I n d . Eng. C h e m . Res. 1987,26, 2079-2086
2079
Kinetics of Bulk Free-Radical Polymerization of Methyl Methacrylate Using Potassium Peroxydisulfate with 18-Crown-6as Phase-Transfer Catalyst K. Y. Choi* and C.Y. Lee Department of Chemical and Nuclear Engineering, University of Maryland, College Park, Maryland 20742
The kinetics of bulk free-radical polymerization of methyl methacrylate with water-soluble initiator (potassium peroxydisulfate) and macrocyclic polyether (18-crown-6) as phase-transfer catalyst has been studied. T h e effects of reaction temperature, initiator concentration, phase-transfer catalyst concentration, and the volume fraction of aqueous phase on the polymerization rate were investigated. T h e poly(methy1 methacrylates) prepared by phase-transfer-catalyzed polymerization were found to have considerably higher molecular weight and more uniform molecular weight distribution than the polymers prepared by conventional organic initiators such as azobisisobutyronitrile under similar reaction conditions. 'H NMR spectroscopy indicated that the crown ethers were incorporated into the growing polymer chains. It was also shown that the phase-transfer-catalyzed bulk polymerization of MMA can be conducted in the absence of aqueous phase and that the polymerization rate decreased with a n increase in the volume of water. The free-radical polymerization of acrylic and vinyl monomers in solution or bulk phase with water-soluble initiators in conjunction with phase-transfer catalysts is a relatively new area of research. Although water-soluble initiators such as potassium peroxydisulfate and ammonium peroxydisulfate have been used extensively for emulsion polymerization processes for many years, the insolubility of water-soluble initiators in organic solvents and monomers makes their uses impractical for solution and bulk polymerizations. Recently, it has been found that such water-soluble initiators can be used effectively for bulk or solution free-radical polymerization when used with certain phase-transfer agents such as quaternary ammonium salts or macrocyclic polyethers (e.g., crown ethers) (Rasmussen and Smith, 1981a,b; Rasmussen, 1982). In phase-transfer catalysis, a substrate (e.g., monomer) in an organic phase is reacted with a reagent (e.g., water-soluble initiator) in another phase which is usually aqueous or solid. Reaction is achieved by means of a phase-transfer catalyst (PTC) which is capable of solubilizing and complexing the inorganic salts such as water-soluble initiators into the organic media. Thus, solution- or bulk-phase free-radical polymerization can be carried out by water-soluble initiators in the presence of properly chosen phase-transfer catalysts. The phasetransfer-catalyzed free-radical polymerization differs from emulsion polymerization in that the former requires no micelle-forming materials (emulsifier) and the polymerization takes place in the bulk phase. The use of water-soluble initiators for bulk or solution polymerization offers some unique advantages. For instance, the organic-insoluble initiators (i.e,, water-soluble initiators) which are generally more stable than organic initiators present fewer storage and handling problems as opposed to many organic-soluble free-radical initiators which, in general, require refrigeration. There is also a report that the polymerization with phase-transfer catalyst can be conducted a t low temperature with high reaction rate for some monomer systems (Rasmussen, 1982). Some authors also reported that the polymers prepared by phase-transfer-catalyzed free-radical polymerization *To whom all correspondence should be addressed. 0888-5885187 / 2626-2079301.SO/ 0
showed improved properties. For instance, Rasmussen and Smith (1984) observed a 10-fold increase in shear strength of isooctyl acrylate and acrylic acid copolymers prepared by phase-transfer-catalyzed free-radical polymerization. Since this polymerization technique has been introduced only recently, there remains a lot of unanswered fundamental problems associated with the characteristics of initiator complex, complexation kinetics, initiation mechanism, phase-transfer rate, and polymerization kinetics. In this paper, we shall present our experimental study of phase-transfer-catalyzed polymerization of bulk methyl methacrylate with potassium peroxydisulfate (K,S,O,) as an initiator and 1,4,7,10,13,16-hexaoxycyclooctadecane (18-crown-6) as a phase-transfer catalyst. The effect of various reaction conditions on the polymerization rate and the polymer molecular weight properties will be examined. Experimental Section The phase-transfer-catalyzedfree-radical polymerization experiments were carried out in a 1000-mL stirred glass reactor equipped with a heating/cooling jacket. The reaction temperature was kept constant by circulating constant-temperature heating fluid through the jacket, and the temperature variation from the set point value was negligibly small (f0.5 "C). A predetermined amount of potassium peroxydisulfate was dissolved in deionized water (8-20 mL) and mixed with a large amount of methyl methacrylate monomer (400 mL, Rohm and Haas Company) before 18-crown-6(Aldrich Chemical Co.) was added to the reactor. The polymerization was conducted under nitrogen atmosphere. Both potassium peroxydisulfate and 18crown-6 were used without further purification, and the monomer was stripped of inhibitors by ion-exchange resins (Amberlyst A-27, Rohm and Haas Co.). Small samples were taken from the reactor during the polymerization and precipitated in excess methanol. The samples were dried in vacuo and monomer conversion was determined by gravimetric method. During the polymerization experiments, the stirrer speed was kept constant at lo00 rpm and the torque required to maintain this constant stirrer speed was recorded. Polymer molecular weight and molecular weight distribution were determined by gel permeation chromatography. 0 1987 American Chemical Society
2080 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
Formation of Potassium Peroxydisulfate-Crown Initiator Complex In order to use a water-soluble initiator (K2S206)to initiate bulk free-radical polymerization, potassium peroxydisulfate is reacted with a phase-transfer catalyst such as 18-crown-6to form the active initiator complex in situ. 18-Crown-6 is a cyclic polyether which dissolves readily in water and many organic solvents including methyl methacrylate monomer. It has been reported that uncharged molecules such as crown ethers have remarkable ability to solvate cations in a nonpolar environment (Pedersen, 1967; Frensdorff, 1971; Izatt and Christensen, 1978). 18-Crown-6is particularly efficient in complexing potassium cation in the aqueous phase: Po?
P?-l
Mandal(1986) discovered that less than 2.5% of the total &OB2- was transferred from the aqueous phase to the organic phase. The transfer of divalent anions from the aqueous phase to the organic phase has been known to be notoriously difficult even in the presence of phase-transfer catalyst (Izatt and Christensen, 1978). Such difficulty is attributed to a high degree of hydration and decreased tendency for highly charged and small sized anions to be phase transferred. Since peroxydisulfate dianion (S202-) is a source of free radicals for the polymerization, it is important to understand how crown ether complexes are involved in the free-radical-formingprocess. The following reactions may take place:
[: :) Po> K+
bJ 18-crown-6
(a)
cr crown complex (a+)
A crown ether molecule surrounds a potassium cation with a number of inward facing oxygen electron lone pairs while a t the same time providing an outer liphophilic exterior which enhances solubility in a wide range of organic solvents. It is believed that the potassium cation lies in the cavity of the ligand on a crystallographic center, and the cation is surrounded by a nearly planar hexagon of oxygen atoms (Izatt and Christensen, 1978). In the aqueous phase, the complexation equilibrium constant (Kl) is 112 L/mol (Frensdorff, 1971), indicating that the potassium cation-crown complex is quite stable. Two major factors contributing to the high stability of 18-crown-6and K+ complex are (i) the largest circular ring possible with the greatest number of evenly distributed oxygen atoms of high basicity with their maximum charge density directed toward the center of the ring; and (ii) a potassium cation (ionic diameter: 2.66 A) that fits the cavity of 18crown-6 (diameter: 2.6-3.2 A) in such a way as to make the total attraction between it and the oxygen atoms a maximum (Izatt and Christensen, 1978). Polymerization with Phase-Transfer Catalyst When crown ethers are added to the aqueous solution of potassium peroxydisulfate, Rasmussen et al. (1983) reported that the thermal decomposition rate of peroxydisulfate to radical species is significantly accelerated. Such an acceleration effect has been attributed to the oxidation of crown ether and to a Coulombic attraction between a cation-complexed crown radical and a peroxydisulfate dianion. The apparent activation energy for the decomposition of K2S206in nonreactive aqueous media was reported to be decreased from 33.5 to 19.9 kcal/mol by the addition of 18-crown-6to the aqueous phase. Takeishi et al. (1981) also discovered that crown ethers can solubilize peroxydisulfate dianions successfully in anhydrous organic media. Rasmussen and co-workers (Rasmussen, 1982; Rasmussen and Smith, 1984) have demonstrated that watersoluble initiators such as K2S208can be used effectively for solution polymerizations of butyl acrylate in anhydrous acetone in the presence of 18-crown-6as the phase-transfer catalyst. However, the details of complex formation phenomena, initiation mechanism, and polymerization kinetics are not well understood. When tetrabutylammonium bromide (Bu,NB,) was used as the phasetransfer catalyst for styrene polymerization, Ghosh and
aqueous phase
+
Po? SO;--
K + H, )
+
HSO4(2)
C
O
J
I organic p h a s e
aqueous p h a s e
I1 organic phase
According to postulation 2, a proton in a crown ether ring is abstracted by sulfate radical anion and the resulting crown radical I is transfeped to the monomer phase. Then the transferred crown radical becomes an initiating species in the monomer phase. Such hydrogen abstraction may be strongly facilitated in the complexed state due to either weakened carbon-hydrogen bond strength upon complexation or increased stabilization of the resultant radical via through-space interaction with the cation. Rasmussen and Smith (1984) suggest that in pure aqueous phase, the accelerated decomposition of potassium peroxydisulfate in the presence of crown ether may be due to Coulombic attraction:
If such a Coulombic attraction is dominant, complex species I1 will be the more probably initiator species. However, it should be noted that the chemistry observed in the aqueous phase in the absence of organic monomer phase may not be directly correlated with the polymerization behavior in bulk or solution phase. For either initiator complex I or 11,the following kinetic scheme of free-radical polymerization can be formulated:
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2081 formation of initiator complex and phase transfer 045'c
+
K+
z
k
H45'C
A7O"C
0.4
L
1
1
,
2
.-,+ H 2 0 - p r o d u c t s .SO4 ,+ Q', Q ' + , + HSOd-
W 5
0.3
kw
SO,
A70"C
Balke and Hamielec (19731
A 2s04*-,
s20:-,
060'C
Xi
Q,
*
0 0
\
z
5
:
0.2
0
I Q+SO;
0
-,
0.1
I1 initiation
+ -
+M
Q'+,
or
Q+SO;-,
k ,
0.0
0
Pl
M
termination P,
+ ,P
-
400
500
If substantial amounts of sulfate dianions (S20E2-) are assumed to be transferred from the aqueous phase to the organic monomer phase, the following initiation mechanism may be written:
k
M
300
Figure 1. Effect of reaction temperature on MMA conversion: [KzSzOslt = 0.0146 mol/L, [18-crown-6], = 0.0292 mol/L, V , = 12 mL, V , = 400 mL.
p1
P 1 + M A p2
+
200
TIME l m l n l
p r o p a g a t ion
p,
100
p,*1
termination
2Q+,
where Q represents the phase-transfer catalyst (crown ether) and Q+ the cation-crown complex. The subscripts w and o denote the aqueous phase and the organic phase, respectively. M is the monomer and P, the growing polymer radicals containing n-monomer units. When the quasi-steady-state approximation is applied to the radical species, the rate of phase transfer of radical (SO4'-) is given by
+ S2082-w_ffl, (Q+)2S20~-o
kh
(Q+)2S2082-0
2Q+S04'-,
(16) (17)
Q+S04*-0+ M propagation (18) This mechanism was proposed by Jayakrishnan and Shah (1983) for the polymerization of methyl methacrylate using ammonium peroxydisulfate initiator and hexadecylpyridinium chloride as a phase-transfer agent in ethyl acetate and water. The polymerization rate expression corresponding to this mechanism takes the form
R, = where k, is the pseudo-first-orderrate constant for reaction 7. Since the initiation in the organic monomer phase is due to phase-transferred crown radicals, the following volume correction is also required: Ri = Rp,tVw/Vo
(13)
where Ri is the initiation rate, Vwthe volume of aqueous phase, and V, the organic phase (monomer). Thus, the bulk polymerization rate takes the following form:
This initiation mechanism was suggested by Ghosh and Mandal (1986) when tetrabutylammonium bromide was used as the phase-transfer agent for styrene polymerization with V,/V, ratios of 0.33-1.33. For some limiting conditions, eq 14 may be simplified as follows:
Note that the sulfate anion radical (SO,*-) is the major contributing factor of initiation and polymerization in the mechanism described above (cf. eq 8a and 8b).
where [&+Itotis the total concentration of potassium cation-crown complex. If 2K2[Q+],[S20,Z-],
