,
the gel effect becomes more pronounced after the disappearance of the pure liquid polymer-lean
phase when the swollen polymer-rich phase becomes more concentrated in PVC with a
continuously decreasing monomer concentration. The increase in monomer conversion and
molecular weight, which is a characteristic of the classical gel effect, is noticeable to a
considerable extent taking into account the opposing factors, such as a decreasing monomer
concentration and chain transfer to monomer as the main mode of termination.
The appearance of the hot spot depends on the mode of agitation, as this has a large influence
on factors such as heat transfer through the aqueous medium, heat loss via the reactor wall, and
the size of the polymerizing droplets. With decreasing droplet size, the enlarged total surface
area of these droplets induces an enhancement of the heat transfer to the surrounding aqueous
medium, which results in a more pronounced heat effect.
No relation was found between the appearance of the hot spot and the formation of defect
structures, and consequently the thermal stability of PVC.
99
CHAPTER 4
4.1 Introduction
The polymerization of VCM is an exothermic process. In our case the polymerization
is carried out in suspension in a 1-l autoclave, as described in Chapter 2. The heating
system consists of an electrical heating coil while cooling occurs by means of water
passing trough a cooling spiral, both situated inside the surrounding mantle. Despite
the exothermic reaction still additional heating is needed during the polymerization
due to heat loss via the reactor wall and lid.
During the suspension polymerization of VCM a heat effect is observed at the
moment the pressure has just started to drop. In literature this effect is known as ‘hot
spot’ 1,2, ‘heat peak’ 3, ‘peak exotherm’ 1,4 and ‘max effect’ 5. At this particular moment
during the polymerization less capacity of the heating system is needed to maintain
the temperature of the reaction mixture at the desired value. The reason for this effect
is an increased heat production inside the polymerizing particles. Some authors
ascribe this increased heat production to the well-known phenomenon occurring
during radical polymerizations called Trommsdorff or gel effect 2,6-9, originating from a
diffusion controlled bimolecular termination process 10. Due to this delay in
termination the average molecular weight increases during the gel effect 2,7,9,11.
However, literature concerning this heat effect in VCM polymerization is very
confusing. Although it seems reasonable to explain this phenomenon by means of the
gel effect, not all observations match with the well-known characteristics of the gel
effect.
In the case of VCM polymerization, growing macroradicals are terminated either by
the bimolecular mode (combination or disproportionation) or by chain transfer to
monomer 11-17.
According to many authors 8,11,17-22 chain transfer to monomer is the main mode of
termination of the growing chains, as it has become clear that transfer to monomer
almost completely controls the molecular weight averages. Razuvayev et al. 14
mentioned that 19 to 40% of all growing polymer chains are terminated by
bimolecular termination. Ten years later Hauss 21 determined the extent of
bimolecular termination during the polymerization of VCM. He found that about 80%
100
The Trommsdorff effect during the suspension polymerization of VCM
of the chains are terminated by transfer to monomer and only approximately 20% by
bimolecular termination, and according to Park and Smith 23 only 25% of all
bimolecular termination occurs by combination. Others confirm the dominating mode
of disproportionation in bimolecular termination 11,24.
Because of the great importance of chain transfer to monomer, and the much smaller
significance of bimolecular termination, the overall mode of termination of growing
PVC chains is inconsistent with the characteristics of the classical gel effect. As a
consequence, the origin of the observed heat effect during the polymerization of VCM
remains unclear and is therefore studied further at our laboratory.
The occurrence of the hot spot is examined in relation to various properties such as
molecular weight, particle size and thermal stability.
4.2 Experimental
The polymerization reactions that were carried out during the optimization of the
suspension polymerization process in the 1-l autoclaves and the conversion study of
this polymerization, described in Chapter 2 and 3 respectively, were used to examine
the observed heat effect.
The polymerization process as well as the different characterization methods have
been described in great detail in the experimental sections of these previous
chapters. Only a few minor variations in the polymerization process have to be
mentioned because of the preparation of some additional samples, comprising the
addition of a larger amount of PVA or the addition of carbon tetrabromide as chain
transfer agent (CBr4, 98%, Acros Organics). This chain transfer agent was added at
the moment of charging the reactor with PVA-solution, buffer and RO-water.
101
CHAPTER 4
4.3 Results and Discussion
4.3.1 Polymerization trends
In Figure 4.1 the trends of pressure and temperature inside the reactor vessel and the
temperature inside the heating mantle are depicted for a standard suspension
66
10.0
64
9.5
62
9.0
60
8.5
58
8.0
56
7.5
54
7.0
52
6.5
50
6.0
48
0
50
100
150
200
250
300
350
pressure reactor (bar)
temperature reactor + heater (°C)
polymerization of VCM at 57.5 °C run up to a monomer conversion of about 87%.
5.5
400
polymerization time (min)
Figure 4.1 Reaction trends during standard suspension polymerization of VCM at 57.5 °C:
temperature inside reactor (dotted line), temperature heater (solid line), pressure inside reactor
(dashed line)
The polymerization of VCM occurs at a constant temperature of 57.5 °C, which is
controlled with an accuracy of ±0.1 °C by the use of a P.I.D.-controller (Chapter 2).
From the course of the temperature of the heater two regimes can be observed. First,
the temperature decreases slightly with increasing monomer conversion. This
decrease could partly be explained by the change in specific heat of the
polymerization mixture as the amount of PVC increases and that of VCM decreases.
102
The Trommsdorff effect during the suspension polymerization of VCM
However, this effect is probably mainly caused by the occurrence of a slightly
increasing degree of the gel effect. As polymerization occurs mainly in the polymerrich phase, bimolecular termination is expected to be retarded to a certain extent
even before Xf. This results in an increase in radical concentration and consequently
in monomer consumption, which is an exothermic process, causing an increasing
heat production. Therefore, less energy is needed from external sources to maintain
the polymerization temperature, which results in a decreasing temperature of the
heating system. The volume fraction of the polymer-rich phase in the polymerizing
droplets increases with increasing monomer conversion, without changing its
composition up to Xf, causing a gradually increasing degree of the gel effect.
Subsequently, just after the pressure drop has started, the heat effect becomes more
obvious, as from this moment on the composition of the polymer-rich phase starts to
change, because the polymer-lean phase is completely consumed. The polymer-rich
phase then becomes more concentrated and therefore the mobility of the chains
decreases dramatically and bimolecular termination will be retarded even more,
resulting in a more distinct gel effect. The increasing heat production is expected to
be the result of an increasing monomer conversion, due to an increase in radical
concentration, which induces acceleration of the rate of polymerization 25,26. However,
no appreciable increase in the growth of monomer conversion as a consequence of
an increasing reaction rate during the hot spot is observed (Figure 4.2).
103
65
90
64
80
63
70
62
60
61
50
60
40
59
30
58
20
57
10
56
monomer conversion (%)
100
0
0
200
400
600
800
1000
polymerization time (min)
1200
temperature heater (°C)
CHAPTER 4
55
1400
Figure 4.2 Development of monomer conversion (dashed line) and temperature inside the
reactor mantle (solid line) during suspension polymerization of VCM at 57.5 °C
The monomer conversion seems to continue growing linearly during the hot spot.
According to Talamini et al. 27 this almost linear growth instead of a dramatic increase
in monomer conversion could be caused by a decrease in reaction rate due to
lowering of the initiator efficiency, which occurs simultaneously with the increase in
reaction rate induced by the gel effect. This explanation sounds reasonable as long
as chain transfer to monomer is completely denied. However, in the case of VCM
polymerization a very large part of the initiation occurs by chain transfer to monomer,
which will not be decelerated, as enough monomer is still present in the dense
polymer-rich phase.
A more plausible explanation might be as follows: As the propagation rate is
proportional to the monomer concentration it is reasonable to expect a decreasing
polymerization rate with the decreasing monomer concentration in the polymer-rich
phase after Xf 28. However, due to the occurrence of the gel effect, in which
bimolecular termination becomes retarded, the propagation rate is expected to
increase as the radical concentration increases (equation 4.1).
104
The Trommsdorff effect during the suspension polymerization of VCM
v p ~ [M][P •]
v tr ~ [M][P •]
(4.1)
v t ~ [P •]
2
where vp, vtr and vt, are the rates for propagation, chain transfer to monomer and
termination, respectively, [M] the monomer concentration and [P•] the macroradical
concentration.
Both effects will probably counteract and therefore the polymerization rate remains
constant with respect to the reaction rate before the heat effect. The almost linear
increase in monomer conversion during the hot spot means that the increase in
reaction rate due to the gel effect indeed occurs as otherwise a decrease in the rate
of monomer conversion would have been observed due to the decrease in monomer
concentration in the polymer-rich phase. Furthermore, the total effect might be minor
with respect to the amount of heat produced (see section 4.3.3).
105
CHAPTER 4
4.3.2 Molecular weight
Due to the occurrence of the gel effect a significant increase in molecular weight is
also expected. When examining the development of the number and weight average
molecular weights of PVC with increasing monomer conversions this is indeed
observed (Figure 4.3), which also resembles results from others 8,20,29.
87%
monomer conversion
66
39%
90000
96%
64
13%
80000
62
70000
60
60000
58
50000
56
40000
54
30000
52
20000
temperature heater (°C)
100000
molecular weight (g/mol)
93%
81%
68%
50
0
200
400
600
800
1000
polymerization time (min)
1200
1400
Figure 4.3 Development of molecular weight of PVC and the course of the temperature of the
heater during suspension polymerization of VCM at 57.5 °C: Mn (), Mw (), temperature heater
(solid line)
It should also be mentioned that this figure presents cumulative values for Mn and
Mw , and at the moment the hot spot occurs about 60% of the initial amount of
monomer has already been polymerized. During the hot spot only about 20% of the
monomer is polymerized, which only counts for a quarter of the determined average
molecular weights. Unfortunately, it is not possible to determine the molecular weight
of the polymer formed instantaneously during the hot spot. Nevertheless, the increase
in the number and weight average molecular weight is evident, although this increase
106
The Trommsdorff effect during the suspension polymerization of VCM
is not as large as for free radical polymerizations in which bimolecular termination is
the only mode of termination 30,31.
The suppression of the expected substantial increase in molecular weight during the
hot spot is probably due to the large importance of chain transfer to monomer as a
mode of termination in the free radical polymerization of VCM 11,12,14,17,18,20. By chain
transfer to monomer, the radical concentration will not diminish, as new chain growth
will be initiated. The kinetic chain length depends on the rate of propagation,
termination and transfer reactions as described in equation 4.2 28.
Pn =
vp
v t + v tr
(4.2)
Both the rate of propagation and chain transfer to monomer are proportional to the
monomer and the radical concentration, while the rate of termination solely depends
on the square of the radical concentration (equation 4.1).
If the gel effect occurs, the number of growing macroradicals will increase due to the
retardation of bimolecular termination. Additionally, the concentration of the initiator
dissolved in the monomer which swells the polymer-rich phase will probably also
increase as the amount of monomer in this phase decreases, causing another
increase in the number of active polymer chains. As a consequence, the rate of
propagation as well as the rate of chain transfer to monomer will increase, as both are
proportional to the radical concentration and still enough monomer is present to
polymerize. Although, the rate of bimolecular termination is even proportional to the
square of the radical concentration, this rate will not increase but conversely decrease
because its reaction rate constant kt is diffusion controlled 10,32. Therefore, the
increase in chain transfer to monomer will probably partly counteract the expected
considerable rise in molecular weight due to the increase in propagation and the
decrease in bimolecular termination.
107
CHAPTER 4
4.3.3 Magnitude of the hot spot
As can be concluded from the results mentioned above, the characteristic effects of
the gel effect are less pronounced for the suspension polymerization of VCM.
Therefore, the magnitude of this gel effect is also expected to be small. To confirm
this, the extra amount of PVC formed during the hot spot was roughly estimated as is
depicted in Figure 4.4.
4
out put (%)
3
A1
A2
2
B
1
0
100
200
300
polymerization time (min)
Figure 4.4 Output of heater during polymerization; regular polymerization (solid line), theoretical
output without reaction (dotted line), heat of polymerization during regular polymerization (area A1
and A2), heat of polymerization during hot spot (area B)
In this figure the output of the heating system in a percentage of the capacity is
presented. Besides this, a dotted line has been drawn representing the capacity that
is needed to heat the reaction mixture to a temperature of 57.5 °C with no reaction
taking place, but by which the heat loss via the reactor wall is taken into account. Due
to a changing composition of the reaction mixture and therefore a decrease in the
108
The Trommsdorff effect during the suspension polymerization of VCM
specific heat, as the specific heat of PVC is much lower than that of VCM (1.05 and
1.59 J·g-1·°C-1, respectively), this line shows a slight decreasing trend with the
supposed increasing monomer conversion. The difference between this theoretical
and the experimental curve corresponds to the reaction heat formed during the
exothermic polymerization of VCM. The areas A1 and A2 represent the amount of heat
developed during the polymerization of VCM, without any heat effect occurring, while
area B stands for the excess of heat formed during the hot spot. By calculating the
ratio between area B and A2, the excess of PVC formed during the hot spot can be
determined, which appears to be approximately 23%. When compared to the total
amount of PVC formed during the entire polymerization, the heat effect corresponds
to 11% excessive PVC. These values are in good agreement with those obtained
from exotherms measured by Meeks et al. 2. These results confirm that, although
some extra PVC is formed during the hot spot, this amount is not as high as would be
expected for a classical gel effect 6-8.
4.3.4 Chain transfer agent
As the development of molecular weight and reaction rate during the hot spot do not
completely give conclusive prove for the occurrence of a gel effect as a consequence
of retardation of the bimolecular termination, a few additional experiments were
performed to support this explanation.
It is well known that the addition of chain transfer agents (CTA) suppresses the
occurrence of the gel effect.
A proper chain transfer agent for the polymerization of VCM is carbon tetrabromide
(CBr4), which is used by many others who studied the kinetics of VCM polymerization
22,33-36
. The amounts of CBr4 which were added to the polymerization system were
calculated using equation 4.3 28, as stated below:
1
= 1
+ Ctr [CTA ] [M]
P 'inst
Pinst
(4.3)
109
CHAPTER 4
in which Pinst and P 'inst are the instantaneous number average degrees of
polymerization in case of absence and presence of CTA, respectively.
The activity of the chain transfer agent is expressed by the chain transfer constant
(Ctr), which is defined as the ratio of the reaction rate constant of chain transfer and
the reaction rate constant of propagation (equation 4.4) 28.
Ctr ≡
k tr
(4.4)
kp
The chain transfer constant for CBr4 in a homogeneous system is 4.7, as determined
by Vidotto et al. 37. As no chain transfer constant for a heterogeneous system could
be found this value was used. In Figure 4.5 the course of the temperature of the
heater with increasing polymerization time for polymerizations of VCM in the
presence of CBr4 is compared with that of a standard polymerization.
64
63
temperature heater (°C)
62
61
60
59
58
57
56
0
50
100
150
200
250
300
350
400
450
polymerization time (min)
Figure 4.5 The effect of [CBr4] on the occurrence of the heat effect; 0 mol·l-1 (dashed line), 4.2·103
mol·l-1 (dotted line), 1.3·10-2 mol·l-1 (solid line)
110
The Trommsdorff effect during the suspension polymerization of VCM
The concentration of CBr4 is measured in relation to the amount of VCM present in
the polymerization system. With an increasing amount of CBr4 the hot spot
diminishes. As the addition of CTA suppresses the occurrence of the gel effect, it can
be concluded that the observed heat effect during the polymerization of VCM is
indeed related to the gel effect and thus retardation of bimolecular termination. The
molecular weight also decreases with the increasing amount of CBr4 (Table 4.1),
which is a second proof for the substitution of bimolecular termination with chain
transfer to CBr4.
Table 4.1 Relation between the addition of CBr4 and Mn
[CBr4] (mol/l)
Mn (g/mol) calculated
Mn (g/mol) measured
0
-
44000
-3
20000
38000
-2
10000
32000
4.2·10
1.3·10
However, this decrease in molecular weight does not correspond to the expected
values based on Ctr = 4.7. Probably a deviating chain transfer constant is valid for this
heterogeneous system. Besides that it is also possible that a part of the CBr4 has not
been distributed over the VCM droplets. Unfortunately, no clear information could be
found about the partition equilibrium of CBr4 over this type of polymerization system.
111
CHAPTER 4
4.3.5 Agitation
By varying the agitation speed, the particle size of PVC changes, as is shown in
Figure 4.6 (see also chapter 2: Minimizing particle size by means of agitation
experiments). With increasing agitation speed the mean particle size decreases
dramatically and the minimal achievable size of approximately 150 µm is obtained
with an agitation speed of 900 - 1000 rpm 38. When higher agitation speeds are used
during the polymerization both the particle size and the particle size distribution again
increase considerably.
500
mean particle size (um)
450
400
350
300
250
200
150
100
300
500
700
900
1100 1300 1500
agitation speed (rpm)
Figure 4.6 Mean particle size of PVC grains as a function of agitation speed
When studying the occurrence of the heat effect in relation to agitation speed, a
remarkable effect was found. The hot spot displays a reverse trend compared to the
particle size, as it increases at first when stirring more vigorously, followed by a
decrease again, as is shown in Figure 4.7.
112
The Trommsdorff effect during the suspension polymerization of VCM
500 rpm
600 rpm
62
60
58
62
60
58
56
56
0
100
200
300
400
500
600
0
100
polymerization time (min)
500
60
58
56
200
300
400
500
60
58
600
0
100
58
56
300
400
300
400
500
100
500
600
58
56
polymerization time (min)
200
300
400
polymerization time (min)
400
500
600
500
600
500
600
1000 rpm
60
58
56
0
100
200
300
400
1300 rpm
64
60
100
300
polymerization time (min)
62
0
200
62
600
temperature heater (°C)
temperature heater (°C)
60
200
200
1200 rpm
64
62
100
0
polymerization time (min)
1100 rpm
0
56
64
62
polymerization time (min)
64
58
polymerization time (min)
56
100
60
600
temperature heater (°C)
temperature heater (°C)
temperature heater (°C)
400
900 rpm
64
62
0
temperature heater (°C)
300
62
polymerization time (min)
800 rpm
64
200
700 rpm
64
temperature heater (°C)
64
temperature heater (°C)
temperature heater (°C)
64
500
600
62
60
58
56
0
100
200
300
400
polymerization time (min)
Figure 4.7 Influence of agitation speed on the appearance of a hot spot
This effect could indicate that diffusion of VCM from the gas phase towards the
particles would be involved in the occurrence of the gel effect, because a decreasing
particle size implies an increasing total surface of the particles. One might expect that
the enhanced diffusion through this enlarged interface would cause a certain increase
of the reaction rate. In this case, diffusion of VCM from the gas phase into the
polymerizing particles would be the limiting factor for the occurrence of the
Trommsdorff effect.
However, this explanation seems implausible, as at the moment of the hot spot a
sufficient amount of VCM is still present in the polymer-rich phase and thus the
continuity of the polymerization is not yet solely dependent on the supply of monomer
from the aqueous and gaseous phase.
113
CHAPTER 4
Nevertheless, the particle size, which in turn is determined by the agitation speed, is
expected to be of influence on the appearance of the hot spot. When particles are
smaller, transfer of reaction heat from the polymerizing particles to the surrounding
aqueous medium becomes easier. This facilitated heat transfer would just result in a
more pronounced hot spot, which reveals itself in a sharper and less spread out dip in
the curve of the temperature of the heater.
Besides the differences in the appearance of the hot spot with varying agitation
speeds, obvious differences are also observed in the initial temperature of the heater,
which is necessary to warm up the reaction mixture to the polymerization temperature
of 57.5 °C. When lower agitation speeds are used the temperature of the heater is
much lower than when stirred at higher speeds.
By imitating the composition of the polymerization mixture at different monomer
conversions in a transparent beaker of similar dimensions as the reactor vessel, the
behavior of this mixture at different agitation speeds could be examined. From these
experiments can be concluded that probably the only reason for the variations in the
initial heating temperatures and the appearance of the hot spot between
polymerization reactions carried out at different agitation speeds, is of physical
matter. With raising the agitation speed, heat loss via the reactor jacket to the outside
is increased due to a larger vortex, and as a consequence a larger contact area of the
reaction mixture with the wall of the reactor vessel. Consequently, more heat input is
necessary to maintain the polymerization temperature of 57.5 °C.
This was demonstrated by carrying out the regular polymerization reaction, with an
agitation speed of 1000 rpm, in a completely insulated reactor. The insulation was
performed by surrounding the heating mantle, bottom and lid with a thick layer of
cotton wool and aluminum foil. This resulted in a lowering of the initial temperature to
a value almost similar to the polymerization temperature and about 2 °C lower than in
case of the non-insulated system. The temperature is even lower than found for an
agitation speed of 500 rpm without insulation. Thus heat loss via the reactor wall is a
significant factor in our set up.
114
The Trommsdorff effect during the suspension polymerization of VCM
The hot spot is present for all polymerization reactions of VCM carried out with
different agitation speeds, however, it’s appearance strongly depends on this mode of
agitation as it influences the heat transport between the polymerizing particles and
the reactor wall via the aqueous medium, and in addition the extent of heat loss via
the reactor wall.
4.3.6 Defect structures
The relation between the formation of defect structures in the polymer chain and the
occurrence of the heat effect was examined. In Figure 4.8 (Chapter 3) the total
amount of various types of branches and the internal allylic structures, which are
expected to influence the final thermal stability of the polymer 39-41, are plotted versus
monomer conversion ranging from 13 to 96%.
9.0
0.9
Xf
8.5
0.8
7.5
7.0
0.7
6.5
0.6
6.0
5.5
0.5
5.0
4.5
4.0
number IA / 1000 VCM
number branches / 1000 VCM
8.0
0.4
3.5
3.0
0
10
20
30
40
50
60
70
80
90
0.3
100
monomer conversion (%)
Figure 4.8 Number of irregular structures per 1000 VCM units as a function of monomer
conversion: number of internal allylic structures (), number of branches (); the region of hot
spot is marked in gray
115
CHAPTER 4
4.3.7 Thermal stability
The thermal stability of these PVC samples was tested by means of measuring the
rate of dehydrochlorination as described in Chapter 3. In Figure 4.9 the rate of
dehydrochlorination is plotted versus monomer conversion. In both figures the stage
of the hot spot is clearly marked.
7.0
Xf
dhc-rate x 1000 (%/min)
6.0
5.0
4.0
3.0
0
10
20
30
40
50
60
70
80
90
100
monomer conversion (%)
Figure 4.9 Dehydrochlorination (dhc) rate of PVC samples produced with increasing monomer
conversion: the region of hot spot is marked in gray
As the radical concentration is expected to increase during the hot spot, it seems
reasonable that the probability of side-reactions would also increase. However, no
significant increase in the number of defects or decrease in thermal stability is
observed for polymers, which are produced up to and including the hot spot.
Therefore, from these results can be concluded that the gel effect probably has no
distinct influence on the formation of defect structures in the polymer chain, and the
resulting lowering of the thermal stability of PVC.
116
The Trommsdorff effect during the suspension polymerization of VCM
4.4 Conclusions
After closer examination of the suspension polymerization process of VCM at 57.5 °C
with regard to the observed heat effect, the so-called hot spot, some important
conclusions can be made. The gel effect is already present early from the start of the
polymerization, as the output of the heating system, necessary to maintain the
constant polymerization temperature, decreases gradually but continuously already
from the beginning. More precisely, the gel effect already exists from the beginning of
the two-phase polymerization system inside the polymerizing droplets, due to
precipitation of the formed PVC chains from the pure liquid polymer-lean phase. The
polymer-rich phase, which consists of PVC swollen in approximately 34 wt % of VCM,
is very viscous, which causes retardation of bimolecular termination. From the
moment the pure liquid polymer-lean phase is completely consumed, the monomer
inside the polymer-rich phase cannot be replaced instantaneously anymore once it is
polymerized. Therefore, the polymer-rich phase becomes more and more
concentrated and as a consequence the appearance of the gel effect will be more
pronounced, resulting in a well observable hot spot. Due to its large contribution,
chain transfer to monomer acts as the main mode of termination at the cost of
bimolecular termination. As a consequence, the gel effect is not as large as in cases
of free radical polymerizations for which bimolecular termination is the only mode of
termination. Therefore, the increase in rate of conversion and molecular weight during
the hot spot, which are the characteristics of the classical gel effect, is only moderate.
The amount of excess PVC formed during the hot spot is about 11% of the total
amount of polymer formed during the entire polymerization.
The appearance of the hot spot depends on the mode of agitation, which comprises
agitation speed, shape and position of baffles, etc. The mode of agitation has a large
influence on factors such as heat transfer through the aqueous medium, heat loss via
the reactor wall, and the size of the polymerizing droplets. By affecting the size of
these droplets and consequently their total surface area, the mode of agitation also
indirectly influences the exchange of heat between these droplets and the
surrounding aqueous medium, as a larger total surface area induces an enhanced
117
CHAPTER 4
heat transfer resulting in a more pronounced heat effect.
No relation was found between the occurrence of the gel effect and the number of
defect structures in the polymer chain, such as branching and internal allylic
structures, and the rate of degradation of PVC. Only after the hot spot, from
approximately 85% of monomer conversion, both the number of these defects and
the rate of degradation start to increase dramatically with increasing conversion. From
these results can be concluded that the gel effect during the polymerization of VCM
does not affect the thermal stability of PVC.
4.5 References
1. Chan, R. K. S.; Langsam, M.; Hamielec, A. E. J.Macromol.Sci.-Chem. 1982, A17, 969-981.
2. Meeks, M. R. Polym.Eng.Sci. 1969, 9, 141-151.
3. Xie, T. Y.; Hamielec, A. E.; Wood, P. E.; Woods, D. R. J.Appl.Polym.Sci. 1991, 43, 12591269.
4. Xie, T. Y.; Hamielec, A. E.; Wood, P. E.; Woods, D. R. J.Appl.Polym.Sci. 1987, 34, 17491766.
5. Ugelstad, J.; Mørk, P. C.; Dahl, P.; Rangnes, P. J.Polym.Sci., Part C 1969, 27, 49-68.
6. Norrish, R. G. W.; Smith, R. R. Nature 1942, 150, 336.
7. Trommsdorff, E.; Köhle, H.; Lagally, P. Makromol.Chem. 1947, 1, 169-198.
8. Bengough, W. I.; Norrish, R. G. W. Proc.R.Soc.London, A 1950, 200, 301-320.
9. Nilsson, H.; Silvegren, C.; Törnell, B. Angew.Chem. 1983, 112, 125-142.
10. Odian, G. Radical chain polymerization; In Principles of Polymerization; A Wiley-Intersience
Publication, John Wiley & Sons, Inc.: New York, 1991; pp 198-334.
11. Abdel-Alim, A. H.; Hamielec, A. E. J.Appl.Polym.Sci. 1972, 16, 783-799.
12. Breitenbach, J. W. Makromol.Chem. 1952, 8, 147-155.
13. Burnett, G. M.; Wright, W. W. Proc.R.Soc.London, A 1954, 221, 28-36.
14. Razuvayev, G. A.; Petukhov, G. G.; Dodonov, V. A. Vyskomol.Soedin. 1961, 3, 1549-1553.
15. Sobue, H.; Kubota, H. Makromol.Chem. 1966, 90, 276-283.
16. Ryska, M.; Kolinský, M.; Lim, D. J.Polym.Sci., Part C 1967, 16, 621-631.
17. Vidotto, G.; Crosato-Arnaldi, A.; Talamini, G. Makromol.Chem. 1968, 114, 217-225.
18. Nozaki, K. Discus.Faraday.Soc. 1947, 2, 337-342.
19. Mickley, H. S.; Michaels, A. S.; Moore, A. L. J.Polym.Sci. 1962, 60, 121-140.
20. Russo, S.; Stannett, V. Makromol.Chem. 1971, 143, 47-56.
21. Hauss, A. F. J.Polym.Sci., Part C 1971, 33, 1-12.
118
The Trommsdorff effect during the suspension polymerization of VCM
22. Ugelstad, J.; Fløgstad, H.; Hertzberg, T.; Sund, E. Makromol.Chem. 1973, 164, 171-181.
23. Park, G. S.; Smith, D. G. Makromol.Chem. 1970, 131, 1-6.
24. Sidiropoulou, E.; Kiparissides, C. J.Macromol.Sci.-Chem. 1990, A27, 257-288.
25. Hamielec, A. E.; Gomez-Vaillard, R.; Marten, F. L. J.Macromol.Sci.-Chem. 1982, A17,
1005-1020.
26. Xie, T. Y.; Hamielec, A. E.; Wood, P. E.; Woods, D. R. J.Vinyl.Tech. 1991, 13, 2-25.
27. Talamini, G.; Kerr, J.; Visentini, A. Polymer 1998, 39, 4379-4384.
28. Challa, G. Polymer chemistry: an introduction; Allis Horwood: New York, 1993
29. Xie, T. Y.; Hamielec, A. E.; Wood, P. E.; Woods, D. R. Polymer 1991, 32, 1098-1111.
30. Bevington, J. C.; Melville, H. W.; Taylor, R. P. J.Polym.Sci. 1954, 14, 463-476.
31. O'Neil, A. A.; Torkelson, J. M. Trends Polym.Sci. 1997, 5, 349-355.
32. Xie, T. Y.; Hamielec, A. E.; Wood, P. E.; Woods, D. R. Polymer 1991, 32, 537-557.
33. Zilberman, E. N. J.M.S.-Rev.Macromol.Chem.Phys. 1995, C35, 47-62.
34. Ugelstad, J.; lervik, H.; Gardinovacki, B.; Sund, E. Pure Appl.Chem. 1971, 26, 121-152.
35. Ugelstad, J. J.Macromol.Sci.-Chem. 1977, A11, 1281-1305.
36. Breitenbach, J. W.; Olaj, O. F.; Schindler, A. Makromol.Chem. 1969, 122, 51-64.
37. Vidotto, G.; Brugnaro, S.; Talamini, G. Makromol.Chem. 1971, 140, 249-261.
38. Özkaya, N.; Erbay, E.; Bilgiç, T.; Savasçi, Ö. T. Angew.Chem. 1993, 211, 35-51.
39. Braun, D. J.Vinyl.Add.Tech. 2001, 7, 168-176.
40. Hjertberg, T.; Sörvik, E. M. Thermal degradation of PVC; In Degradation and stabilisation of
PVC; Owen, E. D., ed. Elsevier Applied Science Publishers: London, 1984; pp 21-79.
41. Starnes, W. H., Jr. Progr.Polym.Sci. 2002, 27, 2133-2170
.
119