IX. IONIC AND COORDINATION CHAIN POLYMERIZATION
A. NONRADICAL CHAIN POLYMERIZATION
In addition to the radical chain polymerization mechanisms discussed above, chain-reaction polymeriza¬tion can also occur through other mechanisms. These include cationic polymerization in which the chain carriers are carbonium ions; anionic polymerization where the carriers are carbanions; and coordination polymerization, which is thought to involve the formation of a coordination compound between the
catalyst, monomer, and growing chain. The polymerization mechanisms of these systems are complex and not as clearly understood as the mechanism of radical polymerization. This is because the reactions are generally heterogeneous, involving usually solid inorganic catalysts and organic monomers. In addition, ionic polymerizations are characterized by extremely high reaction rates. High-molecular¬weight polymers are generated so fast that it is frequently neither possible to establish nor maintain uniform reaction conditions, thus making it difficult to obtain kinetic data or reproducible results. Two essential differences between free-radical and ionic polymerizations are apparent. First, in ionic poly-merization, initiation involves the formation of an ion pair through the transfer of an ion or electron to or from the monomer. This contrasts with the generation and addition of a radical to the monomer in free-radical initiation reactions. Second, termination in ionic polymerization involves the unimolecular reaction of a chain with its counterion or a transfer reaction with the remnant species unable to undergo propagation. In contrast to radical chain polymerization, this termination process in ionic polymerization is strictly unimolecular — bimolecular annihilation of growth activity between two growing chains does not occur. The types of chain polymerization suitable for common monomers are shown in Table 7.1.
While many monomers can polymerize by more than one mechanism, it is evident that the polymer¬ization mechanism best suited for each monomer is related to the polarity of the monomer and the Lewis acid–base strength of the ion formed. Monomers in which electron-donating groups are attached to the carbon atoms with the double bond (e.g., isobutylene) are capable of forming stable carbonium ions (i.e., they behave as Lewis bases). Such monomers are readily converted to polymers by cationic catalysts (Lewis acids). On the other hand, monomers with electron-withdrawing substituent (e.g., acrylonitrile) form stable carbanions and polymerize with anionic catalysts. Free-radical polymerization falls between these structural requirements, being favored by conjugation in the monomer and moderate electron withdrawal from the double bond. The structural requirements for coordination polymerization are less clearly delineated, and many monomers undergo coordination polymerization as well as ionic and radical polymerizations.
B. CATIONIC POLYMERIZATION
Typical catalysts that are effective for cationic polymerization include AlCl3, AlBr3, BF3, TiCl4, SnCl4, and sometimes H2SO4. With the exception of H2SO4, these compounds are all Lewis acids with strong electron-acceptor capability. To be effective, these catalysts generally require the presence of a Lewis base such as water, alcohol, or acetic acid as a cocatalyst. As indicated in Table 7.1, monomers that polymerize readily with these catalysts include isobutylene, styrene, ?-methylstyrene and vinyl alkyl ethers. All of these monomers have electron-donating substituents, which should enhance the electron-sharing ability of the double bonds in these monomers with electrophilic reagents.
Cationic polymerizations proceed at high rates at low temperatures. For example, the polymerization at –100°C of isobutylene with BF3 or AlCl3 as catalysts yields, within a few seconds, a polymer with molecular weight as high as 106. Both the rate of polymerization and the molecular weight of the polymer decrease with increasing temperature. The molecular weights of polyisobutylene obtained at room temperature and above are, however, lower than those obtained through radical polymerization.
1. Mechanism
Based on available experimental evidence, the most likely mechanism for cationic polymerization involves carbonium ion chain carrier. For example, the polymerization of isobutylene with BF3 as the catalyst can be represented thus:
First, the catalyst and cocatalyst (e.g., water) form a complex:
BF3 + H2O H+(BF3OH)- (7.47)
The complex then donates a proton to an isobutylene molecule to form a carbonium ion:
CH3 CH3
H+(BF3OH)- + CH2 C CH3 C+ + (BF3OH)- (7.48)
CH3 CH3
The carbonium ion reacts with a monomer molecule in the propagation step.
CH3 CH3
CH3 C+ (BF3OH)- M (CH3)3C CH2 C+ (BFOH)- (7.49)
CH3 CH3
Since cationic polymerization is generally carried out in hydrocarbon solvents that have low dielectric constants, separation of the ions would require a large amount of energy. Consequently, the anion and cation remain in close proximity as an ion pair. It is therefore to be expected that the growth rate and subsequent reactions (e.g., termination and chain transfer) are affected by the nature of the ion pair.
Termination occurs either by rearrangement of the ion pair to yield a polymer molecule with an unsaturated terminal unit and the original complex or through transfer to a monomer.
Unlike in free-radical polymerization, the catalyst is not attached to the resulting polymer molecule, and in principle many polymer molecules can be produced by each catalyst molecule.
2. Kinetics
For the purpose of establishing the kinetics of generalized cationic polymerization, let A represent the catalyst and RH the cocatalyst, M the monomer, and the catalyst–cocatalyst complex H+ AR–. Then the individual reaction steps can be represented as follows:
K
A RH H AR
?
1
?
H AR M
? ? k i ?
???????HM AR
HM AR M
n ? p
? HM AR
???????n ??1 k ?
k
HM AR
????t M H AR
?
??????
n n
k
HM AR M
??? tr M HM AR
?
????????
n n
The rate of initiation Ri is given by
R i k i H AR M k K A RH M
? ? ???????i ???? ????
? ?
As usual, the square brackets denote concentration. If the complex H+ AR– is readily converted in the second step of Equation 7.48 (i.e., if the complex formation, step 1 Equation 7.48) is the rate-limiting step, then the rate of initiation is independent of the monomer concentration. Since AR– remains in the close vicinity of the growing center, the termination step is first order
R t ??K t ??M ?? (7.55)
where [M+] is the concentration of all the chain carriers [HM+n AR–]. The retention of the terminating agent AR– in the vicinity of the chain carrier is responsible for the primary difference between the kinetics of cationic polymerization and that of free-radical polymerization. Assuming that steady state holds, then Ri = Rt and
? ?? ???? ?? ?
M K k
? i A RH M
k t
The overall rate of polymerization, Rp is given by
R k M M K
? ? ??????
?
p p
k k
i p ????? ?? ??2
A RH M
k
The number-average degree of polymerization, assuming predominance of termination over chain trans¬fer, is
R M M
?
k k
p ?? ? ?????
p? ? ? ???
p M
?
R k M k
i t t
If, on the other hand, chain transfer dominates, then
? ?? ? ????
?
R k M M
p p
R k M M
?
? ???????k
tr t tr
In this case, the average degree of polymerization is independent of both the concentration of the monomer and the concentration of the catalyst. Available kinetic data tend to support the above mechanism.
Example 7.7: Explain why in the cationic polymerization of isobutylene, liquid ethylene or propylene at their boiling points are normally added to the reaction medium as a diluent. How will an increase in the dielectric constant of the reaction medium affect the rate and degree of polymerization?
Solution: Both the rate of polymerization and the molecular weight decrease with increasing temperature in cationic polymerization. These liquids help to prevent excessive temperature increases because part of the heat of polymerization is dissipated through the heat of evaporation of the liquids. In other words, the liquids act essentially as internal refrigerants.
Using the general relation between the rate of reaction and activation energy (k = Ae–E/RT), we note that a decrease in the activation energy, E, increases the rate of reaction while an increase in E has the opposite effect. An increase in dielectric constant increases the rate of initiation, ki, by reducing the energy required for charge separation. On the other hand, an increase in the dielectric constant decreases kt by increasing the energy required for the rearrangement and combination of the ion pair. Since both Rp and Xn are directly proportional to ki/kt, an increase in dielectric constant increases both quantities.
C. ANIONIC POLYMERIZATION
Monomers with electronegative substituents polymerize readily in the presence of active centers bearing
whole or partial negative charges. For example, a high-molecular-weight polymer is formed when
methacrylonitrile is added to a solution of sodium in liquid ammonia at –75°C. Typical electron-withdrawing substituents that permit the anionic polymerization of a monomer include –CN, –COOR, –C6H5, and –CH› CH2. The electronegative group pulls electrons from the double bond and consequently renders the monomer susceptible to attack by an electron donor. Catalysts for anionic polymerization include Grignard reagents, organosodium compounds, alkali metal amides, alkoxide, and hydroxides.
1. Mechanism
Propagation in anionic polymerization proceeds according to the following reactions:
X X X X
Y Y Y Y
Here, M+ represents a counterion that accompanies the growing chain. In most cases, M+ is an alkali metal ion, whereas X and Y are either electron-withdrawing groups or unsaturated groups capable of resonance stabilization of the negative charge.
Initiation may occur in two ways: a direct attack of a base on the monomer to form a carbanion (Equation 7.61) or by transfer of an electron from a donor molecule to the monomer to form an anion radical (Equation 7.62).
X X
M+ B- + CH2 C B CH2 C - M+ (7.61)
Y Y
X X
MO -
+ CH2 C •CH2 CM+
Y Y
anion radical
M+B– may be a metal amide, alkoxide, alkyl, aryl, and hydroxide depending on the nature of the monomer. The effectiveness of the catalyst in the initiation process depends on its basicity and the acidity of the monomer. For example, in the anionic polymerization of styrene, the ability to initiate reaction decreases in the order –CH2 ––NH– 2> NH– 2 @ OH–. Indeed, OH– will not initiate anionic polymerization of styrene. Where the anion is polyvalent, such as tris(sodium ethoxy) amine N(CH2CH2O– Na+)3, an equivalent number of growing chains (in this case, three) can be initiated simultaneously.
The donor molecular in Equation 7.58 represents, in general, an alkali metal. In this case, transfer results in the formation of a positively charged alkali metal counterion and an anion radical. Pairs of anion radicals combine to form a dianion.
•CH2 X
C
Y - M+ X
+ • C -
CH2 M+
Y M+ X
C
Y CH2 X
-
CH2 CM+
Y (7.63)
In carefully controlled systems (pure reactants and inert solvents), anionic polymerizations do not exhibit termination reactions. As we shall see shortly, such systems are referred to as living polymers; however, because of the reactivity of carbanions with oxygen, carbon dioxide, and protonic compounds, termination occurs according to Equation 7.64.
The terminal groups in Equations 7.64a and 7.64b cannot propagate and, consequently, effectively terminate polymer growth. Equations 7.64c–7.64e involv
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