3 Free-Radical Addition Polymerization
3.24 APPLICATION TO EXPERIMENTAL DATA
Equation 3.35 and Equation 3.36 have been tested for all three major types of CRP process, and three examples will be used by way of illustration.
The nitroxide-mediated polymerization of styrene has been studied using a wide variety of nitroxides. If the phosphate nitroxide DEPN is used,
the value of K = 6 × 109 M is larger than the TEMPO reaction, and so it takes longer to reach the stationary state. This allows one to test the power law behavior of the system, in addition to the steady-state analysis
Data for the polymerization of styrene, PS-DEPN (the polystyrene adduct) in the presence and absence of benzoyl peroxide (BPO) are shown in Figure 3.8(a) and Figure 3.8(b). The conversion index is first order with respect to t for systems containing BPO, in accord with Equation 3.35, but in the absence of BPO, a power-dependence, Equation 3.36, of t2/3 is observed.
The basic kinetic features of ATRP are similar to NMP, and this can be demon-strated for the copper-mediated polymerization of styrene. The ATRP of styrene in t-butyl benzene, catalyzed by Cu(I)Br/L, where L is the ligand diheptyl bipyridine, and a (PS-Br) adduct in the presence or absence of a radical initiator 2,2′-azobis (2,4,4-tri methyl pentane) VR110 at 110°C, has been studied. The data are shown in Figure 3.9(a) and Figure 3.9(b). Again, it is seen that for the system containing VR110, steady-state kinetics apply, and the conversion index is first order in t [Figure 3.9(a)], but in the absence of the initiator power law kinetics are obeyed [Figure 3.9(b)].
FIGURE 3.8 Plots of ln([M]0/[M]) vs. (a) t and (b) t2/3 for the styrene/PS-DEPN-(P0-X)/BPO systems (80°C): [P0-X]0 = 25 mM; [BPO]0 = 4.7 mM. The dotted line is the best-fit repre-sentation of the duplicated experiment. (From Yoshikawa, C. et al., Macromolecules, 35, 5801, 2002. With permission of the American Chemical Society.)
N
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Free-Radical Addition Polymerization 91
The situation is less clear-cut for RAFT systems. For a dithiocarbonate-mediated styrene polymerization studied by Goto and co-workers, the steady-state kinetic analysis applied both in the presence and absence of a BPO initiator (Figure 3.10). Similarly, for the solution polymerization of methyl methacrylate, mediated by dithioesters con-taining α-cyanobenzyl groups in the presence of AIBN initiator, pseudo-first-order plots were obtained although a significant induction period was detected.
FIGURE 3.9 Plots of ln([M]0/[M]) vs. (a) t and (b) t2/3 for the styrene/t-butylbenzene/PS-Br(P0-X)/Cu(I)Br/dHbipy /(VR110) systems (110°C): [P0-X]0 = 13 mM; [Cu(I)Br]0 = 10 mM;
[dHbipy]0 = 30 mM; [VR110]0 = 0 (,) and 40 mM (); [styrene]/[ t-butylbenzene] = 50/50 v/v. The experiments are duplicated for [VR110]0 = 0 (,). The line is the best-fit represen-tation of the duplicated experiment. (From Yoshikawa, C. et al., Macromolecules, 36, 908, 2003. With permission of the American Chemical Society.)
FIGURE 3.10 Plot of ln([M]0/[M]) vs. t (60°C) for the styrene/PS-SCSCH3(P0-X)/BPO sys-tem: [P0-X]0 = 0.45 mM; [BPO]0 as indicated in the figure. (From Goto, A. et al., Macromol-ecules, 34, 402, 2001. With permission of the American Chemical Society.)
0 0 0.05 0.1 0.15 0.2 0.25
5 10 15 20 25 30 35 40 t/min
(a) 1n([Mo]/[M])
0
0 2 4 6 8 10 12
0.05 0.1 0.15 0.2
t2/3/min2/3 (b) 1n([Mo]/[M])
0 0 0.002 0.004 0.006 0.008
1 2 3 4 5 6 7
1n ([Mo]/[M])
t/h
0 1 3.3 10 9813_C003.fm Page 91 Friday, June 15, 2007 10:53 AM
92 Polymers: Chemistry and Physics of Modern Materials
In general, steady-state kinetics are successful in describing the majority of the CRP reactions when a conventional radical initiator is present in the reaction, but power law equations may be more appropriate when a conventional initiator is absent.
PROBLEMS
1. Draw structures of all possible configurational isomers resulting from the polymerization of 2-chlorobutadiene.
2. Draw repeat unit structures for the polymers obtained from:
a. 1,4 polymerization of 2,3-dichlorobutadiene b. 1,2 addition of 2-methyl butadiene
What polymer is formed by hydrogenation of (a)?
3. Explain why poly(vinyl alcohol) (PVA) cannot be prepared from its mono-mer. The synthetic route to this polymer involves polymerization of vinyl acetate followed by hydrolysis. Sketch the mechanism. Suggest a reason why PVA made from 100% hydrolyzed poly(vinyl acetate) is difficult to dissolve in water.
4. Write down the mechanism for the free-radical polymerization of vinyl-acetate initiated by 2,2′-azobis(2,4-dimethylpentanenitrile) (Vazo 52) in ethyl acetate including (a) initiation, (b) propagation, and (c) termination by disproportionation.
5. You are asked to select a suitable radical initiator for a polymerization process to be carried out at 50°C and which needs to be completed within 8 h. By considering that the initiator concentration must not fall below half its original value during the polymerization, select the most appro-priate initiator from the list below.
6. Overberger et al. (1949) studied the decomposition rates of a series of aliphatic azonitriles in toluene by measuring the volume of N2 evolved as a function of time, at constant temperature. A set of Vt/V∞ (where Vt and V∞ are the volumes at time t, and t = ∞) data for the decomposition of the azo nitrile compound
Initiator T (°C) kd ×××× 105 (s–1) Ea (kJ mol–1)
Benzoyl peroxide 80 2.5 124
tert-Butyl peroxide 130 2.48 142
Azobisbutyronitrile 50 0.20 129
2,2′-azobis-2,4-dimethyl valeronitrile 70 28.9 121
H3C C CN
N N C CN
CH3 C2H5 C2H5
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Free-Radical Addition Polymerization 93
at 80.2°C are given below. From these: (1) demonstrate that the decom-position reaction follows first order kinetics and (2) determine the rate of decomposition of the azo nitrile compound.
7. The effect of α, α′-azobisisobutyronitrile initiator concentration [I] on the rate of polymerization of methyl methacrylate at 50°C has been studied by Arnett (1952). Use the following data to show that the termination reaction is second order with respect to the growing chains and initiation is first order with respect to the initiator concentration.
The number-average degree of polymerization, xn, was also determined and data are listed in the table above. Use these results to establish a relationship between initiator concentration and xn.
8. 15.45 g of methyl methacrylate were polymerized at 50°C for 55 min using 0.2096 g of α, α′-azobisisobutyronitrile (AIBN) as initiator, to give 1.6826 g of polymer. Radioactivity assay indicated that the polymer sam-ple contained 7.07 × 10–4g of azobisnitrile (Arnett and Peterson, 1952).
Time ×××× 10–3 (s) Vt/V∞∞∞∞
2.86 0.213
5.10 0.381
6.33 0.429
7.35 0.493
9.18 0.577
10.82 0.647
12.04 0.687
13.16 0.733
15.82 0.786
17.04 0.802
18.57 0.831
20.51 0.870
[I] ×××× 102 (mol L–1) ννννp×××× 103 (mol L–1 min–1) xn
0.47 4.66 11090
0.62 5.54 11040
1.15 7.37 7250
1.27 7.92 8390
1.83 9.3 6150
2.56 11.4 8530
3.7 13.31 4880
5.21 15.6 3890
7.45 18.84 3000
10.77 22.45 2660
14.82 26.4 2810
21.06 31.61 2700
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94 Polymers: Chemistry and Physics of Modern Materials
Considering that the first-order rate constant for the decomposition of AIBN is given by:
,
calculate the initiator efficiency. The number-average molecular weight of this polymer was found to be 390,000 from osmotic pressure measure-ments. Establish whether the termination reaction takes place by combi-nation or disproportiocombi-nation of growing chains.
9. Consider the kinetic scheme for free-radical polymerization. Demonstrate that when polymer chains terminate at sites located on the surface of the polymerization vessel:
and this is the only mode of termination, then the overall rate of polymer-ization is given by:
vp = kpol [M][I]
where kpol = 2 kp kd/kwall.
10. The ratio between the rates of termination by disproportionation and combination, kt,d/kt,c, has been evaluated by Berger (1975) for the poly-merization of styrene:
From these data, calculate the difference in activation energy between disproportionation and combination, in the termination step.
11. Consider the free-radical polymerization of methyl methacrylate in toluene solution at 77°C, initiated by AIBN. When the initial monomer concentra-tion was 2.07 M and the initial AIBN concentraconcentra-tion was 2 × 10–4 M, the initial rate of polymerization was determined to be νp = 2.49 × 10–3 M min–1. a. Determine the initial rate of initiation, νi, and kp/(kt)1/2 by considering that the rate constant for the decomposition of AIBN at 77°C is kd = 5.7
× 10–3 min–1, and that virtually all radicals are capable of initiating chains.
T (°C) kt,d/kt,c
30 0.168
52 0.246
62 0.294
70 0.480
80 0.663
logk min1
/ − = −7019T + . 17 806
RM P
wall n
• k→
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Free-Radical Addition Polymerization 95
b. Determine the initial kinetic chain length and the expected number average molecular weight assuming only termination mainly takes place by disproportionation.
c. Ep–0.5 Et gives a measure of the energy required to polymerize a given monomer. Estimate this for methyl methacrylate from your value of kp/(kt)1/2 at 77°C and the value reported at 50°C by Arnett (1952), kp/(kt)1/2 = 0.67.
d. Estimate the time required to polymerize 10% of the monomer.
12. In the polymerization of vinyl acetate with AIBN at 60°C Palit and Das (1954) measured the degree of polymerization in the presence of various chain transfer agents and at different [S]/[M] (i.e., solvent to monomer) ratios and intrinsic viscosities, [η]. A set of data is tabulated below.
Calculate the chain transfer constants, CS. You will need to make use of the relationship:
13. In the polymerization of methyl methacrylate initiated by phenyl ethyl chloride (PECl), in the presence of Cu(I)Br/dinonyl bipyridine complex, the following data were obtained:
Use these results to demonstrate that the reaction has the characteristics of a living radical polymerization, and identify the type of CRP.
14. In the thermally initiated bulk polymerization of styrene (density = 0.906 g cm–3) in the presence of dithiocarbamate chain transfer agent (CTA), the following data were obtained:
Solvent [S]/[M] [ηηηη]
Ethyl acetate 1.076 0.980
1.614 0.880
2.421 0.680
6.456 0.500
Methyl iso-butyl ketone 0.492 0.300
0.738 0.240
1.108 0.190
2.954 0.095
[PECl] (M) [MMA] (M) t (min)
0.10 9.0 0
0.08 7.2 40
0.05 4.5 118
0.03 2.7 200
0.01 0.9 380
logxn=3 4655 1 45. + . log [ ]η
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96 Polymers: Chemistry and Physics of Modern Materials
a Which living polymerization method does this system represent?
b. Calculate the theoretical molecular weight for each of the reactions.
c. Do the data comply with any of the criteria for living polymerization?
REFERENCES
Arnett, L.M., J. Am. Chem. Soc. 74, 2027, 1952.
Arnett, L.M. and Peterson, J.H., J. Am. Chem. Soc. 74, 2031, 1952.
Berger, K.C., Makromol. Chem., 176, 3575, 1975.
Cardenas, J.N. and O’Driscoll, K.F., J. Polym. Sci. Polym. Chem. Ed., 14, 883, 1976.
Chiefari et al., Macromolecules, 31, 5559, 1998.
Dainton, F.S. and Ivin, K.J., Q. Rev., 12, 61, 1958.
Davis, T.P., O’Driscoll, K.F., Piton, M.C., and Winnik, M.A., Macromolecules, 22, 2785, 1989.
Goto, A. et al., Macromolecules, 34, 402, 2001.
Melville, H.W., J. Chem. Soc., 247, 1947.
Olaj, O.F., Bitai, I., and Hinkelmann, F., Makromol. Chem., 188, 1689, 1987.
Overberger et al., J. Am. Chem. Soc., 71, 2661, 1949.
Palit, S.R. and Das, S.K., Proc. R. Soc. London 226A, 82, 1954.
Schulz, G.V., Chem. Ber., 80, 232, 1947.
Yoshikawa, C. et al., Macromolecules, 35, 5801, 2002.
BIBLIOGRAPHY
Allcock, H.R. and Lampe, F.W., Contemporary Polymer Chemistry, Prentice-Hall, 1981.
Allen, G. and Bevington, J.C., Eds., Comprehensive Polymer Science, Vol. 3, 4, Pergamon Press, 1989.
Alter, H. and Jenkins, A.D., Chain-reaction polymerization, in Encyclopedia of Polymer Science and Technology, Interscience Publishers, 1965.
Bamford, C.H. and Tipper, C.F.H., Eds., Free Radical Polymerization, Comprehensive Chem-ical Kinetics, Vol. 14A, Elsevier, 1976.
Billmeyer, F.W., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, 1984.
Blackley, D.C., Emulsion Polymerization, John Wiley and Sons, 1975.
Bouton, T.C., Henderson, J.N., and Bevington, J.C., Eds., Polymerization Reactors and Pro-cesses, ACS Symposium Series 104, 1979.
Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, 1953, chap. 4.
Ham, G.E., Vinyl Polymerization, Vol. I, Marcel Dekker, 1967.
Jenkins, A.D., The reactivity of polymer radicals, in Advances in Free Radical Chemistry, Vol. 2, Logos Press, 1967.
CTA Percentage Conversion
2.5 30
2.5 50
2.5 70
2.5 90
2.0 50
1.5 50
1.0 50
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Lenz, R.W., Organic Chemistry of Synthetic High Polymers, Interscience Publishers, 1967, chap. 9–11.
Margerison, D. and East, G.C., Introduction to Polymer Chemistry, Pergamon Press, 1967, chap. 4.
Napper, D.H., Polymer Stabilization of Colloidal Dispersions, Academic Press, 1984.
Odian, G., Principles of Polymerization, 4th ed., John Wiley and Sons, 2004.
Rempp, P. and Merrill, E.W., Polymer Synthesis, Hüthig and Wepf Verlag, 1986.
Smith, D.A., Addition Polymers, Butterworths, 1968, chap. 2.
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