thermally reversible polymer linkages. ii. linear addition polymers

Thermally Reversible Polymer Linkages. II. Linear Addition Polymers

K. B. WAGENER* and 1. P. ENGLE

Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 3261 1-2046

SYNOPSIS

The stepwise addition polymerization reactions of bisazlactones [ bis (2-oxazolin-5-one) s] and a variety of 4,4’-bisphenols have been studied for the purpose of making thermally reversible linear polymers. Thus polymerization occurs a t or near room temperature, while depolymerization yielding the two monomer species occurs at elevated temperatures. The synthesis of oligomers in solution without the use of catalyst occurs for the reaction of bisazlactones with bisphenols containing an electron-withdrawing moiety between the two phenol groups of the bisphenol. These oligomers regenerate the bisphenol and bisazlactone monomers upon heating to 165-200°C for several hours under dry box conditions. In many cases, these reactions follow the same patterns of reactivity observed in model studies. This chemistry may be useful for forming degradable or recyclable polymers, where short- chain prepolymers, or macromonomers, endcapped with azlactone and phenol moieties could be used to form high molecular weight polymers that are thermoreversible. Such a reaction system might also be used for preventing reactions of bisphenols and/or bisazlactones a t low temperatures, with the desired reaction initiated by formation of the reactive species a t elevated temperatures. Envisioned uses in this case might be thermally triggered crosslinking or polymerization reactions, or temperature-controlled drug release. 0 1993 John Wiley & Sons, Inc. Keywords: reversible thermoreversible azlactone recyclable

INTRODUCTION

Prior research1” has shown that azlactones [ (2-oxazolin-5-one) s ] are suitable electrophiles for thermally-reversible ring-opening reactions with phenols containing electron-withdrawing moieties at the para position. Specifically, the pure 1 : 1 adducts IIIa-c of 2-isopropyl-4,4-dimethyl-2-oxazolin-5- one, I, and three para-substituted phenols, IIa-c, were found to be thermally reversible at temperatures of 300°C and below; thus I and IIa,b, or c were regenerated at the temperatures indicated in Figure 1.

These results suggested that a properly designed system of bisazlactone and bisphenol monomers

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 31,8654’75 (1993) 0 1993 John Wiley & Sons, Inc. CCC 0887-624X/93/040865-11

would produce the corresponding stepwise addition polymers without an external catalyst, which would depolymerize by heating below 300°C. The polymer must undergo depolymerization at temperatures below that of thermal degradation to undergo subsequent repolymerization without purification. Such a system would have potential use as a poly (ester amide ) that would regenerate the monomeric species upon the addition of heat; the monomers then could be used to reform the polymer (recycling), or alternatively used for some other application.

While the concept is clearly useful, only a few other examples of thermally controlled systems of covalent linkages have been described to synthesize recyclable linear polymers. These are the Diels- Alder [ 4+2] addition3 reaction, nitroso dimeriza- tioq4s5 and ionene formation.6 In the case of Diels- Alder chain extension, the reverse reaction (involv- ing the retro-Diels-Alder reaction) is for the most part unsuccessful due to significant degradation as

865

866 WAGENER AND ENGLE

111

&'&"Gx 0 a b

c 111 a: X = NO,

I I I1 a: X = NO,

b: X = CF, It c : X = F

Temp., O C of Reversibility

110-115 235-250 90- 115

I I A

a consequence of the high temperature and pro- longed heating time required for reversal. The nitroso chemistry fares somewhat better, with polymers forming nitroso-functional monomers between 80 and 200°C (depending on functionality) and yielding linear polymer at room temperature. How- ever, these polymers can only be recycled one or two times before thermal degradation becomes a problem in reforming p01ymer.~

Urethane formation has also been studied as a means of thermally reversible chain extension.' In this work, a series of aromatic model urethanes were found to form the corresponding aromatic isocyan- ates and phenols in the pure state. However, linear polymerizations based on this chemistry would rely on the use of aromatic diisocyanates, which have shown a tendency to undergo side reactions in the pastg; thus, this reaction system may not be appli- cable to linear polymerization.

The problems that are inherent to thermoreversible linear polymerizations are difficult to overcome. The reactions described in the literature, as well as the one described herein, rely on stepwise addition reactions. Therefore, yield for the forward reaction must be at least 99% to obtain high polymer." How- ever, thermoreversibility is gained as a result of a large decrease in the equilibrium constant with in- creasing temperature; if equilibrium does not shift significantly under these conditions then depolymerization will not take place at low enough temperatures to avoid thermal degradation of reaction species. Subsequently, due to the dependence of the forward reaction upon the ratio of functionalities, degradation will lead in most cases to failure to re- polymerize after heating the reaction mixture. Thus,

the original purpose of the thermoreversible linear polymerization, to make recyclable polymers, may not be realized.

In the literature, examples of similar reactions used to form thermoreversible crosslinks are tradi- tionally much more successful. Presumably this is because the strict requirements of high conversion for stepwise polymerizations is not of consequence in crosslinking; only a few percent crosslinking are sufficient to produce a gel." Nitroso dimerization 4,5

and the Diels-Alder reaction 11-36 have also been used in this manner. Nitroso dimerization appears to be a good reaction pathway to thermoreversible networks. The Diels-Alder reaction, while much more widely studied, has found mixed success: the limited extent of the reverse reaction at elevated temperatures most often causes these networks to remain as such. However, certain reaction systems have been successfully employed to form networks with good properties that subsequently form soluble or flowable polymers at elevated temperature; in some cases these systems are reported to undergo cycling of the network more than one time.'3-'5336

Cyclic anhydride ring opening reactions with al- cohols to form ester as well as ester ex- change reactions3' have also been used to form thermally reversible covalent crosslinks. However, these polymers undergo a large amount of degradation after only one heating-recrosslinking cycle, as evi- denced by a substantial loss of physical properties.

Ionene network formation is well known to be thermoreversible; several references exist on studies in this area. For interested readers, a book reference is cited as a starting point for further study?' Finally, azlactone-phenol adduct formation has been studied in this laboratory as a means of forming thermoreversible network^.^^.^' In this work, an azlactone functional polymer was crosslinked with various bisphenols. Only one of the bisphenol structures was found to give thermoreversibility; however, this specific reaction was observed to cycle many times under inert atmosphere in the presence of a solvent.

Low yielding thermoreversible linear polymerization systems are still worthy of study as a means of forming latent or protected functionalities that would be triggered to form a target functionality at a certain temperature. For example, such a system might find application in latent polymerization or curing systems, where the reactive functionality is masked until the desired reaction temperature is reached; controlled drug release is another possible use for such a system (depending, of course, on the kinetics and byproducts of the reverse reaction at body temperature).

THERMALLY REVERSIBLE POLYMER LINKAGES. I1 867

cool ! This article describes the linear step propagation,

addition type formation of oligomers using bisazlactones and bisphenols in solution at ambient temperature. Furthermore, it will be shown that these oligomers undergo the reverse reaction to reform the monomers between 165 and 200°C. The oligomer of bisazlactone and one of the bisphenols was reformed at ambient temperature after one heating cycle. Some thermal degradation occurred, principally of the bisazlactone monomer, such that in most cases reformation of the polymer after heating (cycling) could not be accomplished. However, the nature of the forward and reverse reactions exhibited many of the same characteristics that were found during examination of monofunctional model studies. In addition, bisphenols are used both as monomers and crosslinkers in industrial applica- tions." Thus, the present reaction system could be used to thermally "trigger" the reaction of bisphenols when ambient temperature reactivity is unde- sirable.

A reaction scheme, outlined in Figure 2, was chosen such that the effect of various substituents on phenol monomers could be studied. Bisazlactone IV, 1,4-tetramethylenebis- (2-oxazolin-5-one), is the structure upon which the model compound I was based. Bisphenols Va-d were chosen for the purpose of comparing the effects of electron withdrawing groups to bisphenol A (Va, 4,4'-isopropylidene diphenol), which contains no electron-withdrawing capability.

A

EXPERIMENTAL

R

+!-k

Reagents were obtained from the Aldrich Chemical Company and used without further purification ex- cept where noted. Solvents, if not distilled, were generally HPLC grade. All model study and polymerization solutions were manipulated in a Vacuum Atmospheres Dri Lab HE-43-4 model dry box under Argon atmosphere. The atmosphere was maintained at I 10 ppm H 2 0 and O2 as shown by chemical test- ing (diethylzinc) .

NMR spectra were recorded on a Varian XL-200 (200 MHz) Fourier transform instrument. Shifts are recorded as ppm ( 6 ) downfield from tetramethylsi- lane ( TMS ) . Thermogravimetric analyses were performed on a Perkin-Elmer TGA 7 equipped with TAC7 data analysis system and PE 7500 Profes- sional Computer. Temperature ramp was 10"C/min from 50 to 400°C under N2 purge. Dilute solution viscometry was performed using an Ace brand vis-

Compound

Va : 4.4-isopropylidenediphenol. 1 +p 1 Vb : 4,4-sulfonyldiphenoI

1 3 1 Vc : 4.4-hexafluoroisopropylidenediphenol.

+;-(CF2),-:-t Vd : 4.4-( 1,4-disulfonylperfluorobutane)diphenol

Iv: 2,2'-tetramethylenebis(4,4-dimethyl-2-0~-

Figure 2. Step propagation, addition type polymerization scheme of bisazlactone IV with various bisphenols V.

cometer, size 0 capillary, and constant temperature water bath.

Synthesis of Monomers

Bisazlactone IV was synthesized using the procedure of Cleaver and Pratt.43 Modifications were to run the amino acid acylation step at 25-30°C instead of O"C, and to recrystallize the final product bisazlactone from toluene rather than benzene. IV was characterized by 'H-NMR, elemental analysis, and melting point data. It was stored in the dry box to prevent any uptake of water, which interferes as a nucleophile in the ring opening reaction of azlactones.

Bisphenols Va, b, and c were obtained from Ald- rich and were dried by applying a vacuum overnight in the drybox antechamber; storage inside the dry box prevented adsorption of water.

Bisphenol Vd, 4,4 '-disulfonylperfluorobutane) diphenol, was synthesized from the corresponding fluorophenyl adduct, 1,4-bis (4-flUOrO- phenylsulfonyl ) perfluorobutane, which was obtained from E. I. DuPont DeNemours and Company.44 The synthetic scheme is outlined in Figure 3. This compound has been previously re- p ~ r t e d . ~ ' , ~ '


I . MeONa 1 MeOH I reflux 2. ppt. with H,O; CH,CI, recryst.

1 . BBr3 / CH,CI, 1 reflux 2. pH 211. then 52

3. MeOH/H,O recryst. I V d

Figure 3. ( 1,4-disulfonylperfluorobutane) diphenol, Vd.

Reaction strategy for the synthesis of 4,4‘-

Model Study Reaction Solutions

Model studies using V with 2 equiv of model azlactone I were analyzed by HPLC on a Waters Model 590 instrument equipped with a Kratos Spectroflow 757 UV detector at 215 nm, Perkin-Elmer LC-25 refractive index detector, and a Waters Nova-Pak C18 reverse phase column. The mobile phase was 60% aq. CH3CN, at a flow rate of 0.5 mL/min. Peak areas were determined by cutting and weighing. A typical solution was mixed in the dry box as follows:

Reagent Amount

I v c CH3CN

0.115 g (7.42 X lop4 mol) 0.125 g (3.72 X mol) to 0.70 mL

Reactions were run at 30 k 1°C in the dry box for the specified period of time, then heated at the specified temperature for 5-6 h. The solutions were made as concentrated as possible. For some reactions, AlC13 (anh) or 1,8-diazabicyclo [ 5.4.01 undec- 7-ene (“DBU,” fractionated in uacuo) were added in catalytic amounts to the reaction solutions. Heating of the solutions was carried out in the dry box either in a capped vial or in a heavy walled glass vessel equipped with high pressure stopcock.

Equilibration studies were also run for similar reaction solutions. In these experiments, uncatalyzed solutions of V and I were placed in small heavy- walled glass vessels equipped with high-pressure stopcocks. The vessels were allowed to remain at a given temperature inside the dry box until equilibrium was determined by HPLC. In general, equilibrium was reached after 1-2 weeks at ambient tem-

perature and within hours a t temperatures above 100°C.

Polymerization Reaction Solutions

Polymerizations of IV and V were carried out by stirring in the dry box, principally at 26-27°C; CH3CN and CH3CN : THF (4 : 1 to 8 : 1 ) solutions were used. A typical solution was mixed as follows:

Reagent Amount

IV vc CH3CN 0.40 mL THF 0.05 mL Total volume: 0.55 mL

In every case, solutions were made as concentrated as possible. For some reactions, AlC13 (anh) or 1,8-diazabicyclo [ 5.4.01 undec-7-ene ( “DBU,”distilled) were added in specified amounts to the reaction solutions.

Analysis of polymerization solutions of V and IV was by size-exclusion chromatography (SEC) on a Waters Model 6000A Liquid Chromatograph equipped with differential refractometer and Perkin- Elmer Model LC-75 UV detector at 215 nm. The flow rate was 1.0 mL/min with THF as the mobile phase. Calculations for calibration and molecular weight determination were performed using a Zenith Data Systems PC.

SEC was used to monitor the progress of the polymerization reactions as well as to determine molecular weights at apparent equilibrium. UV detection of bisphenol (V) species at the specified wave- length was used to determine approximate molecular weights. A three-column system was used which consisted of Phenogel 100 and 500 A columns and a TSK G3000 column. Column calibration was carried out using both polystyrene standards (molecular weights 1574-7820 g/mol) and the bisphenol monomers (V) themselves. Using this method, the presence of free bisphenol monomer in the solution could be unequivocally determined as well as the approximate molecular weight of the oligomers. Re- fractive index detection was used to confirm the presence of IV.

Apparent equilibrium of the polymerization solutions was determined by monitoring the reaction by SEC over time. Since dilution of the reaction solutions effectively quenched the forward reaction, aliquots over several days could be collected and coinjected as well as separately injected to determine that the reaction was not changing. A typical reac-

0.100 g (3.57 x 10-~ mol) 0.121 g (3.60 X lop4 mol)


tion took 7-14 days to reach apparent equilibrium. Monitoring was then continued for several days (at least the same amount of time as was observed to reach the apparent equilibrium).

The oligomer formed from Vc and IV was also characterized by dilute solution viscometry and 'H-NMR in THF-d8, using standard laboratory practices in each case.

Depolymerization reactions of the oligomers were also examined in solution. Heating of the solutions was carried out in a similar manner to the model studies described above.

Cycling reactions to determine whether polymer would reform after depolymerization via heating were carried out by cooling the depolymerization solutions and allowing sufficient time for reequilibration. SEC was used as described above to determine the products of reequilibration.

RESULTS AND DISCUSSION

The uncatalyzed forward reaction of IV and V followed many of the trends observed previously for the corresponding monofunctional model system, ',' in which the reaction of the phenol nucleophile with the azlactone ring is dependent not on the nucleo- philic strength of the phenol, but rather on the abil- ity of the phenol to protonate the azlactone ring. Thus, it was expected that IV would form relatively high molecular weight polymers with Vb, c, and d, while the reaction of IV and Va should result in lower molecular weight at equilibrium.

Data for the apparent equilibrium molecular weights of the oligomers can be seen in Table I. It should be noted that the values for M , and polydis- persity are probably far from their true values, because the various oligomers are actually separated from each other by the three SEC columns. Since the calibration was performed according to M,, these values should be used as the basis for com- parison of the samples.

At 26-27"C, monomer Va does not undergo any observable reaction with IV after 3-4 weeks; however, monomers Vb-Vd do react. This was the expected result based on the reaction mechanism theory.

The general low percent yield for the reaction was also predicted from earlier model studies.',' It has already been s h o ~ n ~ ' , ~ ' that the azlactone-phe- no1 reaction can be successfully used to form crosslinked networks; this is due presumably to the less stringent yield requirements for the formation of a crosslinked network: only 1-2% reaction is required

Table I. of IV with V, as Determined by SEC: Reaction Solutions Were in CH,CN; Values Represent Apparent Equilibrium for the Solutions

Molecular Weight Data for Polymerizations

M J M , (SEC) M,/M, (SEC) Bisphenol at 27°C after 5-6 h/200°C

monomer monomer Va vb 2147/1415 monomer, dimer vc 6158/3158 monomer

monomer Vd 1853/1341

to gel linear polymers, while 2 99% reaction is required to form a stepwise addition polymer of ap- preciable degree of polymerization."

An additional problem encountered in carrying out these reactions was that the solubility of the two monomer species IV and V, as well as the corresponding oligomers were sufficiently different to create problems in forming concentrated solutions. Although a mixture of THF and CH3CN was found to dissolve both of the monomers, the oligomers often precipitated during the forward reaction, and this may have limited the extent of reaction as well. Several solvent systems were tried, and the THF/ CHBCN mixture was found to be the best for most of the reactions.

The reaction of IV and Vc showed gave the highest degree of polymerization in the uncatalyzed forward reaction. The M , of 6200 corresponds to a weight average degree of polymerization (X,) of about 10, as the molecular weight of the repeat unit is 616. This was a promising result because fluori- nated model adducts 1IJ.b and IIIc exhibited reversibility at the lowest temperatures observed for the model study (see Fig. 1 ) . Therefore, it was expected that the oligomer formed from Vc would give both the highest molecular weight a t ambient temperature as well as the highest degree of reversibility at temperatures below that at which thermal degradation of the monomers occurs.

A similar oligomer synthesized from IV and Vc (M,/M,, = 5600/2100 by SEC) was characterized further. Dilute solution viscosity was carried out in THF to give an [ 771 = 0.06 dL/g at 28.45 k 0.03'C. NMR spectra of the polymer were also run; 13C- NMR did not show several of the quaternary car- bons, but 'H-NMR (shown in Fig. 4) was successful in elucidating most of the protons in the repeat unit. Protons corresponding to unreacted Vc (labelled f ' and g') can also be seen. Although monomeric Vc exists in the polymer mixture, it is a very minor component according to the SEC trace. If it is as-


I C Hydroiyzcd endgroups

' H NMR in dd-TIfF: I

Figure 4. 'H-NMR in THF-d, for the reaction of Vc and IV, after reaching equilibrium at 26-27°C. The solvent was removed upon completion of the reaction and the polymer exposed to air for several weeks to hydrolyze the unreacted azlactone moieties.

sumed that there is no Vc, then the number-average degree of polymerization, X,, as determined by an- alyzing integrations of the reacted versus unreacted phenol protons, is calculated to be 5.8. This corresponds to M , = 3600 and, assuming the polydisper- sity index of 2.7 from the SEC trace is correct, M , = 9600. Since for stepwise polymerizations X , = 1/1-p, the extent of reaction as determined by 'H NMR is 82%.

Experiments with slightly elevated temperatures were conducted to determine the effect of solution temperature on molecular weight. An oligomer of IV and Vc was synthesized using DBU (1,8-diazabicyclo [ 5.4.01 undec-7-ene) as a catalyst. The reaction mixture was allowed to stand a t 27°C for 2 weeks, followed by heating to 43°C for ca. 24 h. The molecular weight decreased upon heating from M , = 1500 g/mol to M , = 1100 g/mol via SEC. This was the expected result based on the exothermic nature of the forward reaction in the monofunctional model studies.','

The uncatalyzed reverse reaction was successful in every case where oligomer was formed at ambient temperature. At 200"C, all the oligomers reversed completely to form IV and V, with the exception of monomer Vb: in this case, dimer and trimer were also apparent in the reaction solution after heating. The lowest temperatures where the reverse reaction was found to be complete were 180-200°C when em- ploying Vb and Vc, and 165°C when Vd was used. SEC traces of the uncatalyzed forward and reverse

reactions of IV with Vb, c, and d are shown in Fig- ure 5.

Attempts were made to cycle these reactions, i.e., reform oligomer after heating the solution sufficiently to obtain monomers. Cycling was accomplished by allowing reequilibration of the reversed reactions a t ambient temperature. In only one case was the reaction successful in reforming oligomer: the SEC traces of the reaction of IV and Vd before heating, directly after heating, and 8 weeks after heating are shown in Figure 6. Vd was the only bisphenol observed to regenerate oligomer with IV after

Figure 5. SEC traces for the reaction of Vb, c, and d with IV: (1) a t equilibrium, after 2 weeks at 26-27"C, ( 2 ) after subsequent heating for 6 h a t 200°C.


81onv6mnl

Figure 6. SEC traces for the reaction of Vd and IV: (1) at equilibrium, 26-27"C, M,/M,, = 2147/1415; (2) after subsequent heating for 6 h at 165°C; (3) Same solution, 8 weeks after heating (22 f 3°C); M,/M,, = 1496/ 1108.

the reverse reaction had taken place. It was thought that this was due to the lower temperature used in accomplishing the reverse reaction ( 165"C), and thus a significant amount of degradation of the monomer ( s ) was avoided.

To determine whether monomer degradation was responsible for the failure of the system to undergo cycling, thermal analysis of the monomers was performed. Bisazlactone IV was subjected to thermogravimetric analysis under N2 atmosphere. The sample exhibited a few percent weight loss between 180 and 200°C; a t 200°C and above a much greater degree of weight loss was seen. IV was then subjected to a temperature ramp in a sealed vial in acetonitrile solution. SEC traces of the heated solution con- firmed minor degradation between 180 and 200°C and major degradation above 200°C. Th' is was an initially unexpected result based on prior studies, as model azlactone I was found to be stable up to 300°C under inert atmosphere. A few percent loss of monomer during heating would be sufficient to cause failure of the subsequent stepwise (forward) reaction.

Thermogravimetric analysis of the bisphenol monomers showed that weight loss of Vc begins a t ca. 180°C with major weight loss occurring at ca. 210°C; Vb and Vd, however, do not exhibit weight loss before ca. 300°C. Thus, it is the degradation of the bisazlactone IV that is probably responsible for the failure of the system in most cases to undergo polymerization after one heating cycle, and it is un- doubtedly the lower temperature of the reverse reaction of IV and Vd that allows this oligomer to reform.

To produce a oligomer that cycles, one of three approaches might be taken. A bisazlactone of greater

thermal stability might be considered, or the use of a bisphenol that allows reversibility a t a lower temperature. Alternatively, the use of a catalyst may aid in the reverse reaction by lowering the activation energy for the reaction and therefore affecting reversibility at a lower temperature.

The use of a more thermally stable bisazlactone, such as one with a phenyl group between the azlactone moieties instead of a C4 chain, would cause greater solubility problems. Since solubility of both the monomers and the resulting polymers have pre- sented a problem in facilitating these reactions in concentrated solution, this was thought to be the path that would have the lowest chance of success.

Catalysts were therefore examined in this system for the purpose of affecting the reverse reaction at lower temperatures, or perhaps in less time than without catalyst. In either case, the amount of degradation in the monomers should be decreased un- less side reactions and/or degradation reactions are also catalyzed. Also of interest is the effect of catalysts on the oligomer molecular weight for the forward reaction. If molecular weight measurements made previously were not in fact measured at the true equilibrium of the reaction, then a catalyst would appear to give higher molecular weights in the corresponding catalyzed reactions. AlC13 a Lewis acid, and DBU ( 1,8-diazabicyclo- [ 5.4.01 undec-7- ene) , a Lewis base, were used in these experiments. Table I1 shows the results of catalyzed reactions analogous to those in Table I.

Although the catalysts' effect in the model studies of I and I1 was both to enhance reaction rate and to reverse the order of reactivity over 24 h of the phenol compounds I11s2 (i.e., phenols I1 with electron-donating substituents were observed to react to a further extent in 24 h with I than those with electron-withdrawing substituents in the presence

Table 11. Polymerizations of IV with V in the Presence of the Indicated Catalysts: Values Represent Apparent Equilibrium for the Solutions

Molecular Weight Data for the

M,/M, (SEC) Bisphenol Catalyst/Solvent at 27°C

Va AlC13/CH&N monomer Va DBU/CHSCN 1224/1111 vb AlCl3/CH3CN, THF Mu = 1046 vb DBU/CH,CN, THF 759/601 vc AlC13/CH&N, THF M , = 1150 vc DBU/CH&N, THF 4361/2280 Vd DBU/CH&N, THF monomer


I K

of DBU and AlC13) , the results in the corresponding polymerizations were not entirely consistent with these findings. General enhancement of reaction rate was observed the solutions of IV and V at ambient temperature had apparently reached equilibrium within 5 min or less of adding catalyst, since no further reaction was observed after 7 days monitoring time. However, the observation that the molecular weights at equilibrium in the presence of the catalysts were in fact lower than those observed in the uncatalyzed reactions seems to suggest that both DBU and AlC13 affect the yield of the reaction in a deleterious sense.

Only monomer Va showed enhancement of reactivity in the presence of DBU to give M , = 1224 with IV. This indicates that the reaction of IV with Va is extremely slow in the absence of DBU, and thus equilibrium measurement of molecular weight was not observed in the absence of catalyst. Upon heating to 200°C, the molecular weight of the polymer of IV and Va with DBU increased to M , = 2454 along with some degradation. Thus bisphenols without electron-donating moieties were not expected to facilitate the reverse reaction below degradation temperatures, which is consistent with earlier findings in model studies with III.1,2

When the other reaction solutions with AlC13 or DBU were heated, no apparent reverse reaction occurred at temperatures below 200°C. At 2 2OO"C, massive degradation occurred; some reverse reaction was also seen, but degradation products were the major components of the reaction solutions after 6 h at 200°C in the presence of either catalyst.

To find out whether differences between the polymerization system and the model studies stem from the use of bisphenols V versus the monofunctional phenols 11, catalyzed reactions were studied for Va-c with 2 equiv of the model azlactone I both with and without catalytic amounts of A1C13 or DBU.

Figure 7. phenols V with model azlactone I.

Reaction scheme for the model study of bis-

The reaction scheme is shown in Figure 7. Quali- tative data only were gathered in this case. A series of reactions were run for Va-c, side by side for each species with 21, with and without AlC13 or DBU; the relative concentrations of unsubstituted, monosubstituted, and disubstituted V were qualitatively compared after 18-24 h in the dry box at 30 k 1°C. Although the concentrations and reaction times varied between series, they were kept constant within each series.

After each forward reaction was analyzed, the reaction solutions were heated for 5-6 h, then analyzed again. As was expected, it was found that < 200°C was generally ineffective in producing significant reversibility in the case of the bisphenols used in the study, so the reverse reaction experiments were typically performed at 200°C. If no reversibility was observed at 2OO"C, the temperature was increased to 300°C. Sample HPLC traces are shown in Fig- ure 8.

It can be seen that in the uncatalyzed reactions, Vc gave the highest degree of substitution at 30°C and formed principally Vc and I at 2OO"C, as expected. Fluorine enhances both the uncatalyzed forward and reverse reactions, which was the case for the reactions of model compounds I and I1 as well as polymerizations of IV and V. Also, Va notably does not undergo significant forward reaction after 24 h at 3OoC, which again is consistent with the reactions of I with 11'~2 and IV with V.

In the presence of DBU, the same reactivity trends are seen as for the polymerizations of IV and V: the reaction of Va gives the highest degree of substitution after 18 h at 30°C. The extent of reaction of Vc with I is actually lesser than in the uncatalyzed reactions, and the reactivity of Vb with I is apparently unchanged. Thus, DBU would not be predicted to give higher yields in the forward reaction of IV with bisphenols Vb-d.

The reverse reaction in the presence of DBU could not be attained in any of these experiments when examined at 200°C. Extensive degradation occurred in all samples heated above 200°C. The failure to observe the reverse reaction may be due to a kinetically fast reaction in the presence of DBU, such that the time required to cool the solutions to sample them after heating is sufficient to allow reequilibration at ambient temperature.

AlC13 catalysis exerted a similar effect upon this model system relative to the corresponding uncatalyzed reactions. The reaction of Va could not be examined by HPLC, as precipitation occurred within minutes after mixing at 30°C (this could be due to


Figure 8. HPLC traces for the indicated reactions in CH3CN. The first eluted peak in each trace is due to V, followed by monosubstituted V, then disubstituted V.

precipitation of disubstituted Va) . Bisphenol Vc reacted to a lesser extent with I than in the uncatalyzed solution, and no notable change of reactivity was observed for Vb with I. AlC13 catalysis did, however, allow for reversibility in the reactions of both Vb and c with I at 2OO0C, although some degradation could be seen in the HPLC traces. In this respect, AIC13 is superior to DBU as a catalyst in this system. Here the model differs from the polymerization reactions of IV and V in that AlC13 did not afford the reverse reaction in the case of the polymerizations.

It would seem, as a result of these model studies, that the best system for the polymerizations of IV and V is the concentrated, uncatalyzed solution of reactants. Without catalysts, the reaction solutions must be allowed to equilibrate for the necessary period of time such as to obtain the highest molecular weight possible. However, the presence of catalysts does not improve the ultimate yield of the reaction for bisphenols containing electron-withdrawing moieties (Vb-d) and in addition causes a higher amount of degradation upon heating.

CONCLUSIONS

The bisazlactone-bisphenol reaction system has been shown to produce thermoreversible oligomers. Oligomers are formed from uncatalyzed solutions of bisazlactone IV and bisphenols Vb-d at room temperature; bisphenol Vc exhibits the highest reactivity under these conditions. Catalysts are not useful in this reaction system with the exception of the reaction of IV with Va, which does not exhibit thermoreversibility at temperatures below that a t which thermal degradation of the bisazlactone monomer occurs. The highest molecular weights are achieved at lower temperatures. All of these findings are best predicted by model studies of V with 2 equivs of model azlactone I. The only difference exhibited by the polymerization reactions relative to the model studies of I and I1 is in the effect of DBU and AlC13 on the behavior of the systems.

Depolymerization of the oligomers in the uncatalyzed solutions occurs a t 200°C for reactions where monomers V contain electron-withdrawing moieties, but cycling (subsequent formation of oligomers ) has only been seen for the uncatalyzed reaction of IV and Vd. This is probably due to the fact that the reverse reaction in this particular case is facilitated at temperatures below that of the thermal degradation temperature of the bisazlactone monomer IV.


Catalysis does not cause reversibility to occur a t lower temperatures, and in addition it causes large amounts of degradation in the solutions upon heating. Reactivity trends for depolymerization were most satisfactorily predicted by the model studies of V and 21.

Although these oligomers are not useful as recyclable polymers, this work shows that similar systems may have potential as useful materials. The crosslinking reaction based on this chemistry has been shown to give a thermoreversible covalent network capable of undergoing several cycles under inert a t m o ~ p h e r e . ~ ~ ~ ~ ~ The observed formation of oligomers in this set of experiments also suggests that a system of bisazlactone and bisphenol prepolymers (or macromonomers) , similar to the work of Kennedy and C a r l ~ o n , ~ could be used to give high actual molecular weights despite a low degree of polymerization.

In addition, since degradation of the bisazlactone monomer appears to be the major side reaction at elevated temperatures, this reaction could conceiv- ably be used as a latent cure or polymerization system for bisphenols: addition of the preformed oligomer to some other reaction system at ambient temperature would result in the release of bisphenol functionalities between 165 and 200°C, depending on the bisphenol structure. Toward this end, other bisphenol compounds might be worth examining.

The authors thank the 3M Company for their financial support of this research and in particular Drs. Heilmann, Jones, Krepski, and Rasmussen at 3M for their helpful advice.

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44. Thanks to Dr. A. Feiring and E. Wonchoba at DuPont Central Research for the starting material and help with synthetic strategy.

Received November 15, 1991 Accepted August 10, 1992

thermally reversible polymer linkages. ii. linear addition polymers

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