Chapter 18

Synthesis of Poly(aminoamides) via EnzymaticMeans†

H. N. Cheng*,a and Qu-Ming Gub

aSouthern Regional Research Center, USDA Agricultural Research Service,1100 Robert E. Lee Blvd., New Orleans, LA 70124

bAshland Inc., Ashland Research Center, 500 Hercules Road, Wilmington,DE 19808

*h.n.cheng@ars.usda.gov†Names are necessary to report factually on available data; however, theUSDA neither guarantees nor warrants the standards of the products,

and the use of the name USDA implies no approval of the products to theexclusion of others that may also be suitable.

Poly(aminoamides) constitute a subclass of polyamides that arewater-soluble and useful for several applications. Commerciallythey are made via chemical reaction pathways. A review ismade in this work of the enzymatic approaches towards theirsyntheses. Lipases and esterases have been found to be suitableenzymes to produce high-molecular-weight polyamides underrelatively mild reaction conditions. A large number of differentpolymer compositions can be synthesized through enzymaticmeans. The design of the polymer structure and syntheticconsiderations are included in this review.

Introduction

Poly(aminoamides) are interesting polymers that have been found to beuseful in many different applications. For example, the poly(aminoamide) ofadipic acid and diethylene triamine (DETA) is well known as a prepolymer for acationic resin that is used to improve wet strength and as a creping aid in paper(1). A quaternized poly(amidoamine) has been reported as a corrosion controlagent (2). Modified poly(aminoamides) are claimed to be retention and drainageaids in paper manufacturing (3). A poly(aminoamide) dendrimer is used for

© 2010 American Chemical Society

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In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

silica scale control in water technology (4). Poly(aminoamide) resins are usedas adhesion promoters of poly(vinyl chloride) plastisols (5). Poly(aminoamides)with UV-absorbing functionalities are used for protection of skin and hair (6, 7).A polyamido-polyethyleneimine has been claimed to be an adhesive coating forpolyester films (8); the same polymer is used as a retention aid for paper (9). Inbiochemical applications, a hybrid siloxane-poly(aminoamide) has been shownto absorb heparin from blood (10).

In the past chemical pathways via condensation polymerization of monomershave been used for the synthesis of poly(aminoamides) (1). Typically a polyamineand a diacid are heated at high temperatures to conduct the polycondensationreaction. Recently there has been a lot of progress to use enzymes to synthesizepolyamides. This latest development is reviewed in this work.

Lipase-Catalyzed Synthesis of Polyamides

For many years there has been a lot of interest in using enzymes for polyamidesynthesis, but earlier work tended to use protease to produce polypeptides (11–13).It has been found that proteases mostly produce oligopeptides (14), with a fewexceptions (15). Earlier, Gu et al (16) used four proteases (chymotrypsin, trypsin,subtilisin, and papain) in attempts to make polyamide from dimethyl adipate anddiethylene triamine, but only oligoamides were found. There have also beenreports on the use of dipeptidyl transferase (17) and cyanophycin synthetase (18)for peptide synthesis.

An alternative approach is to use lipases (and esterases), some of which areknown to catalyze amide formation under suitable reaction conditions. Prior to2000, there have been several publications on the use of lipases (particularlyporcine pancreatic lipase, PPL) to synthesize dipeptides and tripeptides (19–22).In one of these papers, So et al (22) screened 15 different commercial lipases forthe synthesis of dipeptides from D-amino acids, and found PPL to be the onlyeffective lipase.

In a U.S. patent application filed in 2000 (and granted in 2004) Cheng et al (23)reported that several commercial lipases could be good catalysts for the synthesisof polyamides from diesters and diamines. The polyamides thus produced havemolecular weights in the range of 4,000 to 12,000. For these polymerizations,the reactants were reacted with a lipase either in the absence of solvent, or in thepresence of one or more protic solvents such as methanol, ethanol, ethylene glycol,glycerol, t-butanol, isopropanol, or in a water/salt mixture such as water/NaCl.This patent is the first report of the synthesis of high-molecular-weight polyamidesusing lipase.

In a follow-up work, Gu et al (24) reported the use of lipases to facilitate thesynthesis of a family of poly(aminoamides). The polyamides are made byMichaeladdition reaction of a diamine with an acrylic compound (like methyl acrylate) ina 1:2 molar ratio, respectively, in the first step, and polymerization of additionaldiamine with the resulting diester or diacid prepolymer at 70-140 °C or in thepresence of an enzyme at 60-80 °C.

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In 2005, Azim, Sahoo, and Gross (25) reported the use of immobilized lipaseB from Candida antarctica (Novozym® 435) as a catalyst for the formation ofamide bonds between diethyl esters and diamines under mild reaction conditions.Oligoamides were produced.

In 2005, Panova et al (26) filed a patent application (granted in 2009) wherethey carried out a detailed study using lipases to produce cyclic amide oligomersfrom diesters and diamines. The cyclic amide oligomers are useful for thesubsequent production of higher molecular weight polyamides.

Also in 2005, Kong et al (27) filed a patent application on the preparation ofan aqueous polyamide dispersion by lipase-catalyzed polycondensation reactionof a diamine compound and a dicarboxylic compound in aqueous medium. Ina separate patent application (28), they reported the preparation of an aqueouspolyamide dispersion by lipase-catalyzed reaction of an aminocarboxylic acidcompound in aqueous medium.

Recently, Loos et al (29) reported the synthesis of poly(β-alanine) via lipase-catalyzed ring-opening of 2-azetidinone. After removal of cyclic side productsand low molecular weight species pure linear poly(β-alanine) is obtained. Theaverage degree of polymerization of the obtained polymer is limited to DP=8 byits solubility in the reaction medium. A follow-up work has extended the DP to18 (30).

Design of Poly(aminoamide) Structure

From the point of view of applications, it is useful to vary thepoly(aminoamide) structure in order to optimize the properties. Certainlymolecular weight is an important variable. Another important variable is theamount of amine functionality relative to the number of carbons present in thepolymer backbone. With more amine moieties present, the polymer tends to bemore water-soluble and can have higher charge density. A cationic polymer witha high level of amine content is strong in alkalinity at high pH and possesses alarge amount of positive charges at low pH. Amine groups can be alkylated oracylated with a variety of reactive reagents at alkaline pH. Many applicationsof poly(aminoamides) require the quaternization of the amine (1, 2), or otherderivatizations of the amine functionality (3, 6, 7). The ability to vary the numberof amines versus the number of carbons gives more flexibility in polymer design.For example, there is current interest in creating a comb-like polymer architecturefor biomedical applications (31). More amine functional groups on the polymerbackbone should facilitate the design of such materials.

Some of the poly(aminoamide) structures produced from condensation ofDETA and a diacids or diester are shown in Figure 1. In the case of the well-knownpoly(aminoamide) produced from a condensation of adipic acid and DETA,there is one amine functionality (NH) for every 10 backbone carbons (C) in therepeat unit of the polymer (Structure 1 in Figure 1). Through lipase-catalyzedpolymerization, it is now possible to vary this ratio (NH/C) by using differentstarting materials (23) and different chemistry (24, 32).

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Structures 2 and 3 shown in Figure 1 are the poly(aminoamides) from dialkylfumarate and dialkyl malonate, respectively. The composition for structure2 cannot easily be made via chemical synthesis because Michael additionoccurs at the same time as the polycondensation reaction at high temperatures,thereby resulting in a water insoluble material due to crosslinking of thepolymer. Structure 3 shown in Figure 1 cannot be synthesized chemically at hightemperatures because of other reactions taking place at the polymer chain end thatterminate condensation polymerization. Structure 4 in Figure 1 is the copolymerof DETA with dialkyl malonate and dialkyl oxalate; this structure cannot bemade chemically as well. Nevertheless, all four poly(aminoamides) 1-4 shownin Figure 1 can be synthesized readily via lipase-catalyzed polycondensationreactions between a polyamine (e.g., DETA) and a diester. Structure 5 in Figure1 can be made via a two-step synthesis between DETA and methyl acrylate. Thesynthetic details are given in the next section (Experimental Considerations).

An additional handle in structure design is the use of triethylene tetraamine(TETA) and tetraethylene pentaamine (TEPA). The structures of the polyamidesmade with dialkyl adipate are shown (structures 6 and 7) in Figure 2.Enzymatic synthesis of TETA and TEPA with dialkyl malonate, dialkyl oxalate,dialkyl fumarate, or methyl acrylate can potentially produce many morepoly(aminoamides) (23, 24). These structures can provide poly(aminoamides)with an even larger range of NH/C ratios.

In addition to these compositions, other related polyamides with unique andinteresting chemical and physical properties can also be synthesized in a similarfashion (23, 24). Two examples are shown in Figure 3.

Thus, lipase catalysis enables many new polyamide structures to be made.The reactions described herein are effective and entail mild reaction conditionsand less byproducts. These are good applications of green polymer chemistry.

Experimental Considerations

The above structures can be produced through the following syntheticprocedures. For ease of reference, typical procedures are given below. Moreinformation is available in the original patents (23, 24).

Lipase-Catalyzed Polymerization of Aliphatic Diester and Polyamine

This procedure (23) can be used for the synthesis of Structures 1, 2, 3, 4, 6,and 7. The polyamine and diester monomers are oligomerized and then reactedat a mild temperature, in the presence of enzyme, to allow polymerization of theoligomers. The reaction product is dissolved in an aqueous solution such as wateror alkyl alcohol (e.g., methanol), and the enzyme is removed via filtration. Thisprocess allows polymerization of reactants under mild conditions to provide high-molecular-weight reaction products with a relatively narrow molecular weightdistribution. In addition, the reaction products are relatively pure due to the useof enzyme and substantial absence of solvents. Further still, the mild conditions

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Figure 1. Poly(aminoamide) from DETA and diacid or diester

Figure 2. Poly(aminoamides) from polyamine and diacid and diester

prevent denaturation of the enzyme catalysts, and allow them to be optionallyrecycled for further use.

In a typical procedure, dimethyl adipate (43.55 g, 0.25 mol), diethylenetriamine (28.33 g, 0.275 mol) and Novozym® 435 lipase (2.5 g) are mixed in a250-ml flask. The reactants are then heated in an oil bath to 90°C in an open vesselwith a stream of nitrogen Figure 4. Completion of the reaction is indicated by theappearance of a yellowish solid. Methanol (150 ml) is then added to dissolve thepoly(aminoamide) product. The immobilized enzyme is insoluble in the methanolsolution and is removed by filtration. Remaining methanol is removed by a rotaryevaporator under low pressure. The final product is a yellowish solid with a yieldof 48 g, Mw of 8,400 Daltons, and Mw/Mn of 2.73.

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Figure 3. Unconventional polyamide compositions

Figure 4. Enzyme-catalyzed synthesis of poly(aminoamides)

Lipase-Catalyzed Polymerization of Polyamine and Methyl Acrylate

This procedure (24, 32) can be used for the synthesis of Structure 5 in Figure1. As shown in Figure 5, the process used for the current synthesis consists of twodiscrete steps. In the first process step, exactly one mole of polyamine molecule(such as DETA) is gradually added to two moles of alkyl acrylate (such as methylacrylate) to form an amine-containing diester in the absence of a solvent (Michaeladdition). The reaction vessel should be cooled through suitable means becausethe reaction is exothermic. The reaction temperature for this step can be 10-60°C,preferably 15-40°C, and most preferably 20-30 °C. The addition of water to thereaction mixture enhances the rate of the Michael addition.

In the second process step, amidation of the diester with another mole ofeither the same polyamine or a different polyamine gives a high-molecular-weightpolyamide. This reaction can be achieved at 60-70°C with the assistance of alipase as the catalyst. Two preferred lipases are those from the yeast Candidaantarctica (e.g., Novozym® 435) and Rhizomucor miehei (e.g., Palatase®), bothfrom Novozymes A/S.

Alternately, the second step of the reaction can be achievedwithout an enzymeby heating up the reaction mixture to 120-140°C for several hours.

In a typical procedure, methyl acrylate (43.05g, 0.5 mol) is gradually addedto DETA (25.84g, 0.25 mol) at 20°C, and the temperature is gradually increased to40°C with stirring. The addition took about 30 minutes, and the reaction mixturewas stirred further at 24°-30°C for about 60 minutes, whereby the intermediatepre-polymer reaction product was formed. Another portion of DETA (25.84g,0.25 mol) was added, followed by the addition of 4 grams of immobilized lipaseCandida antarctica (Novozym® 435). The reaction mixture was stirred at 65°Cfor 16 hours. The viscous product was dissolved in 100mL of methanol at 65°C,and the immobilized enzyme was removed by filtration. The yield was 75 grams.

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In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Figure 5. Poly(aminoamide) synthesis via a combination of chemical andenzymatic approaches

The molecular weight (Mw) of the final product, based on SEC analysis, was 8,450Daltons and the polydispersity (Mw /Mn) was 2.75.

Lipase-Catalyzed Polymerization of DETA and Phenylmalonate

The synthetic procedure for Structure a in Figure 3 is given here. Diethylphenylmalonate (23.6 g, 0.10 mol), diethylene triamine (10.3 g, 0.10 mol) andNovozym® 435 lipase (1 g) are mixed in a 500-ml flask and heated in an oil bathto 100 °C. The viscous mixture is stirred at 90-100 °C for 24 hrs in an open vesselwith a stream of nitrogen. The mixture solidifies at the end of the reaction. Theproduct is not soluble in most organic solvents and in water at neutral pH. It issoluble in water at pH 3. 150 ml of water are added and the pH is adjusted to 3by adding concentrated HCl. The immobilized enzyme (being insoluble in water)is removed by filtration. The aqueous solution is lyophilized to give the productas a white solid. The yield is 26.9 grams. The molecular weight (Mw) of the finalproduct, based on SEC analysis, is 3600 Daltons and the polydispersity (Mw /Mn)is 2.70.

Polymerization of Dimethyl Adipate and Triethylene Glycol Diamine

The synthetic procedure for Structure b in Figure 3 is described here.Dimethyl adipate (17.42 g, 0.10 mol), triethylene glycol diamine (15.60 g, 0.105mol) and Novozym® 435 lipase (1.0 g) are mixed in a 250-ml open vessel. Thereactants are heated in a stream of nitrogen in an oil bath to 70°C for 24 hourswith stirring. The reaction mixture is then cooled and provides a viscous product.Methanol (100 ml) is added to dissolve the viscous product. The immobilized

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enzyme is insoluble in methanol and is removed by filtration. The remainingmethanol in the reaction mixture is removed by a rotary evaporator under lowpressure. The final product is a semi-solid with a yield of 28 grams. The molecularweight (Mw) of the final product, based on SEC analysis, is 4,540 Daltons and thepolydispersity (Mw /Mn) is 2.71.

Conclusion

Enzyme catalysis has been used to produce many poly(aminoamides).These polymers can be used as is as polyeletrolytes, or functionalized furtherto produce specialty polymers. The advantages of the enzymatic processes(relative to the chemical processes) are: 1) lower process temperature, therebydecreasing energy usage, 2) narrower molecular weight distributions of theproducts, 3) less branching in the products, 4) enzymatic processes allowing somepoly(aminoamides) that cannot be synthesized chemically to be made, e.g., thepolyamides derived from dialkyl malonate, malonate/oxalate, phenylmalonate,fumarate, and maleate. A disadvantage is the cost of the enzyme used, whichcan be partly mitigated if the enzyme is immobilized and recycled (and this ispossible in the case of Novozym® 435 lipase).

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