12 Radiation-Induced Ionic Polymerization
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D. J. METZ Brookhaven National Laboratory, Upton, Ν. Y. 11973
Sufficient
experimental
exist to describe - i n d u c e d ionic monomers
propagation
can be observed.
conductivity reaction;
measurements
scavenger
derant role played polymerization. it is possible process.
data from several laboratories now
the conditions
studies
arrived
the r a d i a t i o n
pure
liquid
vinyl
The kinetic data and
electrical
establish the ionic nature of the appear
to write a simple
to establish
the
prepon
ion in propagating
ion and a vinyl
are compared
at in chemically
initiated
polymerization. are
to describe
of the kinetic
rate
bond.
constants
carbonium
Several shortcomings
the con
reaction
double
with similar
free radical,
the
species,
notably the rate constant for
rate constants
scheme
mechanism
values of several
a bare carbonium
and carbanion present
many
by the carbonium
stants can be estimated, These
under which
On the basis of a single propagating
Limiting
between
of
ion
of the
discussed.
^ l t h o u g h most of the original studies of radiation-initiated liquid-phase vinyl polymerizations pointed unequivocally to initiation and propa gation by free radicals, sufficient evidence is now available to establish that under proper conditions ionic species may also be formed which are capable of not only contributing to but almost entirely dominating the over-all process. The proper conditions under which this can be estab lished are simply purity and dryness (another form of purity), but the degree of purity which must be achieved partially explains why the free radical behavior, and not the ionic process, was the first to be observed. The following discussion outlines briefly the technique of preparing samples, the kinetic evidence on which propagating ions are postulated, some of the evidence for the nature of the propagating ions, and one sug gested mechanism for the radiation-induced ionic polymerization which has limited usefulness. 202 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
12.
Radiation-Induced
METZ
Polymerization
203
During this paper, many questions w i l l be raised for which only partial answers can be given. In an emerging field such as this, that is to be expected. In this way we hope to arrive at a fairly complete understanding of not only radiation-induced polymerization but the broader field of liquid-phase organic radiation chemistry. Sample
Preparation
As we shall see below, the levels of impurity, including the ubiquitous impurity—water, that can successfully destroy the desired ionic contribution to the polymerization process may be as low as 10" 7 M. Therefore, vigorous purification methods for monomer, glassware, and any additives must be used. The particular method of sample preparation which we have used in our laboratory is outlined below and described in detail elsewhere (29, 30). Most other workers in this field use methods comparable in principle but differing in details. The several steps involved are as follows: (a) Monomer purification. Fractionation i n 6-foot packed column under a reduced pressure of dry helium. (b) Bake-out. A l l glassware and drying agent (silica gel) are baked at 5 0 0 ° C . for 36 hours at 10" 7 torr. (c) Monomer degassing. Freeze-thaw cycling; final sealing at > (ktRi)m, where termination of molecular chains is governed by reaction with impurity, it can be shown (22, 27) that X
DP -
^— , ***L*J DP0 ^
(A) ft,[M]
K
}
where D P 0 is the average degree of polymerization i n the absence of impurity. A plot of 1 / D P vs. [ x ] / [ M ] should allow an evaluation of ktx/k , and evaluation of the latter as a diffusion-controlled rate constant i n an ion-dipole interaction on the basis of the Smoluchowski-Debye theory would allow an estimate of k to be made. This has been done for cyclopentadiene (5) and leads to the value of k « 1 0 1 0 M _ 1 sec."1 and corresponding orders of k « 10 8 * M~ sec."1. O n the basis of electrical conductivity, it can be shown (14) that p
tx
tx
p
X
X
1 = 3.6 Χ 1 0 1 2 — Τ
€
(5)
where σ is the specific conductance, 3.6 Χ 10 1 2 represents a conversion factor, and c is the bulk dielectric constant of the medium. This equation
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
12.
METZ
Radiation-Induced
211
Polymerization
is based on the assumption that termination of kinetic chains is by charge neutralization. Thus, measurement of specific conductance allows a cal culation of τ — t h e average lifetime for recombining ions—and, from Equation 6
which assumes that τ > > / . This condition should be approached i n all cases at high enough dose rates and is characterized by the empirical observation that Rp oc I™
Thus, kp can be approximated independently by electrical conductance measurements i n conjunction with kinetic measurements. Combining these two types of experiments—scavenger studies and electrical conductance measurements—it is possible to arrive at limits on the value of kp for any monomer undergoing this process. Williams et al. (36) have done this for several monomers, and the results are shown in Table II along with other data. The values of the propagation rate constants for styryl carbonium (free) ion, carbanion (free), carbanion (associated with N a + ) , free radical, and carbonium ion (associated with gegenions) are shown. The range of values is from 4 χ 10"3 to 3.5 X 1 0 6 M _ 1 sec."1. Based on available data, it is apparent that the free styryl carbonium ion is much more reactive than any of the other styryl species. Also shown in Table II are the propagation rate constants for several other free carbonium ions and one free carbanion. Table II.
Monomer
Styrene
Selected Propagation Rate Constants in Addition Polymerization
Propagation Species
M " 2 sec.'1
carbonium ion 3.5 Χ 10 6 carbanion 6.5 Χ 10 4 1.3 Χ 10 5 80 ion pair free radical 35 17 ion pair
4 X 10' 3 ion pair a-Methylstyrene carbonium ion 3 Χ 10 6 carbonium ion 3 X 10> Isobutyl vinyl ether Cyclopentadiene carbonium ion 6 Χ 10 8 carbanion 6 Χ 107 Nitroethylene
T, °C.
Initiator
Solvent
Ref.
bulk THF THF THF
36 4 16 4
30 30 -30
HC10 4 I2 radiation radiation
bulk C2H4C12 C2H4C12 bulk bulk
21 28 19 17 36
-78 10
radiation radiation
bulk bulk
38
15 25 25 25 20 25
radiation styryl anion styryl anion styryl anion - Na +
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
5
212
ADDITION
A N D CONDENSATION
ι ι ιι ι ι ιιι
id
2
ισ
3
PROCESSES
1I ι ιI ι 111 I1 1 I I M 1
/ [ χ ] = ιο~
id
POLYMERIZATION
/
9
—
4
/ [ X ] = I0"
8
-
IO*
5
-
io
6
-
/[X]=I0"
7
...1
. , ι . . ml ι
DOSE RATE (rod hr" ) 1
Figure 2. Theoretical plots of Equation 3 for vari ous values of impunty concentration, assuming: Gfions)
~
0*1
kt = 2X 1011
M-'secr1
k t x = l X lO^Ur
1
kO = 5X 106 M1
seer1 seer1
A singular contribution that a detailed study of radiation-induced ionic polymerization can make to fundamental processes, then, is that the rate constants for the family of reactions typified by κ
Ion -f- Molecule —> Ion-Molecule may be estimated without the complicating effects of the ever-present gegenions of chemically formed organic ions.
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
12.
Radiation-Induced
METZ
Polymerization
213
Williams et al. (36) have pursued the predictions of Equation 3 further by assuming reasonable values for the rate constants involved and have shown that the observed change of dose rate dependence with increasing dryness (and hence absolute value of the rate of polymeriza tion) for both styrene (30, 35) and a-methylstyrene (24) can be predicted semiquantitatively. For styrene they assumed the following values (36):
(ions)
G = 0 . 1 (based on most published determinations) kt = 2 X 1 0 n M _ 1 sec."1 (based on ion lifetimes measured by con ductivity) ktx = 1 X 1 0 1 0 M _ 1 sec."1 (diffusion-controlled value) kp = 5 X WM"1 sec."1 [see Equation 6]
D O S E R A T E (rod
hr' ) 1
Figure 3. Empirical data for styrene polymerization showing variation of R D vs. dose rate and the dose rate dependence of R D as impurity level decreases (data taken at 0°C.)
Figure 2 shows a plot of Equation 3, using the above assumed values, with the concentration of impurity, [ χ ] , as the adjustable parameter. It is readily seen that at [x] « 10* 7 M, the polymerization rate may be several orders of magnitude below the maximum (impurity free) rate,
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
214
ADDITION
AND CONDENSATION POLYMERIZATION
PROCESSES
and the observed dependence of the rate on the dose rate should be almost unity. Figures 3 and 4 show empirical data, of the above form, for styrene (30) and a-methylstyrene (24), respectively. Only i n the latter case is the predicted square-root behavior observed, but the general predictions of Equation 3 are followed by both monomers. It is interesting to calculate the approximate impurity levels of the samples for which the kinetic data are presented i n Figures 3 and 4. Table III summarizes these approximations, based on Equation 3 and the rate constants given i n Reference 36 for the two monomers i n question. The comparison has been made on the basis of the slopes of the experimental curves between 10 4 and 10 5 rads/hour and the corresponding values of the calculated curves i n the same interval. For both monomers, the values of impurity concentrations over which dramatic changes i n rate constants given i n Ref. 36 for the two monomers i n question, ample explanation of the failure of all early studies, especially for styrene, to uncover any mechanism other than the free radical kinetics.
log
DOSE RATE (rad/hr)
Figure 4. Empirical data for a-methylstyrene showing variation of R D vs. dose rate and dose rate dependence of R D as impurity level decreases (data taken at 0°C.)
Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.
12.
Radiation-Induced
METZ
Table III.
215
Polymerization
Estimated" Impurity Concentrations
Curve
Styrene, M
b
a b c d e f
a-Methylstyrene,
>10^lO"10 —ΙΟ" 8 >10" 8 >10" 8
