Synthesis and Characterization of Easily Processable Polyaramides Containing Pendent Diamantan Moiety in the Main Chain

Three new polyamides were synthesized by direct polycondensation of 1-6 bis(carboxy methyl diamantan with three different aromatic amines in Nmethyl 2-pyrrolidine containing lithium chloride using triphenyl phosphate and pyridine as condensing agent. 1-6 bis(carboxy methyl)diamantan was prepared from 1-6 dibromo diamantan and structure was conformed by H and C NNR spectroscopy. A comparison of the effect for introducing bulky side group in the diamine contribution was evaluated. This polyamide had inherent viscosity of 0.57-1.18dL/g. It was found that all polyamides were soluble in polar aprotic solvents. Thermal stability of polymer was evaluated by TGA analysis. Molecular weights of all polyamides were determined by gel permeation chromatography. These have been found to be thermally stable up to 400 C with 1320% weight loss under nitrogen atmosphere.


Introduction
Aromatic polyamides are a high temperature resistance engineering class polymer which have good thermal stability, good chemical resistance and good mechanical property.Formation of aromatic amides generally carried out by solution polycondensation.Some methods use aromatic diamine that reacts with di acid chlorides 1-6 .and other methods use high temperature, where diamines react with aromatic di-acids or di-acid chlorides as Yamazaki's reaction 7 .Some aromatic polyamide contains intermolecular hydrogen bonding and crystallizes easily.Due to these factors, some polyamides cannot dissolve in most organic solvents and difficult to process.][10][11][12][13][14][15][16][17][18] These study include (i) introducing soft segment CH 2 in the main chain of polymer (ii) introducing bulky group in the main chain of polyamide to break the symmetry of the polymer which suppress crystallization (iii) breaking symmetry and structural regularity makes crystallization impossible (iv) forming a three dimensional structure without sharing the same surface and (v) destroying the hydrogen bonding by substituting hydrogen at amide group.Pali et.al 19 used low temperature poly condensation of pendent bulky group containing diamines with di acid chlorides and found slight increase in solubility with aromatic poly amide chain stiffness.Diamantan is a most general cyclo-aliphatic cage hydrocarbon which contains an extended cage structure made up adamantane structure compound and first time used in polymer in the form of 1-6 diethynyl diamantan monomers which on addition polymerization forms clear thermoset film and have no much use.Charn and Wang 20 have prepared aromatic polyamides using Yamazaki reaction and find that solubility increases with increasing extended cage effect in the main chain.Hence structural modification plays an important on the solubility of aromatic polyamides.In this communication, we present the synthesis of 1-6 bis(carboxy methyl) diamantan along with three different poly amides based in different aromatic diamines.Also, we have attempted to introduce not only cage type structure but also bulky pendent grouping to the aromatic polyamide main chain.We have synthesized three polyamides based on high temperature poly condensation reaction of 4-4' (9-fluorenylidene) diamines, 4-4'(hexa fluoro-isopropylidene) dianiline, 4-4'diamines benzophenone with 1-6 bis(carboxy methyl) diamantane using Yamazaki reaction and studied to understand the influence of thermal property and solubility with the change in structure diamines and di-acids.

Experimental
The diamines used for polyamide synthesis are shown in the Table 1.All amines were further purified by vacuum sublimation and all the solvents used were further purified by vacuum distillation.All the chemicals were procured from Alderich chemical company.Diamantan was prepared by three step synthesis from norbornadiene 21 .1-6,Dibromodiamantan was prepared by the direct bromination of diamantan at ambient temperature 22 .
Table 1  Polymer synthesis 2m mole(0.608g) of 1-6,Bis(carboxyl methyl) diamantan ,4mmole of triphenyl phosphate 15mL of pyridine ,20 mL of N-methyl 2-pyrollidone(NMP) and 0.5g of lithium chloride and 2.5mmole of 4-4'-(9-fluorenylidene) diamines was heated under constant stirring at 130 0 C for ten hours in a reflux assembly.The resultant solution was poured to one liter capacity beaker containing 900 mL of methanol and finally the fibrous white precipitate was washed through methanol and water.The yield was almost quantitative and inherent viscosity 1.18 dL/g was found for this polymer at 0.5g/dL concentration at ambient temperature in the presence 5% w/v lithium chloride in dimethyl acetamide (DMAc).Presence of amide group is conformed IR spectrum of the polymer on KBr which shows absorption band at 3332 cm -1 and 1663 cm -1 .Elemental analyses of polymers are tabulated in Table 1.

Scheme 2
Where R = Pendent bulky group diamine All other polymers were synthesized by the same method.

Polymer characterization
For all the polyamides Infra red spectra were obtained using Fourier transformed infrared spectroscope (Nocolet 380) taking 64 scan per sample using 2%w/w polyamides in KBr.Inherent viscosity of polyamides were measured with Ubbehonde viscometerNo.75.The inherent viscosity was determined at a concentration of 0.5g/dL at 30 0 C in DMAc solvent.Elemental analyses were carried out in elemental analyzer Heraeus Carlo Erba 1180.Thermogravimetric analysis was carried out in TGA -7, Perkin Elmer thermo gravimetric analyzer, under nitrogen atmosphere at heating rate of 10 0 C per minute between 50 and 600 0 C. Gel permeation chromatography (GPC) analysis were conducted on polyamide solution in DMAc.The analyses were performed using distilled DMAc containing 0.1M LiBr.Chromatography was performed on high performance liquid chromatograph (HPLC) Waters-510 equipped with a UV detector and three ultrasryragel/PL gel column ( size 500,1000,& 100000 A 0 ) connected in series in order to increasing pore size at flow rate of 1.0 mL/min.The columns were calibrated with polystyrene (PS) standard and standard of poly m-phenylene isophthalamide with molecular weight 1000 to 310000 g/mol using five point calibration curve.

Results and Discussion
Scheme 2 shows the route for synthesis used in this work based on Yamazaki phosphorylation reaction.The infrared spectra of the polyamide obtained shows the characteristic bands for the amine group between 3270 cm -1 and 3332 cm -1 (-NH) and the carbonyl group at 1662 cm -1 (C=O).The solubility of the three polyamides weres tested in different solvents using 0.1g of polymer in 5mL of solvent.Dimethyl sulphoxide (DMSO), dimethyl acetamide (DMAc), Dimethyl formamide (DMF),benzene (Bz), tetrahydro furane( THF), pyridine (Py), conc.H 2 SO 4 and N-methyl 2-pyrrolidone (NMP) were used.Almost all polymers were soluble in polar aprotic solvents.Solvents such as THF and Bz are unable to dissolve polyamides.These are also soluble in pyridine, but partially soluble in DMSO.Solubility data in different solvents are given in Table 2. Inherent viscosity ηinh were found ranging from 0.57 to 1.18.Values of Inherent viscosity ηinh are comparable to the values of other aromatic polyamide.Films forming capacity was found in all polyamides and film was prepared by casting from their DMAc solution containing 1% LiCl.Film was found colorless and strong.Thermal properties of polymers are summarized in the Table 4.Representative TGA curves are given in the Figure 1 as supporting information.Fluorene substitution in the diamines moiety increases the stability of the polyamide as compared with carbonyl group.However, herxafloro polyamide have slightly higher decomposition temperature than fluorine containing polyamide, but all these polyamides with stand from 375 to 390 0 C without mass loss and 20% mass loss occur at 400 0 C. The same trend was observed by Cassidy et.al. in poly ester synthesis with hexafluoro substitution 23 .Mass loss of the polyamides obtained at 400 0 C ranges between 13-20% as shown in Table 4.The mass loss of the polyamides containing fluorene moiety is less compared with hexa-floro polyamide whereas carbonyl polyamide has maximum weight loss.This is due to fluorene group has the number of phenyl group/maximum stabilization energy and hence have maximum stability.GPC analysis conforms that high molecular weight polyamides are obtained by this method.These polymers are found to be soluble in most of common solvents and can be process in relatively easy way by solvent evaporation method.Further analysis and possible application of this polymer system is under development.