20 Homopolymerization of Epoxides in the Presence of Fluorinated Carbon Acids Catalyst Transformations 1
Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 19, 2013 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch020
J. ROBINS and C. YOUNG
Specialty Chemicals Division, 3M Center, St. Paul, MN 55144 Cationic polymerization of epoxides has been a commercially important means for the preparation of low molecular weight polyethers and curing epoxy adhesives for many years. In order to maximize the molecular weight of the formed polymers, the polymerizations are t y p i c a l l y run at sub-ambient temperatures i n very polar solvents . However, these conditions are not applicable to the curing of polyfunctional epoxy resins i n i n d u s t r i a l applications either as adhesives or coatings. Curing of epoxy resins with cationic i n i t i a t o r s at ambient temperatures t y p i c a l l y results i n some monomer rearrangement, formation of c y c l i c oligomers and catalyst decomposition i n addition to the desired linear epoxy homopolymerization . These side reactions can dramatically affect the ultimate properties of the cured resin, making the use of i n i t i a t o r s with large counterions more desirable. Previous work, however, has been focused on the polymer formation and the fate of the catalysts has been somewhat neglected. We report here on the thermo-kinetic analysis of the homopolymerizations of several epoxide monomers using a unique class of cationic i n i t i a t o r s , bis-trifluoromethanesulfonyl methane and i t s derivatives. In addition we have included standard Lewis and Bronsted acids, which has allowed us to observe some interesting catalyst transformations. Bis-trifluoromethanesulfonyl methane, disulfone (DS), i s a compound having two trifluoromethanesulfonyl groups attached to a methylene group. Because of the strong electron withdrawing properties of trifluoromethanesulfonyl group, the proton on the methylene group becomes very a c i d i c . Disulfones with different substituents on the methyl group are l i s t e d i n Table I. Although the a c i d i t y i s not f u l l y characterized, we believe it to increase i n the following order: methyl disulfone (MDS), disulfone (DS), phenyl disulfone (TDS), bromodisulfone (BrDS), chlorodisulfone (C1DS) and tetrasulfone (TS). The compounds are generally prepared by Grignard reaction, where two moles of an a l k y l magnesium chloride are reacted with two moles of trifluoromethanesulfonyl fluoride i n ether. These compounds and their synthesis have been reported by Koshar (8,9) and Mitsch (8). 1
2
3
1
Author to whom correspondence should be directed. 0097-6156/85/0286-0263$06.00/0 © 1985 American Chemical Society
In Ring-Opening Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
264
RING-OPENING POLYMERIZATION
These catalysts offer some advantages for studying the structure effects of the counter anion i n cationic homopolymeri z a t i o n of epoxides.
Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 19, 2013 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch020
Experimental The epoxides used for study included 1,2-butene oxide, cyclohexene oxide and styrene oxide, which were commercially purchased (Aldrich Chemical Co.). Reagent grade solvents were used: 1,2-dichloroethane (1,2 DEC, MCB), 1,1-dichloroethane (1,1 DCE, Aldrich) and nitrobenzene ( A l d r i c h ) . Acidic catalysts commercially purchased included CF3SO3H (3M Co.) perchloric acid (70% aq., Baker Chem. Co.) HPF (60% aq., Alfa) HSbF #6H 0 (Alfa HBF4 (48% aq., MCB) CF3COOH (3M Co.) and PTSA«H 0 ( A l d r i c h ) . Fluorinated carbon acids were obtained from Koshar prepared according to previously published procedures (8 >9). Calorimetric probes were done as previously described (10), using a 1-1.5M solution of epoxide i n solvent i n the presence of a r e l a t i v e l y small amount (approximately 10~^M) of c a t a l y s t . The reaction was run i n an insulated plastic-coated paper cup f i t t e d with a magnetic s t i r r i n g bar, cover and thermocouple. Temperaturer i s e s were recorded versus time using a Hewlett-Packard 7127A strip-chart recorder. Representative exotherm curves are shown i n Figure 1 where four types of c a t a l y t i c a c t i v i t y are observed: 6
6
2
2
1. 2. 3. 4.
Reaction goes to completion Deactivation that leads to incomplete reaction Induction and then acceleration Poor a c t i v i t y
Results and Discussion It has been reported previously that bis-trifluoromethane-sulfonyl methane ("disulfone") and i t s derivatives are good epoxy homopolymerization catalysts (11,12). Calorimetric studies have shown that l , l , 3 , 3 - t e t r a k i s ( t r i f l u o r o methanesulfonyl) propane ("tetrasulfone") i s probably the only effective catalyst for homopolymerization of an a l i p h a t i c epoxide, e.g. butene oxide, i n a non-polar solvent at room temperature (Figure 2)· The other disulfone catalyst along with Bronsted acids are poor catalysts. On the other hand, cyclohexene oxide i s polymerized more r e a d i l y by a number of acidic catalysts, but some lead to an incomplete reaction (e.g. B F 3 C 2 H 5 O ) and i n general, their a c t i v i t y cannot be correlated to their acid strength, since "disulfones" have a pK around -1 (Figure 3)· Styrene oxide presents further complications and i s polymerized most readily by catalysts which are poor for homopolymerization of cyclohexene oxide such as t r i f l i c acid, but phenyl disulfone, which i s an active catalyst for cyclohexene oxide, i s surprisingly slow for styrene oxide (Figure 4)· The combination of DS and styrene oxide i n 1,2-dichloromethane and 1,1,2-trichloroethane, gave another interesting c a t a l y t i c a
In Ring-Opening Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
20.
Homopolymerization of Epoxides
ROBINS AND YOUNG
265
Table I. Bis-trifluoromethanesulfone methane (DS) and i t s derivatives i n the order of increasing a c i d i t y . ,S0 CF 2
3
CH,-CH
CH DS 3
*SO CF, a
, S0 CF 2
3
Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 19, 2013 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch020
0-C-H
0DS * S0 CF, 2
,S0 CF 2
3
H-CH
DS ' S0 CF
3
, S0 CF
3
2
2
σ ζ
CO
&.-CH
BRDS
S0 CF
3
,S0 CF
3
2
2
CLDS S0 CF 2
3
.S0 CF
2
2
3
CH-CH -CH ^
TS
2
CF S0 3
k
2
SO.CF,
CH S0 H 3
2
ce ο
CL-CH CF,S0
5 ο
