Lu Jianmei Chen Naiyong, Ji Shunjun, Jia Zhun, Sun Zhenrong, Zhu Xiulin, Shi Weiping
(Chemisty Department of Suzhou University, Suzhou, Jiangsu Province; Key Laboratory of Optical & Magnetic Resonance Spectroscopy, East China Normal University Department of Physics, East China Normal University)

Received Dec. 11, 2002; Supported by the National Natural Science Foundation of China (No. 20076031) and Jiangsu Province Natural Science Foundation (No. BK2002042).

AbstractPolycondensation and imidization of m, m‘-diaminobenzophenone (m, m‘-DABP) and pyromellitic dianhydride (PMDA) under microwave radiation were studied, and the polycondensates third-order optical nonlinearities and time responses were measured by phase conjugate forward three-dimensional degenerate four-wave mixing technique for the first time. The effect of microwave radiation time (power), the composition of monomer and the temperature on the imidization and the polymer yield were discussed. At the same time, the results of the microwave radiation polymerization were compared with thermopolymerization. The results showed that the polycondensate prepared had high third-order nonlinear optical coefficients ( PAA⁽³⁾ = 1.642×10^-13esu) and fast time responses ( 24ps), simultaneously the third-order optical nonlinearities of the polymer were improved greatly by microwave radiation. The experimental results also showed that the third-order optical nonlinearities were not only affected by the length of polycondensate chain, but also the molecular structure, which was explained and confirmed by the computation of AMI semi-empirical method, and UV visible absorption spectrum.
Keywords  microwave, third-order optical nonlinearities, polyimide, imidization

In recent years, there has been growing interest in applying microwave energy to synthetic organic chemistry and synthetic polymer chemistry as well. Since the appearance of first article on the use of microwave energy in chemical synthesis, the approach has now developed into a useful technique for a variety of application in organic synthesis, especially, the solventless reactions conducted on solid supports^[1]. In the field of synthetic polymer chemistry, microwave energy has been utilized for the radical polymerization of vinyl monomers such as 2-hydroxy ethyl methacrylate^[2], methyl methacrylate^[3], and styrene^[4], and for the curing of polymers such as epoxy resins^[5-11] and polyurethanes^[12-13], as well as for the imidization of polyamic acids^[14-15]. In most cases, the high heat efficiency gave rise to remarkable rate enhancement and dramatic reduction of reaction time. Recently, the rapid synthesis of aromatic polyamides by microwave-assisted direct polycondensation of aromatic diamines and aromatic dicarboxylic acids with condensing agents^[16-17], and a preliminary study on the microwave-assisted rapid synthesis of polyamides from nylon salts have been intensively investigated by Imai et al.^[18]. However, there is no report so far on the synthesis of benzoguanamine and 2,4-tolylenediisocyanate by using microwave energy except for our studies. During the course of our studies, we have already reported the microwave irradiation solid state copolymerization in binary maleic anhydride and dibenzyl maleate system^[19], copolymerization of maleic anhydride and allyl thiourea in solid state. In polycondensation field microwave irradiation were widely applied. Since polyimides are important as photochemical materials in the third-order optical nonlinearities. So we have tried to afford useful products by using method of microwave irradiation. Herein, we report that the polymeriztion of m, m‘-diaminobenzophenone (m, m‘-DABP) and pyromellitic dianhydride (PMDA) and imidization of m, m‘-DABP-PMDA polycondensate (PAA) under microwave irradiation. Moreover, the third-order optical nonlinearities of the polymers are discussed.

1.EXPERIMENTAL
1.1 Experimental Setup

Scheme 1

Scheme 1:Setup of solid phase polymerization under microwave irradiation
Scheme 2 Setup of solution polymerization under microwave irradiation
Note: Reaction temperatures are measured by alcohol thermometer

1.2 Procedure of Polymerization
m, m’-Diaminobenzophenone (m, m‘-DABP) were synthesized according to the literature, purity 98.5%; Pyromellitic Dianhydride (PMDA), purity≥98%(Shanghai Jiaohua Factory), vacuum dry.
(1) Solution polymerization. m, m’-DABP were dissolved in DMF 10 minutes under nitrogen flow. After completely dissolved, PMDA (m,m’-DABP:PMDA(mol)=1:1)were added in batches. The temperature was kept at 45ºC by controlling the power. Temperature was measured by means of setting alcohol thermometer into the polymerization system, and the reaction time was kept for 2 hours. PAA were produced and precipitated by pouring distilled water, and washed by methanol.
(2) Solid phase polymerization under microwave radiation. PAA powders were set into the reactor under N, and dehydrated PAA was for imidization under continuous microwave radiation. Temperatures were measured by means of setting alcohol thermometer into the polymerization system.
(3) Solid phase thermopolymerization. PAA powders were set into the reactor under N, and dehydrated PAA at the temperature, which was the same as the maximal temperature of solid phase polymerization under microwave radiation.
1.3 Measurement
The confirmation of structure of the polymers was confirmed by means of IR and H-NMR.
H-NMR (D-DMF): 7.44-8.34ppm (=CH-), 10.80 ppm (–COOH), 6.85ppm(–NHCO–).
IR(KBr): 1651cm^-1 ( in the –NHCO–), 1255cm^-1 (–––), 1780 cm^-1, 1725cm^-1in the ).
The intrinsic viscosity of polymer were measured by extrapolation in the solvent: DMF at 30ºC
The imidization degree of PI were measured by using Infrared absorption spectrum that Masatoshi Hasegawa^[20] had used. The stretch vibration of C=O of polyimide were expressed as the imidization degree by using the stretch vibration of C=C in benzene ring as the reference.
The third-order optical nonlinearities of polymer were measured by phase conjugate forward three-dimensional degenerate four-wave mixing (3D DFWM) technique (in Department of Physics, East China Normal University).

2. Results and Discussion
2.1 Synthesis of polymer
This polymer was synthesized by polycondensation of m, m’-DABP and PMDA in two-step. Firstly, PAA polymer was got by polycondensation of m, m’-DABP and PMDA, then the polymer(PI) was obtained by imidization of PAA. In this polymerization, microwave irradiation and regular heating methods were used. The procedure of polymerization was shown in Scheme 3.

Scheme 3The procedure of polymerization.

2.2 The third-order optical nonlinearities of m, m’-DABP-PMDA polymer
The third-order optical nonlinearities of the m, m’-DABP-PMDA polycondensate were measured by phase conjugate forward three-dimensional degenerate four-wave mixing (3D DFWM) technique^[29-30] as depicted in Figure1. After passing through a quarter wavelength plate, the pulse laser beam (wavelength 532 nm, pulse-width 15ps, repetition rate 10Hz, peak irradiance 10GW/cm), from a frequency-doubled picosecond pulse mode-locked Nd:YAG laser, is split into three beams k, k and k with the same energy by use of reflecting beam splitters, then temporally and spatially overlapped in the sample with a 205mm focal-length lens. The angles between the three beams k, k and k are about 1. In the experiment, the intensity I of the phase conjugate beam k is detected by PIN photodiode. According to the following formula, the third-order nonlinear optical coefficients c ⁽³⁾ can be measured by compared the measured signals for solution with that for carbon disulfide as reference under the same experimental condition:

Where I is the intensity of signal peak, L is the thickness of the sample, n is the refractive index, the subscript r refers to carbon disulfide, and the tensor elements⁽³⁾_xxxx = 6.8×10^-13esu and⁽³⁾_yxxy = 7.6×10^-14esu for carbon disulfide. The tensor elements of third-order optical nonlinearities for m, m’-DABP-PMDA polycondensate are calculated.
2.3 The setup for measuring the third-order optical nonlinearities of the polymer

Figure1 The 3D DFWM setup for measuring the third-order optical nonlinearities of the Polymer
(a) The experimental setup for three dimensional DFWM
(b) Configuration for three dimensional DFWM

2.4 The mechanism of the third-order optical nonlinearities
In this paper, the mechanism of the third-order optical nonlinearities of polycondensate were studied by AM1 (AM1 means Austin mode one) semi-empirical method, UV-Visible absorption spectrum and the time response spectrum for the DFWM signal.
2.4.1 The charge density
In order to study the p-p -conjugated structure of m, m’-DABP-PMDA polycondensate (PAA) and their imidization polymer (PI), the charge densities were calculated by the computation of AMI semi-empirical method. The results were shown in Figure 2.

a. PAA

b. PI
Figure 2 The charge density of m, m’-DABP-PMDA polycondensate

The result showed that the polycondensate were the p-p -conjugated system.

2.4.2 The UV-Visible absorption spectrum

Figure 3 The UV-visible absorption spectra of m, m’-DABP-PMDA polycondensate

The UV-Visible absorption spectrums of m, m’-DABP-PMDA polycondensate are shown in Figure 3, where the absorption bands in the region of 230-350 nm of PAA and 236-355 nm of PI are due to p -p transitions of the highly polyconjugated systems, indicative of the distribution range in the p-p -conjugated systems, and the used laser wavelength (532nm) in the experiment is out of the absorption region.
2.4.3 The time response spectrum for the DFWM signal
m, m’-DABP-PMDA polycondensate, with a large quasi-one-dimensional p-p -conjugated structure, contain a high density of p -electron that attributed to the third-order nonlinearities. A delay for any one of the three input beams gives almost the same signal envelope. The time-resolved measurement results of 3D DFWM are shown in Figure 4. The envelope of the signal is fitted very well by a Gaussian function, and the half-width of the fitting envelope (about 25ps) is similar to the autocorrelated pulse duration. The signal profile shows the symmetry about the maximum signal (the zero time delay), which indicates that the response time (in Table 1) of the third-order optical nonlinearity is equivalent to experimental time resolution (PI=25ps). It means that the time response of the third-order optical nonlinearity is quite fast, and the Kerr effect, which arises from the distortion of the large p-p -conjugate electronic charge distribution of m, m’-DABP -PMDA polycondensate, is the main reason for generating DFWM.

Figure4 The DFWM time response spectra for m,m,-DABP-PMDA polycondensate

Table1The time response for the DFWM signal of the m,m,-DABP-PMDA polycondensate

Sample	Response time (ps)
m,m,-DABP-PMDA PAA	27
m,m,-DABP-PMDA PI	25

2.5 The effect factors of the third-order optical nonlinearities of the polymer
2.5.1 Effect of the conjugated chain length
The relationship between the third-order optical nonlinearities and the intrinsic viscosity of polymer were shown in Figure 5(varying the microwave radiation energy) and Figure 6 (varying the monomer composition). It can be learned that the third-order nonlinear optical coefficient⁽³⁾increase as the intrinsic viscosity of polymer (the conjugated length L ) increases in this experimental range. It is known that when the wavelength of strong laser field is the region of materials absorption, the kerr nonlinear effect of resonant strengthening will be produced. On this condition, the molecules are approximately regarded as two energy level system. The nonlinear polarization is relative to the molecular energy level of excitation, especial inverse proportion to the energy gap between the LUMO (Lowest unoccupied molecule orbit) and the HOMO (Highest occupied molecule orbit).

Figure 5Relationship among third-order nonlinear optical coefficient of PAA and intrinsic viscosity with the microwave energy

Figure 6Relationship among third-order nonlinear optical coefficient of PAA and intrinsic viscosity with the monomer composition

The energy gap E values for the various degree of polymerization (n=1-4) conjugated lengths of polymer are calculated by AMI semi-empirical method (see Table 2). The E decreases as the conjugated length of polymer chain increases. It may be that the increase of the conjugated degree results in the decrease of the E value. As a result, the conjugated electrons generate transition quickly and the distortion of conjugated electrons can come into being easily. The results show that the third-order optical nonlinearities of conjugated polymer increase as the length of p-p -conjugated structure increase.

Table2The calculated energy gap E for the various conjugated lengths of the polymer by AM1 semi-empirical method

Sample	(ev)
n=1	n=2	n=3	n=4
m, m’-DABP-PMDA PAA	6.152	711	675	587
m, m’-DABP-PMDA PI	5.921	5.895	5.871	5.869

2.5.2 Effect of microwave radiation and heating on the solution polymerization ( polycondensation of m, m’-DABP and PMDA)
From Figures 7-9, it can be learned that the third-order nonlinear optical coefficient⁽³⁾and the intrinsic viscosity of PAA, which are synthesized by microwave radiation or heating, increase as the temperature increase, then decrease as the temperature go on increasing. The⁽³⁾of PAA which are synthesized by microwave radiation is larger than that by heating. It may be that the microwave radiation contributed to the narrow distribution of molecular weight. The result showed that microwave radiation could improve the third-order optical nonlinearities of polymer, and took on the unheating effect, which may have a particularly active action.

Figure 7Relationship among third-order nonlinear optical coefficient of PAA synthesized in the solution by microwave radiation and intrinsic viscosity with the temperatureFigure 8Relationship among third-order nonlinear optical coefficient of PAA synthesized in the solution by heating and intrinsic viscosity with the temperature

2.5.3 Effect of microwave radiation and heating on the solid phase polymerization (imidization of PAA)
From Figure 10 to 12, it can be learned that the third-order nonlinear optical coefficient⁽³⁾of PI, which were synthesized in solid phase by microwave radiation or heating, increase as the imidization degree increase. The⁽³⁾ of PI which were synthesized in solid phase by heating is smaller than by microwave radiation. It showed that microwave radiation can improve the third-order optical nonlinearities of polymer, and took on the unheating effect, which may have a particularly active action.