Biomimetic polymerization of acrylamide with hydrogen peroxide catalyzed by water-soluble anionic iron(III) 5,10,15,20-tetrakis-(2 ′ ,6 ′ -dichloro-3 ′ -sulfonatophenyl)porphyrin

The biomimetic polymerization of acrylamide ( 1 ) with hydrogen peroxide catalyzed by water soluble iron(III) 5,10,15,20-tetrakis-(2 ′ ,6 ′ -dichloro-3 ′ -sulfonatophenyl)porphyrin ( 3 ) gave high yield of polyacrylamide ( 2 ) in the molecular weight range of 1,30,000-1,50,000 g mol -1 in the absence and presence of 2,4-pentanedione at an ambient temperature in nitrogen atmosphere. The molar ratio of 3 : H 2 O 2 : 1 was 1: 100: 3300. Yields were high at pH 4.5 but dropped at pH 7.2. Finally no polymer was formed at pH 9.2 in the absence of 2,4-pentanedione, whereas the yield raised to 99% in the presence of 2,4-pentanedione.


Introduction
2][3] Various biopolymers and non-natural synthetic polymers have been synthesized under ecofriendly and milder conditions by enzyme-catalyzed polymerizations.A selective enzyme catalyzes a specific polymerization reaction with high selectivity.Glycosidase specifically catalyses the synthesis of various natural and unnatural polysaccharides under much milder conditions with high yields.

4,5
Lipase catalyses the synthesis of optically active polyesters and transesterification of polyesters to produce random copolyesters, 6,7 whereas papain catalyses the polymerization of amino acids.
Oxidoreductases including peroxidase, laccase and bilirubin oxidase are used as catalysts by the production of novel polyphenolics and related compounds.
0][11][12][13][14][15][16] Similarly the vinylic monomers have also been polymerized with hydrogen peroxide catalyzed by HRP and related enzymes under milder conditions with high yields.Lipid-soluble and water-soluble metalloporphyrins mimic the various reactions of cytochrome P450, HRP and other monooxygenases in different reactions. 21,22Herein, we report a novel route for the polymerization of acrylamide with hydrogen peroxide catalyzed by anionic water-soluble iron(III) 5,10,15,20-tetrakis-(2′,6′-dichloro-3′-sulfonatophenyl)porphyrin to understand the molecular mechanism of peroxidase and related oxidoreductase enzymes and their utilization in polymerization reactions.The formation of polymer 2 was indicated in the absence of a peak at 1612 cm -1 for olefinic stretching and a carbonyl absorption from 1672 to 1655 cm -1 in its IR spectra.The disappearance of signals for olefinic protons at δ 5.10 and the appearance of characteristic broad signals at δ 1.59 and 2.16 corresponding to the methylene and methine protons in the 1 H-NMR spectra confirmed the formation of 2. The average molecular weight of 2 was found to be 1,30,000 g mol -1 by the viscosity measurements in water at 25°C. 23The complete thermal degradation of 2 occurred in two stages at 315°C and 423°C when the rate of heating was 5°C/min in their thermogravimetric analysis 24 (Figure 1).The ratio of 3: H 2 O 2 : 1 as 1: 100: 3300 was found to be suitable for the polymerization of 1 at room temperature.The polymer was obtained in 40% and 70% yield when the ratio of 3 and 2,4-PD was 1 : 5 and 1: 50 respectively at pH 9.2.The optimal ratio of 3 and 2,4-PD was found to be 1 : 110 under given reaction conditions to obtain the polymer in 99% yield.Further increase in the concentration of 2,4-PD gave decreased product yield.

Results and Discussion
The effect of pH on the % yield of the polymer in the absence and the presence of 2,4-PD is also summarized in the Figure 2.   At neutral and near alkaline pH, the peroxide anion ( -OOH) from H 2 O 2 is not able to replace the axial ligand OH on -Fe(III)(H 2 O)(OH)-, which is the species when 3 is present in the buffer of neutral and higher pH.Thus the concentration of highvalent oxoiron(IV) radical cation (7)  decreases and hence very low or no polymerization is observed at these pH's.
0 Polyacrylamide of molecular weight 1,24,000 g mol -1 and polydispersity 2.5 has been obtained by the polymerization of 1 with hydrogen peroxide catalyzed by HRP in the presence of 2,4-PD using H 2 O 2 -to-monomer ratio of 1/66 mol/mol at pH 5.1 while no polymer is reported in the absence of 2,4-PD. 17Thus 2,4-PD was added to the reaction of 1 with H 2 O 2 catalyzed by Cl 8 TPPS 4 Fe(III) (3) at pH 4.5 and 7.2 and it enhanced the yield of polymer 2 to 98 and 15% respectively.At pH 4.5, the polymer 2 of comparable molecular weight (1,30,000 g mol -1 ) and polydispersity (2.45) was obtained when 3 was used as the catalyst in place of HRP without any need for the addition of 2,4-PD.The addition of 2,4-PD at pH 9.2 offered a new route for polymerization of 1 at this pH, where the use of HRP is limited owing to its deactivation.The large excess of H 2 O 2 with respect to the monomer resulted in the degradation of the iron(III) porphyrin.Thus, a 1:66 molar ratio of H 2 O 2 to monomer was found to be suitable for the polymerization of 1 both in the absence and the presence of 2,4-PD.

Conclusions
The water soluble anionic Cl 8 TPPS 4 Fe(III) (3) in the presence of hydrogen peroxide is an economical and suitable system to initiate the polymerization of acrylamide at acidic and neutral pH, by using the molar ratio of 3: H 2 O 2 : 1 as 1 : 100 : 3300.The addition of 2,4-PD in this system enhances the yield of the polymer at both acidic and neutral pH and also facilitates the polymerization of 1 at alkaline pH, where HRP is deactivated.Thus the water-soluble anionic iron(III) porphyrin and H 2 O 2 mimic the polymerization reaction of oxidoreductase enzyme HRP in the presence or absence of 2,4-PD under milder conditions with excellent yield.

28,29
The average molecular weight of polyacrylamide was determined from intrinsic viscosity measurements according to Mark-Houwink equations. 23Viscosity measurements were obtained at 25°C in water using ubbelohde viscometer and [η] c→0 was evaluated by the extrapolation of experimental data.
[η] c→0 values were used to calculate the number and weight average molecular weight according to the relations [η] = 6.8 × 10 -4 M n 0.66 and [η] = 6.31 × 10 -4 M w 0.80 . in the presence of 2,4-PD.The procedure and molar ratio of reactant (1) and catalyst (3) used in this case were exactly the same as mentioned above except for the addition of 2,4-PD (0.18 mmol) which was added simultaneously along with hydrogen peroxide.The results obtained in this case are also summarized in Table 1.

Figure 2 .Figure 3 .
Figure 2. Effect of pH on the %yield of the polymer (2) in the absence and the presence of 2,4-PD.

Scheme 2 .
Scheme 2. Proposed mechanism for the formation of reactive intermediate in polymerization of 1 with hydrogen peroxide catalyzed by Cl 8 TPPS 4 Fe(III) (3).

Polymerization of 1 with H 2 O 2 catalyzed by Cl 8 TPPS 4 Fe(III) (3) in different reaction conditions. The reaction of 1 with
H 2 O 2 catalyzed by robust Cl 8 TPPS 4 Fe(III) (3) was carried out in acetate buffer (pH 4.5, 0.2M) at an ambient temperature in a nitrogen atmosphere.The monomer 1 was quantitatively consumed to give the polymer 2 in 85% yield (Table1, entry 3).The yield of 2 dropped to 5% at pH 7.2 while there was no detectable amount of polymer at pH 9.2 under the same conditions (Table1, entry 7 and 11).

Table 1 .
Effect of pH and the presence of 2,4-PD on the polymerization of 1 with H 2 O 2 catalyzed by 3 a Ratio of system 3 : H 2 O 2 : 1 = 1: 100 : 3300.

Effect of 2,4-pentanedione (2,4-PD) on polymerization of 1 at different pH′s.
The addition of 2,4-PD to the reaction of 1 with H 2 O 2 catalyzed by 3 at pH 4.5 and 7.2 enhanced the yield of polymer 2 to 98 and 15% respectively (Table1, entry 4 & 8).The polyacrylamide (2) obtained at pH 4.5 in the presence of 2,4-PD was of comparable molecular weight and polydispersity to that obtained in the absence of 2,4-PD (Table1, entry 3 & 4).At pH 9.2, the addition of 2,4-PD to the above reaction facilitated the polymerization of 1 and produced the polymer 2 of low molecular weight (51,000 g mol -1) and polydispersity 2.90 in 99% yield (

Comparison of polymerization reaction of HRP with water-soluble Cl 8 TPPS 4 Fe(III) (3).
The reaction of 1 with H 2 O 2 catalyzed by HRP at pH 4.5 in a nitrogen atmosphere gave the polymer in 60% yield at room temperature, the concentrations being [HRP]= 2gL -1 , [1]= 0.46M and [H 2 O 2 ]= 11mM.No polymer was obtained at pH 9.2 under the same conditions due to deactivation of HRP enzyme at higher pH.
27termediate 7 is responsible for initiating the formation of acrylamide radical (8a and 8b), which attacks the other acrylamide molecule to form a bigger free radical and hence the reaction further propagates to give the polymer 2. The reaction is terminated either by the combination of two bigger radicals or by abstraction of a proton from the solvent or another monomer molecule (Scheme 2).The stable dimethoxyiron(IV) porphyrin is generated by reaction of iron(III)porphyrin with H 2 O 2 in presence of large excess of methanol.27Thusthepolymerization of 1 is quenched by the addition of large excess of methanol.The formation of intermediate 7 could be explained only by the heterolytic cleavage of H 2 O 2, which leads to the polymerization.This was confirmed as no polymer was obtained in the presence of H 2 O 2 and absence of 3 (Table1, entry 1).
The reaction of 1 with H 2 O 2 in the presence of 2,4-PD in the absence of 3 produced no polymer (Table 1, entry 10).At pH 9.2, the reaction of 3 with H 2 O 2 in the presence of 2,4-PD may produce a stable intermediate which contributes to the facile polymerization of 1 at this pH.

of 1 with hydrogen peroxide catalyzed by water-soluble Cl 8 TPPS 4 Fe(III) (3).
1HNMR spectra were recorded on a Bruker Avance Spectrospin 300MHz spectrophotometer.Perkin Elmer FT-IR Spectrum 2000 Spectrometer was used to record IR spectra.Thermogravimetric analysis was obtained on a Rigaku Thermoflex, PTC-10A.Polymerization The solution of 1 (7 mmol in 10mL acetate buffer of pH 4.5) was charged into a threenecked round-bottomed flask and degassed for 15 minutes.Cl 8 TPPS 4 Fe(III) (2.12 × 10 -3 mmol in 0.5 mL buffer) and 30% H 2 O 2 (0.106 mmol) were successively injected into the above flask under stirring.The reaction was stirred for a predetermined time at an ambient temperature under a nitrogen atmosphere.The polymer was obtained by quenching the reaction with a large excess of methanol and filtered off, washing with methanol and drying under vacuum at 50°C.The yield and molecular weight of polymers are given in Table1.