Study of the Crosslinking Reaction between Bisphenol A Diglycidyl ether ( BADGE ) and a Zinc Porphyrin by Fourier Transform Infrared Spectroscopy

The curing reaction of a system consisting of a diglycidyl ether of bisphenol-A (n=0) and ZnTPyP (Zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphyrin) was studied by Fourier Transform Infrared Spectroscopy (FTIR). A semi-empirical quantum chemistry program (MOPAC) was used to corroborate the experimental data. The tertiary amine ZnTPyP not only initiates the reaction but also reacts with the resin forming pyridone. This study demonstrates that macrocycles can be used as cross-linking agents forepoxy resins and that when metallomacrocycles are used, metal ions can be introduced into the network structure.


INTRODUCTION
Epoxy resins have a position of great importance in polymer industries because of their many different applicationsindustry (electrical, naval, aerospace, construction...) as well as in sports, medicine, and so on.The great success of epoxy resins are related to their light weight compared to other materials, high mechanical characteristics, great resistance to corrosion, and the possibility of different designs for particular applications 1 .On the other hand, porphyrins and related compounds, such as chlorophyll andhemin, are essential to many biological energy transduction processesincluding light harvesting and photocatalytic processes [2][3][4][5] .For this study, the zinc meso-tetra (4-pyridyl) porphyrin (ZnTPyP, a tertiaryamine) has been chosen as the curing agent for the epoxy resin bisphenol-A diglycidyl ether (n=0) (BADGE).
This study is part of a project in which the effects of introducing macrocycles derivatives on the epoxy resinnetwork are investigated 22 .These crosslinking agents improve the stability of the resin at high temperatures, besides they have the ability to strongly bind metallic ions, forming complexes.This could lead to develop epoxy polymers with new unexpected properties.

EXPERIMENTAL Materials and Sample preparation
Diglicidyl ether of bisphenol A (BADGE, n=0) (Resin 332, Sigma Chemical Co., St Louis, USA) and ZnTPyP (Sigma-Aldrich Chemie GmbH, Germany) were used without further purification.The structures of these compounds are shown in Fig. 1.First, epoxy resin and curing agent were carefully and homogeneouslymixed at the 9:1 (mol/mol) ratio at room temperature.Second, a sample ofthis mixture (typically 8-10 mg) was encapsulated in aluminumpans for the differential scanning calorimeter (DSC) analysis.The optimum (BADGE, n =0)/ZnTPyP ratio was determined bycuring experiments at different values of the epoxy/curingagent ratio by the dynamic method.The curing reaction was programmed between 50ºC and 350ºC at a heating rate of 10ºC/min.

RESULTS AND DISCUSSION
The vibration band at 915 cm -1 has been chosen as areference band for the different vibration bands of the epoxygroups (760, 915, and 970 cm -1).Figure 2 shows the infrared spectra of the epoxy resin BADGE n=0 (blue line), the epoxy systemheatedfrom 50ºC to 230ºC (red line) and the epoxy system heated from 50ºC to 350ºC (purple line).It can be seen that the intensity of the vibration band of the terminal epoxy groups decreases during the curing reaction; at the same time v(OH -) at 3450 cm -1 increases in intensity.The ring-opening of the epoxy group generates a strong band at about 1130 cm -1 assigned to the asymmetric C-O-C stretching vibration 14 .The band at 710 cm -1 assigned to the stretching vibrations of C-H bonds in a pyridine ring practically disappears, suggesting that the pyridine ringswere destroyed upon reacting with BADGE n=0.At thesame time, new bandsappear at 1470 and 1650 cm -1 , which could be assigned to the carbonyl of pyridone and the unconjugated carbon double bonds.Therefore the tertiary amine ZnTPyP not only initiates the homopolimerization reaction but also reacts with the resin forming pyridone.It is proposed that epoxy groups of BADGE n=0 could be involved in two reactionsduring the curing process.At the same time that epoxy groups react with the pyridine rings (Figure 3), the homopolymerization mechanism of the epoxy groups could

b)
N appearwhich implies the formation of aliphatic ethers (Figure 4).Both reactions could be catalyzed by zinc, which would act as a Lewis acid.The reaction was initiated by the nucleophilic attackof the tertiary nitrogen atom of ZnTPyP on the epoxide groups of BADGE, forming a salt.In the second step, the oxygen ion immediately attacks the -position ofpyridine ring and the aromatic structure of pyridinering is destroyed, forming pyridone 23 .

Figure 4.Proposed mechanism for the homopolymerization
It is proposed that the homopolymerization of epoxide groups could be initiated by the nitrogen atoms of pyridine rings 24 .Note that the homopolimerizationunder identical conditions but in the absence ofZnTPyP,is a fairly slow process.
A semi-empirical quantum chemistry program was used for verifying the occurrence of the pyridone formation.Various molecular properties of different chemical structures werecalculated for predicting thebehavior of the system.The choice of method (semi-empirical or ab-initio) is a key factor to calculate the structures and properties of molecules and solids.Both ab initio and semi-empirical approaches involve approximations.The ab-initio methods are more accurate but need more computer time; whereas simplifications introduced by semi-empirical approaches assume that the chemical behavior exclusively depends on the valence shell, not including internal electrons.
The software MOPAC2012 25 was chosen in order to make all the calculations needed for this study.Different Hamiltonians available inMOPAC2012 (AM1, PM3, MNDO, PM5, PM6 and PM7) were used for studying the1-methyl-2-pyridone, used as a model compound for assessingthe validity and reliability of the calculated vibration frequencies.Experimental data of 1-methyl-2-pyridone are shown in Figure 5.

Figure5.ExperimentalFTIR spectrum of 1-methyl-2-pyridone( SDBSNo. 4795).
Obtained values for the C=O vibrationare given in Table 1.The best agreement between the calculated and experimental values is obtained withthe RM1 semiempirical Hamiltonian.However, this method is not parameterized for Zinc, so the PM6 model was chosen for calculations with the simplified model molecule shown in Figure 6, which after minimization rendered the structure shown in Figure 7, over which the oscillation frequencies were calculated.The Table 2 shows the theoretical results for the stretching vibration of the lactam carbonyl group.Values fall between 1782 and 1785 cm -1 .It perfectly matches with experimental values for the epoxy resin.

Figure 3 .
Figure 3.Proposed mechanism for the heteropolymerization. Formation of the pyridone.

Table 1 .
Values for the C=O vibrationobtained using the different Hamiltonians available inMOPAC2012.

Table 2 .
Stretching vibration of the carbonyl group inlactamsSimultaneous use of FTIR technique and theoretical calculations by means of standard semiempirical software can help elucidating the curing process as well as the products obtained when macrocycles are used for curing epoxy resins.