Ind. Eng. Chem. Rod, Res. Dev. 1982, 21, 281-284
boride. This is higher than previously reported values for Raney nickel and Adam's platinum. However, because of the complex nature of this reaction, these values should be viewed with caution. In summary, the nickel boride is a less active catalyst for the hydrogenation of nitrobenzenethan is Raney nickel. However, it is more selective for aniline formation throughout the course of the reaction. Unlike Raney nickel, reaction intermediates are always present in less than 2% of the initial concentration of the reactant. Literature Cited
Table I. Aniline Rate Constants Raney nickelb nickel boridea T,"C lo6 x k C 65 80
100
lo6 X k c T, "C min max
1.8d (1.8)e 70 3.8d (3.8)e 10.0d ( 1 0 ) e
85
0.9 1.0
16 15
28 1
mean 4 . 7 d (5.5)e 4.6d (5.3)e
a This work. Burge et al. (1980). Units: g-mol/g Rate for aniline formation. e Rate of Ni, s, atm of H,. for nitrobenzene disappearance.
Brown, C. A.; Brown, H. C. J. Am. Chem. Soc. 1963, 8 5 , 1003. Brown, H. C.; Schleslnger, H. I.; Flnholt, A. E.; Glibrent. J. R.; Hyde,E. K. J . Am. Chem. Soc. 1953, 75, 215. Brown, 0. W.; Henke, C. 0. J. Wys. Chem. 1922, 26, 161, 272, 324, 631, 715. Brown. 0. W.; Henke, C. 0. J. W y s . Chem. 1923, 2 7 , 52. Burge, H. D.; Collins, D. J.; Davis, B. H. I d . Eng. Chem. Prod. Res. Dev. i980, 79, 389. Oharda, K. H.; Siiepcevlch, C. M. Ind. Eng. Chem. 1960, 5 2 , 417. Haber, F. 2.Elektrochem. 1898, 2 2 , 506. Paul, R.: Bulsson, P.; Joseph, N. Ind. Eng. Chem. 1952, 44, 1008. Rihani, D. N.; Narayanan, T. K.; Doralswamy, L. K. Ind. Eng. Chem. Process Des. Dev. 1965, 4 , 403. Ryan, R. C.; Wilemon, G. M.; Daisanto, M. P.; PHtmn, C. U., Jr. J. Mol. Catal. 1975, 5 , 319. Smlth, A. D. M.Eng. Thesis, University of LoulsvUle, 1979. Smlth, H. A.; Bedolt. W. C. I n "Catalysis", Emmett, P. H., Ed., Vol. 111, Reinhold: New York, 1955. Wade, R. C.; Holah, D. 0.; Hughes, A. N.; Hul, 8. C. Catel. Rev. Sci. Eng. 1976, 74. 211. Yao, H. C.; Emmett, P. H. J. Am. Chem. Soc. 1962, 8 4 , 1086.
diates could present processing difficulties under some conditions. Rate Constants. The rate constants, evaluated from the concentration-time data, are given in Table I. Also presented are values for aniline formation using Raney nickel from Burge et al. (1980). The average value is computed by assuming a zero-order process throughout the reaction, reflecting the mean production rate of aniline. The data in Table I show that the overall rate constant for Raney nickel is greater than for nickel boride. In addition, the maximum rate for Raney nickel exceeds the nickel boride rate, even though the nickel boride rate constant is larger than the smallest value for Raney. For nitrobenzene disappearance the rate constants for Raney nickel are similar to the ones for nickel boride. Burge et al. (1980) obtained a very low temperature dependence in the limited range covered; an apparent activation energy of 12 kcal/mol was obtained for nickel
Received for review September 29,1980 Revised manuscript received November 20,1981 Accepted December 31, 1981
GENERAL ARTICLES Effect of Polymer-Bound Amine Accelerators on the Radical-I nitiated Curing of Unsaturated Polyesters with Styrene Charles U. Plttman, Jr.;
and Slvananda S. Jada
Department of Chemkby, The University of Alabama, University, Alabama 35486
Polymer-boundtertiary amine accelerators were compared to their freely added monomeric analogues as catalysts for the curing of poly(diethyiene glycol maleate) prepolymers with styrene. Benzoyl peroxide was used as the initiator. The polymer-anchored accelerators gave shorter gel and curing times and lower energies of initiation than their monomeric analogues. Use of equivalent amounts of the bound accelerators led to resins wlth greater tensile strengths and h w r softening points and Brinell hardness values. Thus, the bound accelerators were more efficient, suggesting that this concept is worthy of further investigation.
Introduction Reagents (Mathuretal, 1980), drugs (Donaruma, 1974), and biocides (Pittman, 1980), when bound to polymers, frequently continue to exhibit their desired properties. However, their physical properties and those of the polymer may change. While many types of species have been chemically bound to polymers (Okawara et al., 1976), no reports exist in the literature where curing accelerators 0196-432118211221-0281$01.25/0
have been attached to polymers and employed. In this paper, tertiary amine accelerators have been chemically bond into poly(diethy1ene glycol maleate) prepolymen. It is well known that tertiary amines react with benzoyl peroxide to accelerate radical-initiated processes (Bemdtson and Turnen, 1954; Maltha and Damen, 1956). An old classic reaction, the curing of polyester prepolymen with styrene, initiated with benzoyl peroxide, was studied 0
1982 American Chemical Society
282
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
Table I. The Influence of Free vs. Polymer-Bound Tertiary Amine Accelerators on the Curing of Poly(diethy1ene glycol maleate) with Styrene Using Benzoyl Peroxide Initiation" accelerator none 1-free
%free 2-bound 2-bound 2-bound 3-free 3-bound 3-bound 4-free 4-bound 4-bound 5-free 5-bound 5-bound 5-bound
prepolymer deg of conv, P
curing temp, "C
pot life, min
gel time, min
0.92 0.35 0.62 0.67 0.92 0.35 0.62 0.67 0.92 0.35 0.62 0.92 0.35 0.62 0.92 0.35 0.62 0.67
60
200 6.0 4.5 4.9 22.0 20.3 6.1 14.6 44.1 25.2 14.8 30.9 13.5 12.2
118 6.4 5 .O 5.2 30.4 28.2 12.5 21.7 64.3 35.5 22.2 50.5 19.0 17.2
71.6 18.0 12.6 14.2
93.8 28.2 20.1 24.3
20 20 20 20 20 20 20 60 50 50 60 40 40 60 50 50 50
cure time, inin 170 7.4 5.5 6.0 43.4 34.2 16.5 28.7 84.3 51.5 29.2 67.5 31.0 25.2 115.8 41.2 30.1 35.3
a Each curing reaction employed a prepolymer which had been previously made at 190 "C from maleic anhydride (1.0 mol) and diethylene glycol (1.1mol) for use of free accelerators, or diethylene glycol (1.0 mol) plus accelerator (0.05 mol), for use of bound accelerators. All curing reactions employed the same total amount of accelerator (0.05 molimol of maleic anhydride in prepolymer). Styrene (30 wt % of the amount of polyester) and benzoyl peroxide (2 wt % of the amount of polyester) were used in each curing reaction.
as the model test reaction to compare the effects of using polymer-bound accelerators vs. the use of these same accelerators when added free to the reactions. Several questions arise. If accelerators are attached to the polymer being cured, will the curing rate increase or decrease relative to the use of free accelerators? In fact will polymer-bound accelerators exert any promoting effects? What effect will accelerator immobilization have on the cross-link density of the resin? Will other resin properties change? Curing kinetics are quite simply discussed in terms of gel times and curing times (Sedor, 1966; Jada et al., 1977). The gel time is a measure of the copolymerization rate (eq 1). (In eq 1,Kiand ni are con(1) log Tge,= Ki + ni log [I] stants and [I]' is initiator concentration.) As previously described (Rybolt and Swigert, 1949), both gel time and curing times were obtained from the exothermic temperature vs. time curves recorded from standard curing reactions. These were analyzed by the standard method (Parkyn, 1967). These quantities together with tensile strengths, values of hardness, and energies of initiation help provide a semiquantitative assessment of the curing process. Experimental Section Poly(diethy1eneglycol maleate) prepolymers were prepared from maleic anhydride (98.1 g, 1.0 mol) and diethylene glycol (111.4 g, 1.1mol) at 190 OC. The preparation of prepolymers containing polymer-bound accelerators employed maleic anhydride (1.0 mol), diethylene glycol (106.3 g, 1.05 mol), and either 2 , 3 , 4 , or 5 (0.5 mol) at 190 "C in the same manner. Curing reactions were carried out in standard test tubes by dissolving the prepolymer (6 g, prepared as prepolymer) and benzoyl peroxide (0.12 g, 2% by weight of the resin). All prepolymer samples contained 0.05% by weight hydroquinone to prevent curing prior to the desired time. Curing temperatures between 20 and 60 "C were employed (Table I) using external thermostatted constant temperature baths. For energies of initiation (Table 111, exothermic curves were obtained at four different temperatures from 20 to
Table 11. Initiation Energies For the Curing of Poly(diethy1ene glycol maleate) Prepolymers with Styrene Using Polymer-Bound vs. Free Amine Accelerators
accelerator
a
polyester energy of prepolymer's initiation, deg of Ei,kcal conv, P mol-'
none dimethylaniline diethylaniline, 1 2-free 2-bound 3-free 3-bound 4-free I-bound 4-bound
0.92 0.92 0.92 0.92 0.67 0.92 0.67 0.92 0.67 0.62 0.35
27.6a 12.4' 15.8 18.7 17.3 28.7 19.3 25.6 16.0 16.4 15.6
5-free 5-bound 5-bound
0.92 0.67 0.35
30.6 20.7 20.8
See Jada et al. (1977).
90 "C. Initiation rate constants, Ki, were evaluated graphically by plotting Tc1vs. initiator concentration. The slope of an Arrhenius plot of log kivs. the reciprocal Kelvin temperature gave the value of the initiation energies (i.e.,
Ei). Several physical properties were examined. Resins were extracted with acetone at ambient temperatures according to ASTM standard method D2765-68 (ASTM, 1972) to determine the extracted fraction. The Brinell hardness was determined by ASTM standard method E10-60 (Kirk-Othmer) and the resin's tensile strength was measured according to ASTM standard method D638-72 (ASTM, 1974a). The softening point was determined by ASTM standard method D1525-70 (ASTM, 1974b). Results Polyesters were prepared at 190 OC from maleic anhydride and diethylene glycol and then cured with styrene
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 283
Scheme 11. Probable Mechanism for the Acceleration of
Scheme I. Synthesis of Polymer-Bound Amine Accelerators
Benzoyl Peroxide-Catalyzed Resin Curing by Amines
R
2- 5 0
0
0
0
I1 II II II ~OCH2CHzOCH2CH20CCH=CHCOCH2CHzNCHzCH20CCH~CHCO~
P"
iH2
T
6
I
R prepolymer containing bound a c c e l e r a t o r
8 cured resin
R
0
-OCCH=CHCOvm
in the presence of benzoyl peroxide and an added tertiary amine accelerator (Jada et al., 1977). The accelerators used were diethyl aniline, 1, N-phenyl diethanolamine, 2, N propyl diethanolamine, 3, N-isopropyl diethanolamine, 4, and N-n-butyl diethanolamine 5. In analogous reactions
II
OR
I1I
-0CCH-GHCOyw
0
II
styrene
curing
~OCH,~HOCH,CH,~
styrene
curing styrene
ICH2CHI;;-
1
I Ph
3
2
5
the amine accelerator was bound into the polyester as a diol derivative in a molar concentration 5% that of maleic anhydride (Scheme I). Polyesters a t conversions of P = 0.35, 0.62, and 0.67 were made. Compounds 2-5 were incorporated into polyesters at 5 mol % relative to maleic anhydride. The pot life, gel times, and curing times for the use of free accelerators 1-5 are given in Table I. The efficiency decreases in the order 1 > 2 > 4 > 3 > 5 and each accelerator exerts a substantial effect. The pot life, gel times, and curing times for the bound accelerators are also given in Table I. Each of the bound accelerators 3-5 was significantly more efficient than its free analogue. This was true at all degrees of polymerization of the starting prepolymer (i-e.,P = 0.35,0.62,0.67). When amines 3,4, and 5 were bound as part of the polymer, they gave even shorter gel times and cure times at 40 "C (for 4) and 50 "C (for 3 and 5) than they did when used free at 60 "C. Amine 2 was more efficient at 20 "C when bound than when used free at 20 "C. The polymer-bound accelerator efficiency order was 2 > 4 > 3 E 5. Aniline derivatives 1 and 2 were far more efficient than aliphatic amines 3-5. Energies of initiation, Ei,obtained from Arrhenius plots of initiation rate constants, are shown for both polymerbound and free accelerators in Table 11. The initiation energies for polymer-bound accelerators 3, 4, and 5 are lower by 9-10 kcal mol-' than those obtained when they were added free. Lower values of Ei were also found for polymer-bound 2 vs. free 2 but the magnitude is less. The weight fraction of the resins extractable with acetone was always lower when the polymer-bound accelerator
(attached to polyester if initiated by bound 8)
8b
was employed. Also, the Brinell hardness, the tensile strength, and the softening points were always higher when polymer-bound amines 2,3,4, and 5 were used and compared to resins prepared with the identical amount of free accelerator. These results (Table 111) suggest a higher cross-link density was obtained with the bound amines.
Discussion Amine accelerators facilitate the formation of radicals from benzoyl peroxide. A probable mechanistic sequence is shown in Scheme I1 for the polymer-bound accelerators. Horner and Schloenk (1949) showed that an initial amine complex with benzoyl peroxide is formed with free amines. The polymer-anchored analogue of this complex is represented by 6 in Scheme 11. Higher electron density at nitrogen favors the formation of salt 6. The accelerating effect of amines on curing derives from the decomposition of 6 to 7 and 8. Both 7 and 8 may initiate styrene polymerization or form new radical sites along the prepolymer chain, either %y addition to the double bonds remaining from maleic anhydride moieties or by hydrogen abstraction a to ether oxygens. Abstraction of hydrogen atoms by 8 gives a quaternary ammonium counterion to
284
Ind. Eng. Chem. Prod. Res. Dev., Val. 21, No. 2, 1982
Table 111. Physical Properties of Styrene-Cured Poly(diethy1ene glycol maleate). Comparison of Properties Induced by Polymer-Bound vs. Free Acceleratorsa _ I _ _ _
accelerator
none 1-free 2-free 2-bound 3-free 3-bound 4-free 4-bound 5-free 5-bound
Brinell hardness, extracted kg. fraction m m - 2 0.11 0.10
8 10
0.10 0.07 0.11 0.05 0.1 5 0.06 0.17 0.11
18 24 12 15 9 16 15 18
tensile
strength, kg.cm-2
softening point, “C
1400-1450 1550-1570 1650-1700 2000-2150 1500-1540 1710-1750 1470-1490 1870-1900 1640-1670 1790-1859
95 100 120 19 5 113 161 102 16 5 130 172
See footnote a in Table I for standard curing reaction.
benzoate. Also, radical cation 8 may directly initiate a polymerizing styrene chain giving a quaternary ammonium salt site a t the N. This mechanism accounts for the low initiation energies of the aryl amines dimethylaniline, 1 and 2 (12.4, 16.5, and 18.7 kcal mol-’, respectively) vs. trialkylamines 3, 4, and 5 (28.7, 25.6, and 30.6 kcal mol-’, respectively). Nitrogen centered radical cations (Le., 8) will be stabilized by resonance (see 8b) into the phenyl ring in the three aniline derivatives but not in 3-5. Thus, faster initiation with aniline derivatives is expected. The fastest initiation was achieved with dimethylaniline. Dimethylaniline more readily forms monomer complexes of the type represented by 6 since 1 and 2 are somewhat more hindered amines. The -I effect of the 0-C bonds of 2 also lowers the base strength of 2 relative to 1which also retards the formation of complexes such as 6 from 2. The reason for the remarkable 10 kcal mol-’ decrease in Eiobserved going from the use of 3 , 4 , and 5 free to their use when polymer-bound is unexplained. The solution microenvironment at the polyester chain might exert different solvation effects in the steps forming 6 or 8 than do the bulk styrene/polyester solutions. The specific effects of the free -OH groups in 2-5 relative to the corresponding ester functions in the polymer-bound analogues of 2-5 are not known. These effects are not thought to be large based on the use of diacetate of 3. Esterification of 3 with acetyl chloride gave its diacetate 9. When this
9
was used as a curing agent, the results were quite similar to those exhibited by free 3 in Tables I and 11. Polymer-bound accelerators 2-5 gave faster cross-linking rates per mole of accelerator employed as evidenced by the higher hardnesses, tensile strengths, and softening points (Table 111) of the resins cured with bound accelerators. Bound accelerators also give smaller extracted fractions since fewer polystyrene segments are formed which do not bind to polyester segments during curing. Possibly, higher concentrations of initiation sites are generated along the polyester chain when polymer complex 6 decomposes to PhC02. and 8. PhCO,. may abstract hydrogen from the backbone more efficiently before complete diffusion from the solvent cage occurs when generated from a bound complex 6 (relative to the action of PhC02.generated from benzoyl peroxide and the free amine accelerators). This would lead to a higher density of growing polystyrene side chains, shorter average polystyrene lengths, and higher cross-link densities. If nitrogen cation radical, 8, can also initiate styrene polymerication, this would also result in a higher styrene side chain density along the polyester with the use of bound accelerators. In conclusion, the use of polymer-bound curing accelerators appears to be worthy of further serious study. The bound accelerators were clearly more efficient curing agents than their monomeric analogues, 2-5, as defined by the classic measurements of curing temperature, pot life, gel time, cure time, energy of initiation, Brinell hardness, tensile strength, softening point, and the fraction of polymer that could be extracted (i.e., uncured polymer). This study is more indicative than definitive because the structure of the polymer-bound accelerators (diesters) was not functionally identical with the free accelerators (diols). Therefore, it should not be concluded that bound accelerators will necessarily be more efficient than their free analogues. Literature Cited American Standard Testing Materials Standard, D2765-72, 1972). American Standard Testing Materials Standard D638-72 (1974a). American Standard Testing Materials Standard D1525-70 (1974b). Berndtsson. 8.; Turnen, L. Kunstoffe 1954, 44, 430. Donaruma, L. G. Prog. Po&m. Scf. 1974, 4 , 1. Horner, L.; Schloenk, E. Angew. Chem. 1949, 67, 711. Jada, S . ; Kutepov, D. F.; Valgln, A. D. Plest. Messy. 1977, No. 7, 69. Kirk-Othmer Encyclopedia of Chemical Technology, 2nd ed; Vol. 10, p 810. Maltha, P.; Darnen, L. Kunstoffe 1956, 46, 324. Mathur, N. K.; Narang, C. K.; Williams, R. E. “Polymers As Aids in Organic Chemistry”; Academic Press: New York, 1980. Okawara, M.; Takemoto, K.; Harada. T. “Application of Polymers in Synthetic Chemistry”; New Japan Printers, Inc.: Tokyo, 1976. Parkyn, B. “Polyesters”; American Elsevier, Inc.: New York, 1967, Vol. 2, pp 49-52. Plttman, C. U., Jr.; Stahl, G. A,; Winters, H. J . Coat. Techno/. 1978, 4 0 , 636. Pittman, C. U.,Jr. In “Polymer-Supported Reactions in Organic Synthesis”; Hod@, P.; Sherrlngton, D. C.; Ed.; Wlley: New York, 1980. RyboR, C. H.; Swlgert, T. C. Mod. Plast. 1949, 26, 97. Sedov. L. N. Plast. Massy. 1988, No. 12, 16.
Receiued for review October 26, 1981 Accepted December 17, 1981
