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Table of Contents

A. Khan, S. K. Sharma, A. Kumar,A. C. Watterson, J. Kumar, V. S. Parmar*

379 – 390

Novozym 435-Catalyzed Syntheses ofPolyesters and Polyamides ofMedicinal and Industrial Relevance

Walk around green approaches:ACHTUNGTRENNUNGEcofriendly and efficient synthetic ap-proaches involving chemoenzymaticmethodologies based on the principlesof “green chemistry” for waste reduc-tion are discussed. These lead to theformation of unique nanomaterials andbiomaterials for diverse applications,such as drug/gene delivery systems,flame retardant materials, conductingpolymers, controlled release systems,diagnostic agents, and polymeric elec-trolytes for nanocrystalline solar cells.

DOI: 10.1002/cssc.201300343

Novozym 435-Catalyzed Syntheses of Polyesters andPolyamides of Medicinal and Industrial RelevanceAbdullah Khan,[a] Sunil K. Sharma,[a, b, c] Amit Kumar,[a] Arthur C. Watterson,[b] Jayant Kumar,[c]

and Virinder S. Parmar*[a, b, c]

Dedicated to Professor Goverdhan Mehta, FRS on the occasion of his 70th birthday.

1. Introduction

Polymeric nanoassemblies, such as micelles of various mor-phologies, torsoidal assembled polymersomes, nanofibers, andnanoscale tubes have attracted considerable attention inrecent years. These self-organized materials ranging from thenano to the micro scale have found broad applications inareas such as bioengineering, biomedicine, cosmetics, materi-als science, and pharmaceutics.[1–6] Among these diverse appli-cations, nanostructured polymeric assemblies for drug deliveryand gene therapy are of special interest.[7, 8] Several formula-tions based on polymeric micelles have been extensively stud-ied for cancer therapy and their efficacy has been well demon-strated.[9] Recent advances have made the delivery of thera-peutic agents such as small molecules, peptides, proteins, plas-mid DNAs and siRNAs possible in aqueous media.[10, 11] Further-more, nanoparticle formation and their size can be controlledin an environment-friendly manner.[12] Also considering thechemical, economic, and social advantages of biocatalysis overtraditional chemical approaches, biotechnology holds tremen-dous opportunities for realizing functional polymeric materials.Biocatalytic pathways to polymeric materials are an emergingresearch area with not only enormous scientific and technolog-

ical promise but also with a tremendous impact on environ-mental issues. In recent years, some interesting reviews[13] andbooks[14] have been published that give a useful introductionto the field of enzymatic polymerization. Among the enzymesused successfully for polymer synthesis, Candida antarcticalipase B (CAL-B) is by far the most well-known enzyme in litera-ture. In most of the enzymatic polymerizations reported, it isused as an immobilized enzyme because of additional advan-tages, for example, ease of separation and its robust nature.Novozym 435 is a commercially available heterogeneous bio-catalyst that consists of CAL-B physically immobilized withina macroporous resin of poly(methyl methacrylate) and is mar-keted by Novozymes.[15] Owing to its in vitro transesterificationpotential, Novozym 435 has been extensively utilized for syn-thesizing polyester architectures. In this Minireview, we havesummarized some of the most useful enzymatic polymerizationreactions forming amphiphilic polyesters and polyamides forvarious applications.

2. Nanomaterials for Drug and Gene Delivery

The search for new drug-delivery approaches and new modesof action are the major driving forces in polymer therapeutics.Pharmaceutical and biotech startup companies are engaged inthe development of drug-delivery systems (DDSs) for new aswell as already existing drugs.[16] Targeted and controlled DDSsensure the commercial success of these bioactive molecules interms of stability, absorption, easy metabolic inactivation, andneed to cross cell and nuclear membranes to reach intracellu-lar targets. Polymers that can self-assemble into micellar nano-particles can be effectively used as vehicles for drug delivery.[17]

Our interest has been in synthesizing amphiphilic polyestersand polyamides that aggregate in aqueous media and thusform nanospheric particles, the surface of which results ina nonimmunogenic response; therefore, these nanospheres

The adverse impact of chemical and biochemical waste on theenvironment and human health poses a serious challenge intoday’s World. The best way to address these challenges is toreduce the waste by developing more efficient processes andtechnologies, based on the principles of “green chemistry”.Some of these synthetic approaches involving the chemoenzy-

matic synthetic methodologies are discussed herein. Theselead to the formation of unique nanomaterials with diverse ap-plications, such as drugs/gene delivery systems, flame retard-ant materials, conducting polymers, controlled release systems,diagnostic agents, and polymeric electrolytes for nanocrystal-line solar cells.

[a] A. Khan, Prof. S. K. Sharma, A. Kumar, Prof. V. S. ParmarBioorganic LaboratoryDepartment of ChemistryUniversity of DelhiDelhi-110 007 (India)E-mail : [email protected]

[b] Prof. S. K. Sharma, Prof. A. C. Watterson, Prof. V. S. ParmarDepartment of ChemistryInstitute of Nano-Science and Engineering Technology(INSET)University of Massachusetts LowellLowell, MA 01854 (USA)

[c] Prof. S. K. Sharma, Prof. J. Kumar, Prof. V. S. ParmarCenter for Advanced Materials (CAM)University of Massachusetts LowellLowell, MA 01854 (USA)

Ó 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2014, 7, 379 – 390 379

CHEMSUSCHEMMINIREVIEWS

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Highlight

can be used as carriers for drugdelivery. Initially we attemptedto synthesize polyesters basedon dimethyl 5-hydroxyisophtha-late and polyethylene glycol(PEG) moieties through transes-terification–polycondensationreactions using dibutyltin diace-tate as catalyst (Scheme 1).[18]

According to control experi-ments, a C-5 hydroxyl moietyinhibits the reaction and itsprotection became necessary.However, prior protection of thephenolic moiety restricts thefreedom of post polymerizationmodification. Also very harsh re-action conditions (330 8C, undervacuum, toxic tin-based catalyst)had to be adopted; dark coloredoligomers were obtained thatwere contaminated with the tincatalyst, which was difficult to remove completely.

Our experience in using lipases to achieve chemo-, regio-,and enantioselectivity on a variety of substrates[19–26] encour-aged us to develop an enzymatic route for the synthesis ofpolymers. We have successfullyused Novozym 435 to synthesizecopolymers of dimethyl 5-hy-droxyisophthalate and PEGs ofdifferent sizes through transes-terification (Scheme 2) under sol-ventless conditions.[27] Thisgreener approach has an addedadvantage of post-polymeri-zation modification (Scheme 2)as the enzymatic polymerization

is feasible without protecting the hydroxyl moiety (C-5 OH) ofthe isophthalate moiety. Thus, polymers obtained by this bio-catalytic method can be used for the delivery of drugs, pro-teins, or polysaccharides as these can be easily attached or en-

capsulated. Moreover, the alkoxy chain at the C-5 position ofthe isophthalate moiety tends to retard the polymerizationwith increase in size of the alkyl chain and no polymerizationwas observed when the alkyl chain was longer than eightcarbon atoms.[28] The aggregation behavior studied by meansof 1H NMR longitudinal relaxation time (T1) and light-scatteringdata revealed that polymers with alkyl chain lengths of C9–C12

form nanospheric particles of about 20–50 nm with a hydro-phobic core surrounded by hydrophilic PEG units (Figure 1 and2).[29, 30] Data from toxicity studies suggest that polymer 3 isnot toxic (uptake up to 60 g per kg body weight) and that itcan be used for both transdermal and oral delivery of medici-nally active materials.[30]

The drug delivery potential of amphiphilic polymeric nano-spheres was studied by encapsulating the anti-inflammatoryagents aspirin and naproxen. The application of these encap-sulated materials resulted in a significant reduction in inflam-mation in a mouse model. This nanosphere-mediated encapsu-

Scheme 1. Copolymerization of PEG and dimethyl 5-hydroxyisophthalate.

Scheme 2. Chemoenzymatic copolymerization of PEG and C-5-substituted isophthalates.

Virinder S. Parmar received his B.Sc.

(1968), M.Sc. (1970), and PhD (1978)

from the University of Delhi and has

Postdoctoral/Visiting Scientist research

experience of nearly ten years in sever-

al US American and European Universi-

ties. He joined the University of Delhi

as lecturer in chemistry at St. Ste-

phen’s College in 1970 and was ap-

pointed Reader in the Department of

Chemistry in 1984 and Professor of

Chemistry in 1996. He has served the

University of Delhi as Chairman of the Board of Research Studies in

Science during November 2007 to August 2008 and as Head of the

Department of Chemistry from May 2007 to April 2010. His re-

search interests include nanotechnology, synthetic organic chemis-

try, bio-catalysis, nucleic acid chemistry, medicinal chemistry, green

chemistry, advanced materials and chemistry of natural products.

Figure 1. Functionalized PEG–(5-hydroxyisophthalate) copolymers.


CHEMSUSCHEMMINIREVIEWS www.chemsuschem.org

lation and delivery of equal doses of aspirin and naproxenhave improved drug efficacy by a factor of 1.8–2.0 comparedto their aqueous preparations (Figure 3).[30]

An encapsulation study of a series of aromatic guests withvaried electron donor/acceptor properties demonstrated thatelectronic complementarity between polymer and encapsulat-ed drug contributed significantly towards encapsulation.[31] Theeffect of hydrophilic block, PEG length, linker, concentration,and temperature on these nanomicellar structures and interac-tions by static light scattering techniques has been studied.[32]

In addition, amphiphilic polymers (5) were synthesized by co-polymerization of 5-aminoisophthalate/5-hydroxyisophthalateand PEG followed by conjugation of acyl chains as amidic/ester linkages to the 5-NH2/5-OH moieties, respectively(Figure 4).[33]

2.1. Bio-catalytic synthesis of pluronic- and guanidine-basedpolymers

A poly(ethylene oxide)–poly(propylene oxide) (PEO–PPO–PEO)tri-block copolymer family of pluronics with different numbersof hydrophilic and hydrophobic units were synthesized andcharacterized by us. Polymers 6 and 7 (Figure 5) could be usedto encapsulate hydrophobic drugs, for example curcumin, inthe range of 2.7–5.7 % with regard to polymer weight, thus en-hancing its aqueous solubility.[34, 35] Results from in vitro andanimal studies suggested that curcumin has antitumoral, anti-

oxidant, antiarthritic, antiamyloid,anti-ischemic, and anti-inflamma-tory properties. However, its lowaqueous solubility delays its po-tential use. Thus, our study maybe useful in enhancing the bio-availability of curcumin.

Arginine-based compoundshave been reported to enhancecellular permeability and interac-tion with nucleic acids caused bythe presence of a guanidinemoiety. We have developeda guanidine-based polymericsystem 8 (Scheme 3) and such

systems may serve as gene/small interfering ribonucleic acid(siRNA) delivery vehicles.[36]

2.2. Biocatalytic synthesis of nonproteinogenic amino acid/amino acid diesters and PEG-based copolymers

The high cellular permeability and chirality of amino acidsprompted us to use them as building blocks for polymer syn-thesis.[37] We have performed the copolymerization of nonpro-teinogenic amino dicarboxylic acids with PEG (Mn = 600) inbulk. The free amino moiety of the copolymers was then func-tionalized by using acyl chloride, resulting in the formation ofthe amphiphilic polymers 9[37] and 10[38] (Figure 6).

Figure 2. Self-assembly of the amphiphilic polymers 3 in aqueous media toform polymeric nanomicelles.

Figure 3. Anti-inflammatory properties of PEG nanospheres containing a) aspirin, and b) naproxen.

Figure 4. Functionalized 5-hydroxy and 5-amino moieties of the basepolymer.

Figure 5. Pluronic-based polymers.



2.3. Biocatalytic synthesis of glycerol-PEG co-polymers andpolyglycerol esters

Glycerol exhibits good chemical stability and inertness underbiological conditions.[39] Our initial efforts to utilize glycerol re-sulted in a highly efficient chemoenzymatic approach to syn-thesize aliphatic polyester/polyamide ester dendritic buildingblocks by using structured triacyl glycerol (TAG); this approachcould be successfully applied to a variety of cores (Scheme 4).The synthesis of a unique classof structured TAG-based star-shaped and linear dendriticbuilding blocks was achieved byusing Novozym 435.[40] The car-boxylic moiety of the TAG esters12 and 13 was used to couplethe esters with hydroxyl/aminofunctionalities of various coremoieties (Scheme 5).

Following this, we subse-quently developed a biocatalyticmethod to synthesize polymericsystems using glycerol and PEGdimethyl ester[41] regioselectivelythrough reaction of the primaryhydroxyl moieties of glycerol,thus leaving the secondary hy-droxyl moieties available forpost-polymerization chemicalmodifications through attach-ment of alkyl chains simply byacylation (Scheme 6). The amphi-

philic polymers 18 a–c aggregat-ed in aqueous media to formnanosized particles. We success-fully attempted the encapsula-tion of vitamin E in these amphi-philic polymers (18 a–c) up to22 % with regard to polymerweight. This study shows prom-ise in enhancing the bioavailabil-ity of vitamin E, a well-knownlipophilic antioxidant; its limitedaqueous solubility severely ham-pers its antioxidant efficiency.

Recently, a new class of non-ionic dendronized multiamphiphilic polymers have also beenprepared by us starting from a biodegradable (AB)n-type di-block polymer synthesized from 2-azido-1,3-propanediol (azidoglycerol) and polyethylene glycol (PEG-600) diethyl ester usingNovozym 435 as a biocatalyst (Scheme 7). These polymers arefunctionalized with dendritic polyglycerols (G1 and G2) and oc-tadecyl chains at different functionalization levels through clickchemistry.[42] Surface tension measurements and dynamic lightscattering studies revealed that all of the multiamphiphilicpolymers spontaneously self-assemble in aqueous solution.Cryogenic transmission electron microscopy further proves theformation of multiamphiphiles as monodisperse spherical mi-celles of about 7–9 nm in diameter. The evidence from UV/Visand fluorescence spectroscopy suggests the effective solubili-zation of hydrophobic guests such as pyrene and 1-anilino-naphthalene-8-sulfonic acid within the hydrophobic core ofthe micelles.[42]

We have also explored the chemoenzymatic modification ondendritic hyperbranched polyglycerol (dPG) that led to amphi-philic polymeric architectures with easily hydrolysable ester

Scheme 3. Synthesis of guanidine-functionalized polymers. DIPEA = diisopropyl ethylamine; DCM = dichlorome-thane; FMoc = fluorenylmethyloxycarbonyl; DMF = N,N-dimethyl formamide.

Figure 6. Copolymerization of non-proteinogenic amino acid/amino acid die-sters and PEG.

Scheme 4. Chemoenzymatic synthesis of structured TAG 12–14. DMAP = 4-dimethylaminopyridine.



linkages. The Novozym 435-catalyzed pegylation of dPG oc-curred regioselectively at the primary hydroxyl moieties in thepresence of secondary hydroxyl moieties (Figure 7). The re-maining hydroxyl moieties were acylated to obtain amphiphilicdPG architectures. The amphiphilic polymeric architectures 26–28 had transport capacities for guest molecules, thus demon-strating their suitability for the solubilization of hydrophobicdrugs. These architectures were studied for Nile Red solubiliza-tion, which showed a capacity of up to 5.6 mg L¢1 at 0.1 wt %polymer concentration. The release of Nile Red from thesepolymers was observed with a half-life of 8 h at pH 5.0, where-

as no release was found atpH 7.4. The cell viability studiesof our polymeric architecturesshowed them to be relativelynontoxic.[43]

We have developed a highlyefficient temperature-dependentchemoenzymatic methodologyfor the regioselective synthesisof glycerol esters, G1 triglyceroldendrons and related esters forthe first time using 4-nitrophenyl2-(tert-butoxycarbonyl) acetate(Boc-gly-Ph-pNO2) as the acylat-ing agent. The immobilizedlipase Lipozyme TL IM in dioxanewas the most efficient biocata-lyst for the regioselective trans-esterification on glycerol and af-forded the mono- and di-esteri-fied products 29 and 30(Figure 8).[44] The regioselectivityachieved in case of glycerol wasthen extended to bifunctionalG1 glycerol dendrons bearingfour hydroxyl moieties (two pri-mary and two secondary hydrox-yl moieties). It was demonstratedthat glycine loading occurred se-lectively on the primary hydroxylmoieties and that the secondaryhydroxyl moieties remained un-affected (Figure 8).[44]

Encouraged by these results,multivalent polyglycerol-den-dron-based amphiphiles withwell-defined molecular struc-tures expressing controlled gly-cine arrays on their surfaceswere synthesized. The structure–activity relationships with re-spect to siRNA/DNA complexa-tion, toxicity, and transfectionprofiles with the synthesizedpolycations were recorded. Asecond-generation amphiphilic

dendrimer (G2-octaamine) with eight amine moieties on itssurface and a hydrophobic C18 alkyl chain at the core acted asan efficient vector to deliver siRNA inside the cell and achievedpotent gene silencing as demonstrated by the knockdown ofnormalized luciferase activity and also for glyceraldehyde 3-phosphate dehydrogenase in HeLa cells. The amphiphilicvector is nontoxic even at a higher ratio of N/P 100 both in vi-tro and in vivo.[45]

Scheme 5. Synthesis of G1 dendrimers from structured TAG. EDC = 1-ethyl-3-(3-dimethylaminopropyl)carbodii-mide.

Scheme 6. Chemoenzymatic synthesis of amphiphilic glycerol–PEG-based copolymers.



2.4. Synthesis of amphiphilic polymers for magnetic reso-nance imaging and selective targeting in cancer therapy

The use of fluorescent dyes andperfluoro compounds for imag-ing and selective targeting iswell known. To enhance the che-motherapeutic efficiency and se-lectivity for binding, we have at-tached a peptide to polymer 35through a hydrophilic linker (tri-ethylene glycol). Subsequentlyfluorescent dyes, a perfluoroalkylmoiety (for 19F NMR imaging),and a hydrophobic side chainwere attached by performingsimple chemical modifications(Figure 9). Initial in vitro studiesindicate cellular uptake of thesenanocarriers ; radioactive labelingand analysis have shown someselectivity for targeted (pancreat-ic) cancer cells over non-target-ed cells.[46]

Perfluorinated amphiphilicpolymers were also synthesizedby using a chemoenzymaticmethodology (Scheme 8). The

supramolecular organization of polymer 36 in aque-ous and organic media was studied and observed toform nanomicelles (in the range of 50–60 nm at25 8C) in aqueous media; however, no micellizationoccurred in organic media.[47]

Recently, similar carrier molecules 37 a–c and38 a–c were synthesized and studied for multiple ap-plications, including drug encapsulation, drug deliv-ery, and disease diagnosis (imaging) (Figure 10).These polymers showed 4–14 % curcumin encapsula-tion in water.[48]

2.5. Biocatalytic route to sugar-PEG-based poly-mers for drug delivery

Sugar-PEG-based polymers were synthesized by enzy-matic copolymerization of 4-C-hydroxymethyl-1,2-O-isopropylidene-b-l-threo-pentofuranose/4-C-hydroxy-methyl-1,2-O-benzylidene-b-l-threo-pentofuranose/4-C-hydroxymethyl-1,2-O-isopropylidene-3-O-pentyl-b-l-threo-pentofuranose with PEG-600 dimethyl esterusing Novozym 435 (Figure 11).

Results of aggregation studies on the copolymersrevealed that in aqueous solution, those polymersbearing a hydrophobic pentyl/benzylidene moiety(39–41) spontaneously self-assembled into supra-molecular aggregates. The polymeric aggregateswere further explored for their drug encapsulationproperties in buffered aqueous solutions of pH 7.4(37 8C) using Nile Red as a hydrophobic model com-pound by means of UV/Vis and fluorescence spec-troscopy.[49]

Scheme 7. Biocatalytic synthesis of dendronized multi-amphiphilic polymers 25 a–f.

Figure 7. Biocatalytic synthesis of amphiphilic hyperbranched polyglycerol 26–28 and its Nile Red encapsulation/release.



3. Biocatalytic Synthesis of PEGylated Curcu-min Block Copolymers

Curcumin is known as one of the Nrf2 activators, which isa central transcription factor regulating the antioxidant de-

fense system and acts as a modi-fier for several inflammatory dis-eases. Curcumin is used as a di-etary supplement, but its hydro-phobic nature renders it ineffec-tive.

To circumvent these issues,PEG–curcumin copolymers weresynthesized and evaluated aspotent Nrf2 activators. Amongthe copolymers 42 a–d, copoly-mer 42 a predominantly activat-ed Nrf2-driven antioxidant geneexpression (Figure 12). Thisstudy opens a new path for en-hancing the efficacy of variousexisting hydrophobic drugsthrough their PEGylation.[50]

4. Biocatalytic Synthesisof PEGylated CoumarinBlock Copolymers

Coumarins constitute an impor-tant group of natural productsbelonging to the flavonoidfamily.[51, 52] To enhance their bio-availability, potent antioxidantcoumarins were encapsulated aswell as covalently attached tobase polymers. These newclasses of polymeric materialshave superior antioxidant prop-erties in comparison to the start-

ing monomers. We also synthesized coumarin-and-PEG-basedblock copolymers (Scheme 9).[53] These PEGylated coumarin de-rivatives were evaluated for their anti-inflammatory activitieswith respect to their ability to inhibit the tumor necrosis factor

Figure 8. Lipozyme-catalyzed transesterification of glycerol and G1 glyceroldendron.

Figure 9. Amphiphilic polymers for MRI and selective targeting.

Scheme 8. Synthesis of perfluorinated polymer 36.

Figure 10. Amphiphilic polymers for theranostic applications.



TNF-a induced intercellular cell adhesion molecule-1 (ICAM-1)expression on human endothelial cells.[54] Both PEGylated 4-methyl- and 4,8-dimethylcoumarins have shown improved abil-ity to inhibit the TNF-a induced ICAM-1 expression in compari-son to the corresponding monomers.

Coumarin derivatives have also been extensively investigat-ed for electronic and photonic applications. We have used cou-marins for sensor (fluorescence-quenching sensors) applica-tions, that is, detection of nitro aromatics/explosive materials.However, coumarins themselves are not suitable for the prepa-ration of solid devices because of their low molecular weight

and aggregation in solid thin films, which reduces the fluores-cence quantum yield. To overcome this problem, we have co-polymerized the diester of 4,8-dimethylcoumarin with PEG andpolydimethylsiloxane (PDMS; Figure 13). The obtained poly-mers 44 and 45 have good solubility in a wide range of sol-vents, thus making them suitable candidates for thin film fabri-cation.[55]

5. Biocatalytically Generated Polysiloxane-Based Copolymers and their Nanocompositesas Flame Retardants

Flame retardants (FRs) comprise a diverse group of chemicalswidely used at relatively high concentrations and have manyapplications, including in the manufacture of electronic equip-ment, textiles, plastics, and polymers as well as in the aviationand automobile industries.[56] Owing to environmental con-cerns in the synthesis of nanocomposites and polymer/layeredsilicate composites, we used biocatalysts to synthesize FR ma-terials.[57] 5-Hydroxy- and 5-aminoisopthalates were polymer-ized with siloxane and evaluated for FR properties(Scheme 10). In addition to this, siloxane-based aliphatic poly-amides were also synthesized (Scheme 11).

The flammability of copolymers 46 a and b is comparable tothat of Kevlar or polyether ether ketone (PEEK), two commer-cial products of DuPont. Polymers 47 b and c may be used forapplications that require ultra-fire-safe polymers, as the heat-release capacity of these polymers is under 100 J g¢1 K¢1.

Furthermore, we have prepared composites of titanium di-oxide (TiO2) nanoparticles and biocatalytically synthesized di-

methyl siloxane co-polyamidesand co-polyesters, and evaluatedtheir thermal and FR proper-ties.[58, 59] A number of other si-loxane polymers, siloxane copo-lyimide (siloximide E) 48(Scheme 12) and siloxane co-polymers 49, with coumarin inthe backbone were also synthe-sized (Figure 11) for FR applica-tions.[60–62]

Figure 11. Synthesis of sugar–PEG-based polymers.

Figure 12. Amphiphilic PEG–curcumin conjugates.

Scheme 9. Synthesis of PEGylated C-4 methylcoumarins.

Figure 13. Copolymer of 4,8-dimethyl coumarin with a) PEG and b) PDMS.



In another study, cross-linking of enzymatically synthesizedpolydimethyl siloxane copolymers with aromatic dianhydridesyielded the cross-linked polymer 51 as cross-linked FR(Scheme 13).[63]

6. Polymeric Electro-lytes for NanocrystallineSolar Cells

Dye-sensitized solar cells(DSSCs) offer the advantage ofsignificant reduction in the costof production of solar electricityowing to the inexpensive rawmaterials and simple fabricationprocess involved in the produc-tion of DSSC-based solar mod-ules. An alternative approach fo-cused on the development of

polymeric or quasi-solid matricesfor efficient operation of alreadywell-known redox electrolytesfor DSSCs.[64–71] The chain mobili-ty and ionic conductivity of thepolymer electrolyte can be in-creased by adding organic sol-vents and polymer gellingagents to the liquid electrolyteto promote its solidification.[70, 71]

6.1. Biocatalytic approach forpreparing quasi-solid electro-lyte systems for DSSCs andtheir photovoltaic performance

Quasi-solid electrolytes preparedfrom the biocatalytically syn-thesized polymers 52 a–c(Scheme 14) by adding at least25 wt % polymer to an ionic-liquid-based electrolyte showeda photovoltaic (PV) efficiency ofaround 4.3 %; the measuredionic conductivity of the formu-lation used in these devices wasapproximately 2 Õ 10¢5 S cm¢1.[72]

Studying the intensity-depen-dent PV efficiency suggestedthat the best PV performancewas achieved at around 0.5 Sun(50 mW cm¢2) intensity level. Anefficiency of over 4.6 % was ach-ieved by a polymer 52 c-basedgel-incorporated flexible cell at55 mW cm¢2, which is about10 % higher than the 1 Sun con-dition.[72] The solar conversion ef-

ficiency of solar cells incorporating quasi-solid electrolytes de-pended strongly on the polymer’s microstructure used in for-mulating the redox electrolyte. Continuing this work further,we synthesized new polymers and these polymeric materials

Scheme 10. Silicone-based aromatic polyesters and polyamides.

Scheme 11. Silicone-based aliphatic polyamides.

Scheme 12. Siloxane-based polyimides.

Scheme 13. Cross-linking of polydimethyl siloxane copolymers with aromatic dianhydride.



showed photovoltaic efficiency of up to 9.0 % in the laborato-ry.[72]

6.2. Biocatalytic synthesis and ion-transport properties ofPEGylated polyphenolics

We used Novozym 435 to catalyze the highly chemoselectivemonoacylation of the alcoholic hydroxyl moiety of 4-hydroxy-methylphenol with PEG diacid under solvent-less conditions(Scheme 15). The resulting acyloxy macromer 53 was thenpolymerized using horse radish peroxidase (HRP)[73] to form thePEGylated poly(hydroxymethylphenol) 54.[74]

7. Miscellaneous Applications

Various amphiphilic copolymers were also synthesized thatself-assemble into nanomicellar aggregates in aqueous mediaand were used for the encapsulation and controlled release ofcarbofuran, a systemic insecticide–nematicide (Figure 14).[75–77]

The chemoselectivity shown by biocatalysts was also utilized in

synthesizing serinol-based surfactants[78] avoiding the protec-tion/deprotection chemistry.

8. Conclusions and Outlook

Use of enzymatic and chemoenzymatic methods for the prepa-ration of polymeric structures has expanded rapidly in recentyears as the commercial availability of enzymes has increaseddramatically in the same time period. In addition to selectivity,factors such as energy reduction with lower temperature of re-action, reduction of toxic solvent use, and reuse of catalyst areadditional advantages of enzymatic reactions. The stereo-,regio-, and chemoselectivity of enzymes observed in small-molecule reactions have also been observed in the synthesis ofpolymeric materials. But the applications of biocatalysis inpolymer science still lag behind the use of biocatalysts in otherareas.

Our efforts to exploit the array of lipases to develop newmethodologies, reactions, and processes in polymer synthesishave led to the identification of Novozym 435 being theenzyme of choice, with a wide versatility for the synthesis ofpolyesters and polyamides. The successful and easy prepara-tion of these polymers allows the application of our method tothe easy synthesis of many functionalized polymeric materials.However, for biomedical applications, the challenge still re-maining is the optimization of biocatalytic synthesis of amphi-philic polymers that form aggregates with sizes of 20–100 nm,show a high targeting potential, and can be cleared by the kid-neys (molecular weight cut-off ~40 kDa). Furthermore, newchemoenzymatic approaches to prepare polyether architec-tures need to be addressed in the future; this may open newarenas to enhance the versatility of polyether-based amphiphil-ic scaffolds. Thus, significant research is still needed to expandthe possibility of using enzymes to modify and tailor polymericarchitectures to fit future demands of the biomedical and in-dustrial sectors.

Acknowledgements

We thank all PhD students and postdoctoral fellows, in particularDr. Rajesh Kumar (who was an instrumental initiator that led tothe foundation of the work included in all areas in this manu-script), Dr. Mukesh Pandey, and Dr. Ravi Mosurkal for their greatcontributions, inspiring and dedicated hard-work in obtaining all

Scheme 14. Chemoenzymatic PEGylation of the biocatalytically derived basepolymer 2. DIEA = diisopropyl ethylamine.

Scheme 15. Biocatalytic synthesis of PEGylated macromer 53 and its oxida-tive polymerization using HRP.

Figure 14. Copolymers used for carbofuran encapsulation.



the results in our laboratories in the past thirteen years. Wewould also like to thank all colleagues at various institutionsworldwide included in the publications/research papers men-tioned in the “Reference Section”, for their active collaborationsand contributions, in particular Dr. Lynne Samuelson and Dr.Ashok Cholli at the University of Massachusetts Lowell (UML, MA,USA), Professor Ashok Prasad at the University of Delhi (India),and Professor Dr. Rainer Haag at the Free University of Berlin(Germany). S.K.S. and V.S.P. thank the Department of Biotech-nology (DBT, New Delhi, India), Department of Science and Tech-nology (DST, New Delhi, India) and the University of Delhi for fi-nancial assistance under the DU-DST PURSE Scheme and DBT-CREST award to S.K.S. . A.K. thanks the Council of Scientific andIndustrial Research (CSIR, New Delhi, India) for the award ofJunior and Senior Research Fellowships.

Keywords: biocatalysis · micelles · nanomaterials ·photovoltaic cell · polymerization

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Received: April 16, 2013

Published online on January 21, 2014



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