6 Branching and Crosslinking in
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Styrene-Butadiene Polymerizations G. M. BURNETT and G. G. CAMERON University of Aberdeen, Old Aberdeen, Scotland
A method based on partial conversion properties for studying crosslinking during polymerization of dienes is described. With the method, the competition between crosslinking and processes that produce new polymer molecules can be studied with considerable sensitivity. This competition depends upon polymerization variables such as temperature, and modifier type and concentration. The problem of controlling crosslinking during styrene -butadiene copolymerization is considered, and it is shown that incremental addition of modifier is more satisfactory for this purpose than is a single initial charge. Mechanisms of branch and crosslink formation during copolymerization of styrene and butadiene are discussed. A c t i v e free radicals w i l l almost inevitably react i n some way with molecular species with which they come i n contact. T h e interaction of radicals i n polymerizing systems with either monomer or solvent is well recognized as a transfer step that brings about the formation of inactive polymeric molecules. A t the same time, though, new radicals are generated; the kinetic chain reaction is usually not seriously i m peded although, i n some instances, this is not true. As the amount of polymer i n the system increases, the probability of interaction of growing free radicals with polymeric (inactive) species either by transfer or, sometimes, by copolymerization through residual double bonds must increase. Because such reactions automatically give rise to branched and crosslinked species, reliable experimentation aimed at studying these processes is difficult to achieve. This follows from the fact that the region of the reaction i n which study is essential is precisely that region i n which there are rapid changes i n the characteristics of the polymer produced, particularly i n solubility. This is of 102 Platzer; Polymerization Kinetics and Technology Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
6.
BURNETT A N D C A M E R O N
Branching and Crosslinking
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significance in the methods described here i n that these depend on rapid, reliable, and precise molecular-weight measurements. Although fundamental studies of these phenomena have been slow to develop, the deleterious effects of branching and crosslinking in com mercially produced polymers have been recognized for many years. In particular, when significant crosslinking occurs during polymer manu facture, the processing characteristics of the resulting polymer are sud denly, and catastrophically, altered in many instances. W i t h styrenebutadiene copolymers, the usual indicator of good processability of the polymer is the delta Mooney index, a negative value being characteristic of a processible rubber. The change from negative to positive values takes place over a very narrow conversion range. The objective of this paper is to outline methods by which control of the deleterious reactions can be achieved, bearing i n mind that any solution to the problem should not interfere with those reactions that lead to a normally acceptable product. Because of the solubility difficulties, methods that can get as close to the insoluble situation as possible must be used. Although the pro cessability index undergoes a very sharp change, there is no doubt that the branching and crosslinking reactions are possible from the time that polymer forms; these reactions grow i n importance as the reaction pro ceeds. What is required, therefore, is a reliable method of assessing the balance between the normal ' productive" reactions and those leading to crosslinks at all stages of the reaction. About 25 years ago, F . T. W a l l proposed such a method, which depended on the measurement of the so-called "partial conversion molecular weight" ( I ) . If Ν is the number of polymeric molecules and W the weight of polymer, then the number average molecular weight M is given by W/N, from which it is easy to show n
dN/dW = M -*(M -W n
_
= In"
n
dMJdW)
(1)
1
where M is the partial conversion molecular weight. During a reac tion, dN/dW is the measure of the rate at which the population of poly meric molecules varies with the extent of reaction, as defined by W . Normally, this quantity w i l l be positive. However, each intermolecular crosslinkage formed reduces the number of polymeric molecules by one. Consequently, when crosslinking becomes of importance, dN/dW w i l l tend toward negative values. When dN/dW = 0, the production rates of new macromolecules and of reduction caused by crosslinking are equal. The application of this technique demands accurate determinations of a large number of molecular weights. This is no longer a formidable n
Platzer; Polymerization Kinetics and Technology Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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P O L Y M E R I Z A T I O N KINETICS A N D T E C H N O L O G Y
task with the advent of automated osmometers. Figure 1 shows the sort of results that have been obtained using this technique ( 2 ) . N u m ber-average molecular weights are determined at various conversions, and from plots of these against conversion, dN/dW can be calculated from Equation 1. The correlation of the results of this approach with the conventional delta Mooney procedure can be assessed from Figures 2 and 3 ( 3 ) . In Figure 2, dN/dW is plotted as a function of W for two series of commercial samples withdrawn from an experimental continuous reactor line i n which butadiene and styrene were copolymerized i n emulsion using a 1500-type recipe at 5°C. The conversion marking the onset of predominant crosslinking is given by the point at which the curves cut the W axis—that is, at 53 and 57% conversions.
i l l
20
40
60
80
100
CONVERSION %
Figure 1. Rate of change in macromolecular population d N / dw as a function of conversion in emulsion copolymerization of styrene and butadiene (styrene: butadiene = 29:71 by wt); concentration of tert-dodecyl mercaptan (TOM) modifier in phm: A — run 4, 0.10; Ο — run 6, 0.20; X - run 7, 0.23
Figure 3 shows that the delta Mooney indexes show an abrupt change from negative to positive values i n the same region. Thus, there is a clear relationship between the onset of dominant crosslinking and processability. The deterioration i n physical characteristics is not coin cidental with the formation of gel since, i n both of these examples, the gel point occurs at conversions greater than 60%. Although the serious effects of crosslinking on the properties of commercial polymers have long been recognized, systematic study of the phenomenon has been somewhat neglected. It is true that Flory (4) outlined the basic kinetics of the crosslinking process for the homopolymerization of dienes which gave the proportion of crosslinks ν as a function of the fraction conversion a as: dv/da
= 2KN [a/(l-a)]
(2)
0
where the density of crosslinks, p, defined as v / « N is given by 0
ρ = - 2 Κ [ 1 + α ~ 1 η ( 1 - α)] 1
Platzer; Polymerization Kinetics and Technology Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
(3)
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Branching and Crosslinking
Chemistry and Industry (London)
20
30
40
50
CONVERSION
60
Figure 2. dN/dw as a function of conversion for series A (A)
%
and
(3)
Β (O)
where N is the number of monomer molecules originally present. In this derivation, it is assumed that crosslinks are formed by the addition of propagating radicals with either internal or pendent double bonds on polymer molecules. Κ is the ratio of the rate constant k for this essentially copolymerization process and the propagation rate con stant k . While this is logical enough, there must be some doubt as to its ultimate validity in that the internal double bonds are essentially 1,2-disubstituted ethylenes where reactivity is notoriously low. There is, indeed, little evidence to suggest that these bonds disappear in the later stages of the reaction. So it may be that crosslinking (even in diene systems) is preceded by branching that results from transfer to the polymeric molecules followed by coupling of branched radicals. For the SBR case, the numerator « in Equation 2 should be re placed by a , the fraction of consumed monomer, which is butadiene. The analysis of this situation is complex but may be simplified by the experimental observation that 0
x
p
B
a /(l B
(4)
— a) = A + Ba
2
where A and Β are constants at a fixed temperature. (5). Flory has also shown that at the gel point (5)
/a = ρ = Ύ^-1
y
where Y is the weight-average degree of polymerization of the primary chains. Substituting Equation 4 in Equation 2 modified as indicated and integrating between « = 0 and (the gel point) gives wg
F*,-
1
= 2K(A + ( £ / 3 K ) . 2
When Y and a are known, Κ can be found and thus its dependence on temperature. In this analysis, however, the 'parameter
