Synthesis and Applications of Polymeric Reagent p-Substituted Triphenylamine

: Chemical modification of chloromethylstyrene - styrene copolymer throughout reaction of p-substituted carboxylic acid group of bis-(4,4`-dibromo)-4``-triphenylamine carboxylic acid with the chloromethyl group attached to a phenyl group was carried out on soluble copolymer and polymeric cross-linked copolymer. Chemical oxidation of the neutral p-substituted triphenylamine with antimony pentachloride in dichloromethane solvent gives the corresponding cation - radical salt with the counter ion antimony hexachloride (SbCl 6- ). The isolated deep blue color cation radical salt is soluble or in insoluble (resin) form in the copolymer was used as a thermal cationic initiator for the polymerization of epoxy and vinyl monomers at room temperature. The cation radical resin showed good activity and stability compared to the soluble polymeric cation radical, both can initiate the cationic polymerization of cyclohexene oxide and N-vinylcarbazole in dichloromethane at room temperature.


INTRODUCTION
Chloromethylated polystyrenes are key intermediates in the preparation of amino-exchange resins, supports solid-phase peptide synthesis, and supports for polymeric reagents and catalysts [1]. The polymer support might be a linear soluble species, or porous cross-linked polystyrene prepared by copolymerization of styrene with divinylbenzene, having a different density of crosslinks. The chemically bound functional group might be a reagent, a protecting group, a ligand, a metal complex catalyst or some other catalytic species e.g., an acid or base, or a phase transfer catalyst [2].
Polymeric reagents are easily separable from low molecular weight compounds; with cross-linked insoluble polymers simple filtration is usually sufficient; selective precipitation removes soluble polymers. This enables the polymeric reagent to be used in either columns or batch processes, and it may be regenerated several times. Polymeric reagents also often lack many inconvenient properties of the corresponding low-molecular-weight reagents, such as liability, volatility, toxicity or color. The excess starting material is easily separated from the product and may often be reused, and reaction side products are often easily removed [3].
*Address correspondence to this author at the Department of Chemistry, University of Sharjah, Sharjah P.O. Box 27272, UAE; E-mail: abuabdoun@sharjah.ac.ae Polymers having triphenylamine units have been extensively used as excellent hole transport materials and photoconductors based on their ability to form stable cation radical and high drift mobility [4,5] and in advanced materials for various optoelectronic applications, such as electrochromic, electro-fluoro chromic, and polymeric memory devices [6].
In our early works with cation radical salts of psubstituted triphenylamine, phenothiazine and tetraphenylenediamine have been used to accomplish extremely efficient one electron transfer, to induce cationic polymerization reactions of cyclohexene oxide, THF and N-vinylcarbazole monomers [7][8][9]. and also in electrochemical modification of electrodes [10].
Thus, our interest in the polymeric cation radical of para-substituted triphenylamine leads us to the synthesis of triphenylamine containing polymeric reagent, through coupling reactions of the copolymer of chloromethyl styrene -styrene with 4-carboxyl-4', 4"dibromotriphneylamine. The obtained copolymer in this system carrying p-substituted triphenylamine can be oxidized chemically to give the polymeric cation radical salt, which is found to be useful for the cationic polymerization reactions of epoxides and vinyl monomers and other isomerization reactions.

Chemicals and Solvents
Substituted triphenylamine (4 and 5) were obtained as reported early [11]. Monomers and solvents were purified according to standard procedures. Other chemicals were of reagent grade and were used without further purification.

Spectroscopic Measurements
Ultraviolet spectra were obtained on a Unicam SP1800 Ultraviolet spectrophotometer. Infrared spectra were recorded using potassium bromide (KBr disc) with a Perkin -Elmer grating infrared spectrophotometer (Model 577).

Polymerization
The selected amounts of monomer and initiator in dichloromethane were placed in a reaction tube and placed in a holder and left for the required period immersed in the water bath at 25 o C. The polymer was precipitated in methanol, in case of polymerization by the resin cation radical; to the selected amount of resin dichloromethane was added to allow swallow the resin. Directly after, the monomer was added and left for a selected time, the filtered polymer solution was precipitated in methanol, later filter and dry the obtained polymer, this process was repeated several times and between each process swelling of the resin in dichloromethane was repeated too. The yield was filtered, dried and weighed. The polymer yield and the rate of polymerization were determined gravimetrically [12]. While in the case of soluble polymeric cation radical the polymerization was carried out as in the case of homogeneous polymerization system where after polymerization time the polymerized solution powered in 5-folds excess methanol, filtered and dried to recover the obtained polymer.
Typical Procedure for the Reaction of Pendant Chloromethyl Group in the polymer with carboxylic Acid Scheme 1: Synthesis of the soluble polymeric reagent from chloromethylstyrene copolymer and p-substituted triphenylamine. PCMS (7) (2.0 gm) was dissolved in 30 ml of DMF, and then carboxylic acid triphenylamine (4) (1.35 gm) and DBU (1.5 gm) were added into the solution. The mixture was stirred at 60 o C for 4 hours, cool to room temperature and then poured into methanol to precipitate the polymer, which was reprecipitated twice from dichloromethane into methanol, filtered, and dried under vacuum at room temperature to give slightly yellow plates. Yield = 2.8 gm. ν max (KBr disc), 1720 (C=O), 1605 (N-Ar), 1580, 1500, 1480, 1435, 1375

Oxidation of the Polymeric Triphenylamine Coupling Product (6)
A sample of the polymer (5) obtained above was treated under nitrogen in dichloromethane with SbCl 5 in the same solvent dropwise, after cooling the reaction was instantaneous and the blue colored mixture was left stirring for about 5 minutes, then treated with a 4fold excess of cold benzene which precipitated the deep blue salt. It was then quickly filtered and washed with cold benzene twice, then dried, and stored in a stoppered flask in a cold place.
The salt (6) was stable and soluble in dichloromethane and acetonitrile, but it decomposed to a colourless solution in alcohol, petroleum ether, and acetone.
The isolated salt was found to be partially soluble after few days when stored at room temperature either as a solid or in solution without losing its blue color. Figure 1 shows the UV spectra of the soluble neutral amine (5) in dichloromethane, and Figure 2 shows the absorption spectra of the polymeric soluble cation radical (6). The polymerization solution of cyclohexene oxide in dichloromethane before precipitation is shown in Figure 3.
The reaction vessel was fitted with a reflux condenser, a nitrogen inlet and a mechanical stirrer, and loaded with 350 ml deionized water and 1.5 gm poly(vinylpyrrolidene) (PVP), 1.0 gm calcium sulfate, 0.4 gm calcium orthophosphate, and 30 gm sodium chloride. The vessel was placed in a water bath at 80 o C and the mixture was flushed with oxygen-free nitrogen for approximately 30 minutes before a slow stream of nitrogen was allowed to pass over the reaction mixture and the stirrer was adjusted to an appropriate speed of 300 r. p.m. The monomer mixture contained 10 gm styrene, 0.25 gm divinylbenzene, 2.0 gm p-chloromethyl styrene and 0.3 gm azobisizobutyronitrile (AIBN) free radical initiator. This was added through the sampling arm. After 5-6 hours the vessel was removed from the constant temperature bath.
Polymer beads were purified and recovered according to the following washing-decantation sequence: a.
The reaction mixture was stirred in three litres of water for 10 minutes, allowed to stand and the less dense dust-like fine suspension was decanted out from the polymer beads. This process was repeated a few times until no finer particles separated. b.
c. The polymer beads were stirred and decanted successively with three litres of water-methanolacetone (7:2:1), one litre of methanol-acetone (1:1) and finally pure methanol and then dried in a vacuum oven at 50 0 C. Yield = 7.0 gm.

Reaction of the Resin (7) with (4) to Prepare (8)
A 0.2 gm Chloromethylstyrene resin (7) and 0.5 gm of the 4-carboxyl-4'-dibromotriphenylamine mixture was heated at 60 O C in 20 ml DMF in presence of 0.2 gram DBU as reported in preparation of (5) for 4 hours. After cooling out the reaction to room temperature, the reaction mixture was filtered through a glass sinter, provided with suction tail and tap through a Buchner flask, and washed with excess dichloromethane twice, followed by aqueous methanol, and then washed with acetone (3 -4 times). The resin was allowed to stand for at least two minutes with each washing step to ensure that no unreacted amine was left inside the polymer particles. The polymeric resin was dried at 50 O C. Elemental % analysis was carried out and presented below: Expected: Carbon = 75.15, found 81.12, Hydrogen Expected = 5.6, found 6.73. Nitrogen: Expected 1.30, found 1.08

Oxidation of the Polymeric Triphenylamine Resin, Preparation of (9)
A sample of the copolymer obtained above was treated under nitrogen in dichloromethane, with SbCl 5 in the same solvent. The reaction was instantaneous, and the blue colored mixture was left stirring for about 10 minutes, washed several times with cold benzene, then filtered and dried at room temperature invacuo. The deep blue salt was stored in a cold place, without losing its color for more than one year. The salt (9) was stable and insoluble in dichloromethane and acetonitrile, but decomposed to a colorless solution in alcohol, petroleum ether and acetone inless rate than (6).
TGA analysis of the copolymer 17% Cl (7) under oxygen atmosphere showed significant weight loss in the temperature range of 374 -414 o C as illustrated in Figure 4. This weight loss is mainly due to the halogen atom (Cl) splitting which leads to cross-linked dimeric form which decomposes further at 556 -577 o C.

Attempts in Polymerization of Selected Monomers
Polymerization was carried out as explained early in the experimental section 2.3. For the purpose of comparison in reactivity and stability of polymeric resin and soluble polymeric reagent prepared cation radical salts, the type and concentration of each monomer was fixed, and only varies the time of polymerization. The concentration of the polymeric initiator salt was in the range 5.0 x10 -3 M, and 2.0 grams of the monomer was used.
a. Polymerization of cyclohexene oxide (CHO) at room temperature ( Table 1): It is clear from Table 1, the polymeric crosslinked resin (9) is more active as a polymeric reagent for the polymerization of cyclohexene oxide than (6) in dichloromethane at room temperature Scheme 2: Coupling of Chloromethylstyrene polymeric resin p-substituted triphenylamine.  Table 2.
Also, Table 2 shows different reactivity, as the polymeric crosslinked resin (9) is less active as a polymeric reagent for the polymerization of NVC than (6) at room temperature and in dichloromethane. This variation in reactivity could be due to the effect of the counter ion stability on the propagated chain type for both monomers.

CONCLUSIONS
p-substituted triphenylamine coupled to the chlomethylated styrene copolymer in a soluble and crosslinked form prepared and found to be useful as the polymerization of cyclohexene oxide and Nvinylcarbazole. The resin form of the obtained polymeric triphenylamine cation radical salt was found to be more stable than the soluble polymeric one. Heterogenization of homogeneous catalyst in this work is well demonstrated and can be applied to many organic or polymerization reactions in future work.