15 Mechanisms of Propagation of Ionic
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Polymerizations M. SZWARC State University College of Forestry at Syracuse University, Syracuse, Ν. Y. 13210
Ionic polymerization is propagated by various species (free ions, ion pairs, triple ions, etc.). Solvents, counterions, solvating agents, etc. affect in different ways the reactivities of free ions and of ion pairs. The existence of different types of ion pairs is revealed by ESR spectroscopy. The applica tion of ESR for such studies is reviewed, and the potentiali ties of the method illustrated by suitable examples. The role of water in cationic polymerization is discussed. Water can act as a terminating agent of cationic polymerization either as single H O molecules or as agglomerates such as (H O) . It is suggested that a single H O molecule acts as a Brönsted base, while (H O) , or higher agglomerates act as Lewis bases. This changes the kinetics of termination and affects profoundly the behavior of the system. 2
2
2
2
2
2
V V T h i l e radical homopolymerization involves one species only—viz., a ^ growing free radical—several distinct entities may contribute to ionic propagation. In a classic ionic polymerization the growing polymers are endowed with electrically charged active ends, the charge being negative i n anionic systems and positive i n cationic processes. However, because the reacting solution must be electrically neutral, an equal number of oppositely charged ions, referred to as counterions, must be present i n the system. In solvents of high dielectric constant and strong solvating power the ions may remain free, especially i n dilute solutions, and the polymerization is performed then by free ions—e.g., by carbanions or carbonium ions. In less powerfully solvating media of relatively low dielectric constant the oppositely charged ions combine into ion pairs and these could be the carriers of the reaction. In a general case both free ions and ion pairs may contribute to the propagation, 236
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
15.
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Ionic
237
Polymerizations
and if kf denotes the rate constant of the free ion and k± that of ion pairs, the observed propagation constant kp is given by *ρ = γ * , + ( 1 - γ ) *
±
where y denotes the fraction of growing ends present as free ions. Ions and ion pairs remain usually in equilibrium with each other— P , M + * ± — P - + M+,
Kdiss
and then y2C/(l — y) = K d i s s , C denoting the concentration of all the growing species. For y < < 1 , a case often encountered in practice, the approximation y=
Kûln™/c™
is valid. One finds then that the observed propagation constant kp is given by kp = k±
+
(kf-k±)Kdies^/C^9
which was verified for many anionic polymerizations. Its reliability is illustrated i n Figure 1 (6). Other examples are discussed in recent monograph by the writer (31). In the presence of another electrolyte, sharing a common counterion with the growing polymer, the above relation must be modified (26). For example, the addition of N a + , B P h f to living sodium polystyrene, — S " , N a + , retards its polymerization, and the propagation constant is then independent of the concentration of living polymers—viz., kp = k±
+
(kf-k±)Kdiss/[Nï]
This result is caused by the fact that most of the sodium ions are formed through the dissociation of the salt, and thus, γ / ( 1 — y) = Kdiss/CNa*]. It follows that the kinetic results permit us to find kf and K d i s s be cause i n the absence of the boride the slope of the line kp vs. 1 / C 1 / 2 gives (kf — k± )Kdiss1/2, whereas the slope of the line kp vs. 1/[counterion], ob tained in the presence of the salt gives (kf — k± )Kdiss. The propagation constant k± may be found from either plot; it is given by the intercept of the respective lines. The equilibrium between ions and ion pairs is not maintained in all polymerizing systems. For example, the cationic polymerization induced by ionizing radiation produces the positive and negative ions, the latter initiating a free carbonium ion propagation which is terminated by their collision with negative ions. Such a collision destroys the free ions, and hence their stationary, but not the equilibrium, concentration is deter mined by their rate of formation and destruction.
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
238
ADDITION AND CONDENSATION POLYMERIZATION PROCESSES
Journal of Physical Chemistry
Figure 1. Linear dependence of the observed propagation constant, k D , of living polystyrene polymerization on [living polymers]'1'* for Li+9 Na+, K+, Rb+, and Cs+ salts in THF at 25°C.
The cation-anion annihilation provides the main mode for termination i n extraordinarily dry systems. Traces of water terminate propagation i n a most efficient way, and then the termination is first order with respect to the growing polymers. It is interesting to digress here and discuss this writer's ideas about the role of water i n such processes. Under normally achieved states of "dryness" the concentration of water in a hydrocarbon monomer is probably sufficiently high to maintain the dissolved water i n its dimeric form, ( H 2 0 ) 2 or, at least, to allow a second molecule of water to collide with carbonium ion associated with one H 2 0 prior to the decomposition of such an associate. Thus, the termination involves processes such as
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
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Polymerizations
-CH2 · C+ + (H20)2 -»
C H 2 · COH+ HaO+
(1)
or
!
I
C H 2 · C + H 2 0 -> C H 2 · C—O.
(2)
+
I
I
H
followed by the reaction —CH
2
·k
—
+
H20 -»
C H 2 · COH+ H30+
(3)
When the concentration of water is exceedingly low, Reaction 2 is not followed by 3, but the associate decomposes—i.e.,
I • /H I C H 2 · C — — C H = C + H„0*
I
I
H
(4)
The eventual collision between H 3 0 and a negative center regenerates then the water molecule and destroys the negative ion. Hence, ( H 2 0 ) 2 or higher agglomerates react with carbonium ions as Lewis bases, alcohol is formed, and one water molecule is destroyed. Therefore, the termination of polymerization proceeds simultaneously with the destruction of the terminator. However, at very low water concentration, the single H 2 0 molecule eventually reacts as a Bronsted base—i.e., as a proton acceptor (Reaction 4). In this process the polymerization is terminated, but the terminator is not destroyed. This accounts for the experimental results—viz., the last trace of moisture cannot be removed from the monomer by prolonged irradiation (24, 37). To what extent are the reactivities of free ions influenced by the nature of solvent? The required information is available at present for anionic systems only although it is hoped that some data pertaining to cationic polymerization w i l l be available soon. Extensive studies of polymerization of living polystyrene showed that kf is affected only slightly by the solvent; the relevant values are listed i n Table I. Similar observations were reported for living poly-a-methylstyrene (11). In fact, even the activation energies of propagation are virtually unaffected by the change of solvent—e.g., for the — S ~ growth the activation energies appear to be in the range 5-6 kcal./mole i n many different solvents (31). In this respect the behavior of ion pairs dramatically contrasts with that of free ions. Table II provides some data for various salts of — S " . Its inspection shows that k± may vary by a factor as large as 1000 (k± for the propagation of S",Na + at 2 5 ° C . is about 4 M ' 1 sec."1 i n 1 dioxane but about 3 6 0 0 M ' sec."1 in dimethoxyethane ). Moreover, the +
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
240
ADDITION
A N D CONDENSATION
POLYMERIZATION
PROCESSES
pattern of reactivities for a series of alkali salts may depend on the solvent—e.g., — S " , L i + salt is the least reactive, and the — S " , C s + salt is the most reactive when the polymerization takes place i n dioxane. The order of reactivities is reversed i n tetrahydrofuran. This observation, as well as other facts, led to the conclusion that ion pairs may exist i n various forms which could differ greatly i n their re activities. The nature of ion pairs is revealed by various studies (34), and the E S R technique applied to paramagnetic ion pairs provides prob ably the most penetrating and powerful tool for such investigations. Indeed, the very first direct evidence for the existence of ion pairs as independent molecular species was furnished by Weissman's E S R studies of sodium naphthalenide (1, 2). In the following years the E S R method revealed many interesting phenomena which acquaint us with the thermoTable I.
Absolute Rate Constants of Propagation by Free Polystyryl Anions in Different Solvents at 2 5 ° C .
Solvent
Tetrahydrofuran Dimethoxyethane 2-Methyltetrahydrofuran Tetrahydropyrane Tetrahydrofuran-dioxane mixtures Tetrahydrofuran-benzene mixtures Table II.
k_, liter mole'1 sec.'1
Ref.
63,000-65,000 -40,000 27,000-30,000 (?) 59,000-72,000 —60,000 40,000-70,000
26 27 15 14 36 39
Propagation by Living Polystyrene Salts in Various Solvents at 2 5 ° C . k ± , liter mole'1 sec:
Solvent
Na+
u+
K+
Rb+
Ref.
Cs+
Living Polystyrene Salts 0.9
Dioxane Benzene (nonassociated) Tetrahydropyrane
