Synthesis of Aromatic Polyisophthalamides by in Situ Silylation of Aromatic Diamines † Angel E. Lozano,* Javier de Abajo, and Jose ´ G. de la Campa Instituto de Ciencia y Tecnologı ´a de Polı ´meros, C.S.I.C. Juan de la Cierva 3, 28006 Madrid, Spain Received October 21, 1996 Revised Manuscript Received January 29, 1997 Introduction Wholly aromatic polyamides are thermally stable polymers, with high molecular rigidity and high transi- tion temperatures. Therefore, these polymers cannot be prepared in the molten state and are synthesized by solution condensation methods in polar organic solvents, usually from diamines and very reactive species such as diacyl chlorides. Aromatic diamines are less basic and less nucleophilic than aliphatic diamines, and therefore, their reactivities are very low in some cases, mainly when there are electron-withdrawing groups connected to the phenyl rings. In 1983, Bowser and co-workers showed that aliphatic amides could be prepared in excellent yields from silylated amines and acid chlorides. 1 In the field of polymer chemistry, the use of silylated amines became important with the synthesis of aro- matic polyamic acids and polyimides by Korshak 2 and the synthesis of aromatic polyamides and polyimides by Imai and co-workers. 3-6 In spite of the advantages of this reaction, 3 the main drawback of using silylated diamines as condensation monomers is their great ability to hydrolize, which hinders the isolation and purification of these mono- mers. Therefore, we have considered the formation of silylated diamines in situ by adding trimethylchlorosi- lane (TMSCl) to the diamine solutions that, after the addition of a diacid chloride, give the polyamides. This method has been used previously by Becker and Schmidt, 7 who synthesized poly(amic alkyl esters) as intermediates of rodlike polyimides, by reacting 2,5-bis- (ethoxycarbonyl)terephthaloyl chloride with diamines in the presence of TMSCl. Kaneda et al. had earlier reported 8 the synthesis of polyterephthalamides in the presence of inorganic salts and TMSCl. These authors observed an increase of the viscosity when adding TMSCl, but they did not consider the formation of silylated diamines as reponsible for that positive effect. Experimental Section Materials. Trimethylchlorosilane (TMSCl) was twice dis- tilled at normal pressure under nitrogen. Isophthaloyl chloride (IPC) was recrystallized from hexane and finally distilled at reduced pressure. p-Phenylene diamine (PPD) and m-phe- nylene diamine (MPD) were purified by vacuum distillation; 4,4′-oxydianiline (DDE), 4,4′-methylenedianiline (DDM), and 4,4′-sulfonyldianiline (DDS) were sublimed twice before use. 4,4′-Hexafluoroisopropylidenedianiline (D6F) was kindly sup- plied by Hoechst Celanese and was used as received. N,N- dimethylacetamide (DMAc) was vacuum distilled twice, the first time over phosphorus pentoxide and the second time over calcium hydride. Subsequently, it was stored in a dark glass bottle over molecular sieves (4 Å). Polymer Syntheses. A flask equipped with a mechanical stirrer and in a nitrogen atmosphere was flame dried and charged with 15 mL of DMAc and 0.01 mol of diamine. The mixture was stirred at room temperature until all solids had dissolved. Then the solution was cooled to -5 °C and the required amount of TMSCl was slowly injected. The temper- ature was raised to room temperature, and the solution was stirred for 15 min to assure the formation of the silylated diamine. After this time, the solution was once more cooled to -5 °C, and 0.01 mol of IPC was rapidly added using a funnel which was subsequently rinsed with 5 mL of DMAc. The reaction mixture was then stirred for 1 h at that temperature, after which it was heated to room temperature and left for a further 3 h. The resulting polymer solution was precipitated into 500 mL of water, washed with hot water and hot methanol, and extracted in a Sohxlet extractor with acetone to remove solvent and oligomers. The polymer from D6F required extraction in methanol, since it manifested swelling in acetone. All of the polymers were dried overnight under vacuum at 120 °C. Yields over 95% were obtained. Measurements. The inherent viscosities were measured at 25 °C with an Ubbelohde suspended level viscometer using DMAc as solvent for all the polymers. The polymer concentra- tion was 0.5 g/dL Results and Discussion The synthesis of aromatic polyamides was carried out by reacting isophthaloyl chloride and silylated diamines prepared in situ, as shown in Scheme 1. The polymer- ization was performed by dissolving the diamine in DMAc and adding different amounts of TMSCl into the solution. Finally, the required stoichiometric amount of IPC was poured into the reaction mixture. To investigate the effect of the different reaction conditions on the ability to obtain high molecular weight polymers (the higher the reaction rate, the lower the possibility of potential side reactions and the higher the molecular weight), a set of reactions was performed by varying the amount of TMSCl, the diamine, and the reaction temperature. Inherent viscosity values were taken as a measure of the molecular weight of the polyamides obtained. Although this relationship can be rigorously established only when the viscosimetric equation is known, we considered that it could be used to compare the results for the same type of polymers. With the purpose of optimizing the reaction, six different diamines were used, DDE and PPD, with † This paper was presented, in part, at the New Orleans Meeting of the American Chemical Society, March 1996. Scheme 1. Synthesis of Polyisophthalamides from Silylated Diamines Prepared in Situ. 2507 Macromolecules 1997, 30, 2507-2508 S0024-9297(96)01543-4 CCC: $14.00 © 1997 American Chemical Society