ISSN 1070-4272. Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 1, pp. 133!136. + Pleiades Publishing, Inc., 2006. Original Russian Text + I.L. Beilin, V.P. Arkhireev, S.S. Galibeev, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 1, pp. 135!138.
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MACROMOLECULAR CHEMISTRY ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ AND POLYMERIC MATERIALS
Copolymerization of Cyclic Carbonates with Isocyanates under Anionic Initiation Conditions and Structure of the New Copolymers I. L. Beilin, V. P. Arkhireev, and S. S. Galibeev Kazan State Technological University, Kazan, Tatarstan, Russia
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Received June 20, 2005
Abstract The possibility of preparing polyamido esters from propylene carbonate and 2,4-toluene diisocyanate in the presence of N-methylpyrrolidone was examined. The mechanism of the reaction of the starting monomers and the structure of the resulting copolymers were determined. The properties of the copolymers were studied. DOI: 10.1134/S1070427206010277
The reaction was monitored by 1H NMR spectroscopy with a Tesla-100 spectrometer operating at 100 MHz (solvent and internal reference acetone-d6). The middle-IR spectra of the copolymers were recorded on a Bruker FT-IR spectrophotometer; samples were prepared as KBr pellets. The elemental analysis was performed with a CNH-3 analyzer (oxidation at 1050oC). Gel-chromatographic studies were made with a Waters-150 C gel chromatograph equipped with Styrogel HR 1033106 A columns at 25oC; eluent THF, flow rate 2 ml min!1.
Along with urethane formation, much attention is given today to homo- and copolymerization of isocyanates under the conditions of anionic initiation. Of particular interest are copolymers of isocyanates with compounds of other classes (aldehydes, ketones, alkenes, etc.) [1]. Such polyamido esters (PAEs) can be used as coatings on glass, steel, and fabric supports, as films, and as fibers [2, 3]. Existing procedures for preparing polycarbonates and their copolymers are complex and time-consuming, require strict observance of the prescribed conditions and also high vacuum or elevated temperatures, and involve the use of organic solvents [4], which negatively affects the cost of the polymer and items thereof. At the same time, these materials have unique properties and in many cases are indispensable. Therefore, an urgent problem is development of procedures for preparing PAEs at atmospheric pressure and moderate temperatures. We showed in [537] that isocyanates in the presence of alkali metal lactamates and tertiary amines can induce ring opening in many heterocyclic compounds (lactams, lactones, oxiranes) with the formation of alternating copolymers.
The reaction rate constants were determined according to [8]. To optimize a kinetic study of the ternary copolymerization, we used the simplex-lattice experimental design. As approximating polynomial we used the third-order Scheffe model. Thermal analysis of samples was performed on an MOM Q-1500 derivatograph (Hungary) at a heating rate of 3 deg min!1. To determine the physicomechanical characteristics of the materials based on PC, TDI, and N-MP, we prepared films of these materials by chemical forming. As solvent for the monomers we used acetone. The breaking stress sb was determined with an MR-500T2 tensile-testing machine at 20 + 2oC. The velocity of mobile clamps was 100 mm min!1.
In this study we examined the copolymerization of propylene carbonate (PC), 2,4-toluene diisocyanate (TDI), and N-methylpyrrolidone (N-MP). The synthesis was performed at 20oC; the monomer ratio in the starting mixture was widely varied. The catalyst (triethylamine, TEA) concentration was 0.01 mol %. The amount of the unchanged monomer was determined by extraction of the polymer in a Soxhlet apparatus for 8 h.
The IR spectrum of the TDI3PC3N-MP terpolymer contained absorption bands absent in the spectrum of the starting monomers, cm!1: 1704 and 1412 (stretching vibrations of the carbonyl group in tertiary amides formed by cleavage of the isocyanate group across 133
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(a)
N-MP
PC
TDI (b)
N-MP
PC
TDI Fig. 1. Lines of (a) equal rates ( 106 mol l!1 s!1) and (b) equal rate constants ( 104 l mol!1 s!1) of copolymerization of PC, TDI, and N-MP (mole fraction scale).
Table 1. Copolymer yield in the reaction of TDI with PC and N-MP
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ Monomer ratio in the starting mixture, mol % ³ Yield, ÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ´ TDI ³ PC ³ N-MP ³ wt % ÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ 60 ³ 20 ³ 20 ³ 95 40 ³ 40 ³ 20 ³ 84 40 ³ 20 ³ 40 ³ 81 20 ³ 40 ³ 40 ³ 46 20 ³ 60 ³ 20 ³ 48 20 ³ 20 ³ 60 ³ 40 ÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄ
the N=C bond), 1304 (C3N stretching vibrations in amides), 1216 (C3O stretching vibrations in esters), 1066 (C3O3C stretching vibrations in aliphatic esters; this band is indicative of the cleavage of PC), and also 2926 and 2860 (CH stretching vibrations in CH2 groups). This pattern shows that the copolymerization involved both monomers. The spectrum does not contain bands at 1692 (C=O stretching vibrations in five-membered lactams), 1270, and 865 cm!1 (C3O vibrations in cyclic ethers). Homopolymerization of isocyanates in the presence of tertiary amines is possible, but homopolymerization of PC and N-MP under these conditions was not observed. However, the anion generated by cleavage of the isocyanate group is highly reactive and apparently attacks the carbonate or lactam ring, causing its cleavage. The spectrum does not contain bands characteristic of uretidinedione (1780 cm!1) and triisocyanurate (1720 cm!1) rings, which indicates that dimers and trimers of TDI are not formed in the course of the copolymerization [9311]. The highest rates and rate constants at low (5310%) conversions (Fig. 1) are observed at the monomer molar ratio in the starting mixture PC : TDI : N-MP = 25 : 65 : 10. This kinetic pattern suggests formation of an alternating macromolecule in the copolymerization. The copolymer is formed in a high yield (Table 1). With an excess of TDI in the starting mixture, polyisocyanate and also (as TDI is a bifunctional agent) branched and partially cross-linked structures are apparently formed on the ends of the macromolecule. This is indirectly confirmed by the fact that the copolymer solubility in organic solvents (dimethylformamide, dimethyl sulfoxide, etc.) becomes worse with an increase in the isocyanate content. With TDI taken in deficiency, excess PC and N-MP do not participate in the copolymerization and are removed in the course of the product purification. With a deficiency of one of the heterocycle, after its exhaustion the reaction continues as binary (PC3TDI or N-MP3TDI) copolymerization. The number-average molecular weight of the copolymers, according to gel permeation chromatography, is 2900 321 400. The relative changes in the content of functional groups, monitored by 1H NMR spectroscopy, apparently determine the composition of the reaction product. At equal excess of both heterocycles relative to TDI in the starting mixture (molar ratio TDI : PC : N-MP = 10 : 45 : 45), three carbonate rings are opened per lactam ring. The analytical data (percentage of C, H, N, O) are virtually fully consistent with the assumption that the copolymer contains three PC units
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Table 2. Characteristics of the thermal stability of the copolymers obtained*
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ Monomer ratio in starting mixture, mol % ³ T,m, 5 ³ T,m, 10 ³ T,m, 50 ³ To.d ³ Td ÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ TDI ³ PC ³ N-MP ³ oC ÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ 100 ³ 0 ³ 0 ³ 294 ³ 316 ³ 413 ³ 288 ³ 356 20 ³ 0 ³ 80 ³ 254 ³ 281 ³ 406 ³ 270 ³ 325 20 ³ 80 ³ 0 ³ 266 ³ 294 ³ 404 ³ 269 ³ 326 20 ³ 40 ³ 40 ³ 288 ³ 317 ³ 419 ³ 310 ³ 375 20 ³ 60 ³ 20 ³ 291 ³ 322 ³ 428 ³ 318 ³ 386 20 ³ 20 ³ 60 ³ 284 ³ 316 ³ 415 ³ 308 ³ 370 ÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄ * (T,m,5, T,m,10, T,m,50) Temperatures of 5, 10, and 15% weight loss, respectively; (To.d) temperature of degradation onset; and (Td) degradation temperature.
per N-MP unit, with the isocyanate acting as a link: H Found, wt % 6.0 Calculated, wt % 5.7
C 54.4 53.9
N O 7.5 32.1 (by difference) 7.3 33.1
Thus, our results in combination with quantumchemical calculations suggest that the isocyanate component in the ternary system initiates the chain propagation, and PAE is formed by the following mechanism. In the first step (initiation), nucleophilic addition of an anionic catalyst to the electrophilic carbon atom of the isocyanate group yields a chargetransfer complex. This interaction results in a shift of the electron density in the cumulated bond system and formation of anion A with the negative charge on N: .. 3 + N + C = N 76 N . . . C = N .
:9> :9> :9> :9> :9> 9
99
R
O
:9> QS <99 QS 9
9
O R
A
The isocyanate anion adds with the ring opening to propylene carbonate, forming an alkoxide anion: ! + N . . . C 7 N + H 2 C 7 CH7 CH 3 99
O
9
R
+
9
9
O
O
3
76 N . . . C 7 N 7 C 7 O 7 CH 7 CH27 O , 99
O
9
99
R
O
9
CH 3
or to the electrophilic carbon atom of lactam: + 3 CH 2 N . . . C 7 N + H2C 99
9
R
O
76
H2 C
C
99
9
R
N 7 CH 3
99
99
O
9
CH 3
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99
O
76
9
R
99
O
9
R
+
3
N . . .C 7 N 7 C 7 N . 99
9
O
R
99
O
9
R
The anions formed in the process can further react with isocyanate, PC, or N-MP to form an alternating structure which can be represented, to a first approximation, as follows: 7 [ C 3 N3 C 3 O3 CH3 CH 23 O7 [ 3 N3C3 (CH 2)3 3N 7 7 ]7 ]n 7 . n C /3
9 99 9 99 9 99 9 9 9 O R m CH O CH3 O R O 3
The termination in such polymerizing systems occurs, as a rule, by exchange reactions of propagating macroanions with compounds containing a labile hydrogen atom or by chain transfer to the monomer with the formation of terminal amino groups.
The feasibility of preparing ready items (including polymer films) by chemical forming is very important. Examination of the strength characteristics of these materials shows that it is possible to prepare materials with the preset properties. In particular, the highest breaking tensile stress of the films, exceeding 40 MPa, was attained at the molar ratio of the components in the starting mixture TDI : PC : N-MP ; 27 : 67 : 6 (Fig. 2).
O + 3 N . . . C 7 N 7 C 7 (CH 2 )37 N ; O
3 + + 3 N . . . C7 N + C 7 N
The differential thermal and thermogravimetric studies of the copolymers show that their thermal stability depends essentially on the monomer ratio in the starting mixture and is, on the whole, somewhat higher than that of TDI homopolymers and of TDI3PC and TDI3N-MP binary copolymers (Table 2).
C
99
O
:9>9> :
it can also add to another isocyanate molecule with the formation of a tertiary imide fragment:
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10, and 50% weight loss; degradation onset and degradation temperatures) and physicomechanical characteristics (e.g., breaking tensile stress of films) of the copolymers can be controlled by varying the monomer ratio in the starting mixture. PC
N-MP
REFERENCES
TDI Fig. 2. Composition!property diagram for the breaking tensile stress (MPa) of PC!TDI!N-MP copolymers (mole fraction scale).
CONCLUSIONS (1) Terpolymers of propylene carbonate, 2,4-toluene diisocyanate, and N-methylpyrrolidone were prepared in the presence of triethylamine. Although propylene carbonate and N-methylpyrrolidone do not homopolymerize under the action of tertiary amines, they readily copolymerize with isocyanates. (2) The 1H NMR and IR spectra, kinetic data, and elemental analysis suggest that the copolymerization mainly yields high-molecular-weight alternating block copolymers. (3) The thermal stability (e.g., temperatures of 5,
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