Anna MIELAŃCZYK, Dorota NEUGEBAUER ? Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, Gliwice, Poland

Abstract:
This article is devoted to carbohydrates utilization, in particular acting as initiators, to obtain synthetic polymers using controlled methods, including radical polymerization (CRP). The use of CRP nables the synthesis of macromolecules with different compositions and complex architectures, whereas control over their structures is maintained. Moreover, the influence of sugar moieties on the roperties of the polymers will be shown, with particular emphasis on their potential uses.

Please cite as: CHEMIK 2013, 67, 3, 232-241

Introduction

Whenever there is a need, the ideas for its satisfaction arise. For more than a hundred years, the polymer science is involved in continuous competitions towards a better future. It has been started in 1909 with a Belgian scientist and industrialist Leo Hendrick Baekeland, who obtained the first artificial plastic material called Bakelite which has been used on an industrial scale as construction and insulating material for a long time. However, recently in 2011 Professor Krzysztof Matyjaszewski was awarded with the prestigious Wolf Prize for the discovery and advance of new method of controlled free radical polymerization (ATRP), having a huge impact on the development of chemistry and technology of polymers. Probably each of us have heard the slogan ?Chemistry feeds, cures, builds up,? which summarizes the daily struggles of polymer chemists looking for unique solutions, and thus attempting to create new materials with properties fitted to expectations of the XXI century as the ages of ?smart polymers?. Currently the design of new polymer with special properties is in close correlation with its potential application, which requires the selection of proper methods of synthesis as well as reagents (e.g., catalyst, or combination of various monomer units).

According to The Twelve Principles of Green Chemistry published in 1999 by Anastas?a and Warner?a [1], the scientists are increasingly turning to natural materials from renewable sources, fully biodegradable and harmless for humans and the environment. Carbohydrates represent all these values and therefore it is not surprising that their usage for the synthesis of polymers with special properties are constantly increasing. There are three main routes to synthetic polymers containing sugar units:
? polymerization initiated by sugar compounds and their derivatives, where the sugar unit depending on the number of initiating groups can be located at the beginning of the linear polymer or in the center of branched polymer
? polymerization of sugar monomers, where the sugar molecules act as monomer units of glycopolymers [2] ? polymer modification by the introduction of sugar units at the end of the main chain or as substituents by means of an esterification, nucleophilic substitution, addition-elimination etc [3].

It is worth to remember that there are a number of polymerization methods, which differ in mechanism yielding a polymer with more or less complex architecture. Therefore, the selection of a suitable method, which provides a controlled polymerization process of a particular monomer, leading to a well-defined polymer with required properties, plays the key role. For many years the conventional free radical polymerization was used on an industrial scale for the production of macromolecular compounds. Recently, the development of new methods, which eliminate or reduce its weakness points, such as lack of control over the average molecular weight and its distribution related to the heterogeneity of polymeric chains, and lack of possibility to prepare block copolymers due to the large contribution of side reactions terminating chain growth. These include controlled radical polymerization (Controlled Radical Polymerization (CRP) [4] with a ?pseudo-living? nature (active centers of chains are retained), and a reversible phenomenon of generation and deactivation of unpaired electron at the end of the growing macroradical. The decrease in concentration of propagating macroradicals leads to lower contribution of chain termination and thus allows to maintain control over polymerization process.

The most commonly used controlled radical polymerization methods include:
? nitroxide mediated polymerization (NMP)
? reversible addition fragmentation chain transfer polymerization (RAFT)
? atom transfer radical polymerization (ATRP).

ATRP seems to be the most versatile method among the mentioned above, probably due to the wide range of monomers (with the exception of the acidic and cyclic monomers), temperatures (25?120?C), nonpolar and polar solvents (including water) and possibility to obtain a block copolymers. In addition, a modified version of ATRP allows the use of the small quantities of catalyst (transition metal complexes), which expands application area to biomedical ones. The importance of other methods, such as NMP and RAFT, should be also respected due to advantages related to no metal mediated ecological conditions as well as the possibility of application of the monomers containing acidic groups and the synthesis of polymers with a very high degree of polymerization (ultra-high molecular weight polymers), respectively [5]. Pointing out the controlled polymerization methods it should be mentioned that the anionic polymerization is the only one which completely has got the ?living? character and it successfully proceeds with heterocyclic and unsaturated monomers. However, the extraordinary rigorous reaction conditions are required to keep living chains, which makes this method extremely difficult to use in the industrial scale [6].

This article aims to present polymers prepared by the abovementioned controlled polymerization methods that are initiated by the means of sugar, in which the number of initiating groups has a significant influence on the macromolecular topology (linear, star-shape and branched structures) and moderation of the properties according to their application.

Natural precursors of initiators used in the chain polymerization

The initiation process in the chain polymerization can undergo with the action of physical factors (i.e., thermal energy, UV radiation, ultrasounds and others) and chemicals (compound decomposition into free radicals or ions) [6]. Considering the latter case, the chemical compound must have an appropriate structure in order to serve as an initiator, whereas an initiator precursors are compounds, which only after suitable chemical modification can be used as the polymerization initiators. In this matter sugars are very interesting group of compounds, since they can be successfully treated either as initiators due to the presence the hydroxyl groups served as the initiating groups for the ringopening polymerization (ROP) and the precursors of the initiators in controlled radical polymerization (NMP, ATRP, RAFT). Moreover, sugars belong to the group of polyols, which contain from several to several hundred hydroxyl groups. This phenomenon opens the possibility for the preparation of polymers with a branched structures of a stars, brushes or combs (Fig. 1).

In the case of controlled radical polymerization techniques the initiators are compounds, which possess specific groups such as alkoxyamines (NMP), thiocarbamates (RAFT), haloesters (ATRP). Their preparation on the base of sugars mainly involves transformation of the hydroxyl groups to the groups that are active in the polymerization process. ATRP and RAFT initiators are mostly obtained by esterification reactions, [7, 8] whereas the procedure for NMP sugar initiators is more complex, because the precursor should have a primary amino group(s). Therefore, the initiator is obtained by the carbodiimide method involving reaction between carboxyl and amine group in the presence of N,N?-dicyclohexylcarbodiimide, which acts as a coupling agent [9].

Synthesis of star polymers initiated by cyclic oligosaccharides

The structures and properties of cyclodextrins (CD) make them a very attractive compounds. They are cyclic oligosaccharides composed of 5, 6 or 7 ?-D-glucopyranoside units, referred as ?-, ?- and ?-cyclodextrins respectively [10]. CDs present a unique shape of toroid, where primary hydroxyl groups are placed on its narrower side, whereas secondary hydroxyl groups are located on the opposite broader side (Fig. 2). Because of that the hydrophobic cavity is formed, while CD as a compound is hydrophilic and water-soluble. Moreover, the presence of the cavity favors formation of guest-host inclusion complexes with a number of hydrophobic compounds, where CD is a host molecule. Such complexes can be used either to trap toxins (e.g. trichloroethane or heavy metals), absorb odors or gradually release aromas (such as air fresheners), and to release drugs (e.g. hydrocortisone, nitroglycerin). Biomedical applications of CDs are possible due to the their biocompatibility and lack of immune response in both animals and humans.

The literature reports that a-CD and ?-CD were used repeatedly for the synthesis of star-shaped macromolecules with well-defined structures via controlled polymerization methods. [11,12] Figure 3 illustrates the possibility of employing ?-CD as a universal precursor of ATRP, NMP and RAFT initiators as well as ROP initiator.\

Linear cationic CD-based polymers with low immunogenicity have found an application as carriers of genetic material in gene therapy, which is a method involving the introduction of foreign genetic information in the form of DNA or RNA into cells (known as transfection) to change their properties. Therefore, the synthesis of polycations containing amidine units was performed by condensation reaction of dimethyl suberimidate and cyclodextrin modified with diamines [13]. On the other hand bromoisobutyryl functionalized CD?s were used in ATRP yielding the star-shaped polycations, in which arms were made of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) or its block copolymer with poly(poly(ethylene glycol) ethyl ether methacrylate) (PPEGMA). These structures showed the ability to form nanoparticle complexes of 100-200 nm and the zeta potential equal to 25-40 mV. The PPEGMA segments in the polymer chain significantly reduced the cytotoxicity of PDMAEMA, and the presence of the CD introduced as the initiator enhanced the transfection efficiency. This allowed to produce a new kind of gene carriers that can be used in gene therapy [14].

It is also worth to mention that the cyclodextrins have also found applications in nanotechnology. In 1980 the first synthetic pores (partial ion channels) based on CDs were prepared. In the following decades, the several attempts were performed to improve model of synthesis of molecules capable to create ?channels?. Finally, in 2009 the star-shaped macromolecules based on modified CD (with O-heptyl substituents at C-2 and C-3 carbon atoms), where the arms were made of poly(ethylene oxide) (PEO), were successful to form channels in lipid bilayer after one hour. The synthesis of such structures presents an original approach to issue of biomimetics [15].

Synthesis of star-shaped polymers initiated by monosaccharides

D-glucose is the well-known in nature and the most important simple sugar (monosaccharide). For most of the organisms, it acts as a storage of an energy. The importance of glucose for the human body is proved by the effects of persistent hypoglycemia (too low sugar level in blood), which finally can lead to death from starvation of the brain taking energy only from D-glucose. In addition, from a biological point of view it is a substrate of many important and interesting processes, which undergo in the cell. [10] Chemically D-glucose is a heterocyclic natural compound, which contains 5 free hydroxyl groups with different orientations in space. In 1999 fivearmed star-shaped polymers of styrene (PS)5 and methyl methacrylate (PMMA)5 were obtained on the basis of D-glucose modified with 2-bromoisobutyryl bromide [8].

The modified glucose ethyl glucopyranoside was used in enzymecatalyzed regioselective ROP of ?-caprolactone (CL) and trimethylene carbonate. The major advantages of this method with the participation of enzymes concern the lack of by-products during the monomer ring opening and the possibility of enantioselective polymerization of chiral lactones. In effect the biodegradable, linear polyesters with monosaccharide terminal group were obtained. Differences in the activity of the lipases in polymerization were also observed. The highest monomer conversion (>78%) was observed with PPL (porcine pancreatic lipase) and Novozym-435 (Immobilized lipases from C. antarctica). The reaction was performed selectively only with the primary hydroxyl group of ethyl glucopyranoside, whereas the untouched secondary hydroxyl groups may be used in another step of polymerization with different monomer. This kind of procedure leads to formation of star-shaped copolymers, which consist of arms with different structures, called as mikto-arm polymers [16].

In the case of another glucose derivative such as methyl ?-D-glucopyranoside, four-armed star polymer was obtained by ROP of D,L-lactide in the presence of Sn(NMe2)4 as catalyst [17]. In another studies the primary hydroxyl group of methyl ?-D-glucopyranoside was initially protected by tert-butyldiphenylsilyl ether (TBDPS) (Fig. 4), then the ROP polymerization of CL on three free hydroxyl groups of monosaccharide, and then the terminal hydroxyl groups at the chain ends were esterified by acetic anhydride. In next step the selective hydrolysis was conducted to remove tert-butyldiphenylsilyl group as protecting primary hydroxyl groups and converted via esterification into 2-bromisobutyrate moiety able to initiate ATRP reaction of sugar methacrylate monomer 1,2;3,4-di-O-isopropylidene- 6-O-methacryloyl-D-galactopyranose (MAIGP). It resulted the welldefined amphiphilic mikto-arm A3B polymers with low polydispersity (1.19-1.34) which additionally in aqueous solution were able to create micelles with diameter 50-70 nm [18].

Synthesis of star-shaped polymers initiated by disaccharides

The synthesis of star polymers by core-first method, based on at least three-functional compounds able to attach monomer molecules, was also performed with the use of disaccharides, including sucrose (?-Dfructofuranose linked with ?-D-glucopyranose by ?,?-1,2-glycosidic linkage) or lactose (?-D-galactopyranose linked with D-glucopyranose by ?-1,4-glycosidic linkage), which are potential eight-functional initiators leading to eight-armed stars (Fig. 5).

The star-shaped polymers with core saccharide and poly(acrylic acid) arms, which were obtained by two step procedure that is ATRP polymerization of tert-butyl acrylate and acidic hydrolysis of tert-butyl groups, exhibited polyelectrolyte character. [19] The other interesting star-shaped teroligomers used as a photoresist materials were obtained in ATRP of ?-gamma butyrolactone methacrylate, methyl adamantyl methacrylate and hydroxyl adamantyl methacrylate initiated by bromoester derivatives of sucrose [20].

The properly modified lactose was applied to synthesis of statistical copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA), fluorescently tagged monomer derived from hostasol and methyl methacrylate. The studies on the resulted polymers in the wide pH range (1,2-12) indicated no influence on P(MMAco- Hostasol)-bl-PDMAEMA fluorescent properties of the formed aggregates/micelles, where the fluorescent marker was located in the hydrophobic part of macromolecule. In case of PMMAbl- P(DMAEMA-co-Hostasol) copolymers, the emission intensity was twice higher (in comparison to the mentioned above), which gradually decreased with increasing pH. It was also concluded that the polymer became more hydrophobic affecting the aggregation of fluorophores and consequently the emission intensity was reduced [21].

Brush copolymers based on multifunctional polysaccharides

Regarding to the structure, where the side chains are attached to the main chain with different composition, the polymer brushes with unique properties are also interesting. They can be prepared by one of three methods: grafting onto, grafting through and grafting from. Parameters such as the type of polymer backbone and side chains, the degree of polymerization of both kinds of chains, and the degree of grafting are important for the basic characteristics of graft copolymers [22].

Polysaccharides due to the presence of a large amount of hydroxyl groups can act as multifunctional ROP or ATRP macroinitiators (after -OH groups modification) in graft copolymer synthesis. Major advantages of polysaccharides are related to biodegradability, biocompatibility and wide distribution in nature resulting low cost of purchase. Cellulose, starch and their derivatives are a great examples of polysaccarides, which were used for synthesis of presented below brush copolymers with complex architectures and specific properties (Fig. 6).

In case of L-lactide [LLA] polymerized on cellulose by ROP the copolymer containing a hydrophilic backbone and hydrophobic side chains was prepared, thus it exhibited the ability to self-organization in water forming nanomicelles at 37°C. These properties of cellulose-graft-PLLA macromolecules were utilized in design of polymeric carriers for hydrophobic drugs such as prednisone (antiinflammatory drug) [23].

The graft copolymers based on other polysaccharide, that is starchgraft- poly(vinyl acetate) and its modified version starch-graft-poly(vinyl alcohol) were synthesized by RAFT polymerization. The first one presents an amphiphilic character, thus in aqueous medium it can be self-organized to micelles. Moreover it can be modified by saponification to a different structure, which in water forms a hydrogel. Because of biocompatibility of the starch and polyvinyl alcohol this kind of grafted copolymer can find biomedical applications [24].

Synthesis of the polymers based on modified saccharides

The group of synthetic sugar initiators includes the acetal derivatives of monosaccharides. Both, 1,2:5,6-di-O-isopropylidene-Dglucofuranoside, and 1,2:3,4-di-O-isopropylidene-D-galactopyranoside were served as the precursors of monofunctional ATRP initiators used for synthesis of amphiphilic block copolymers based on hydrophilic PEGMA and hydrophobic benzyl methacrylate [25]. The acetal protective groups in the copolymer were removed in via acidic hydrolysis to get hydroxyl groups in the sugar moiety. The formed in aqueous media aggregates/micelles of sugar coated amphiphilic block copolymers show the binding properties toward the RCA-1 lectin. The studies indicated that only copolymers with galactose units were able to demonstrate interaction with RCA- 1 lectin. This opportunity can be used to design precisely drug delivery system.

Acetalization reaction of methyl ?-D-glucopyranoside and D(-)-salicin with mono- and dialdehydes resulted in formation of new, chiral sugar initiators with acetal groups. The main function of the acetal groups was not the protection of hydroxyl groups, but the increase in their number, as well as to improve the possibility of hydrolytic degradation of the core in the star-shaped polymers. The esterification reaction performed on O-acetal derivatives of glycosides led to sugar compounds with three, four, and six groups which were able to initiate copolymerization of glycidyl methacrylate with MMA by ATRP. As the result the well-defined star-shaped polymers with a statistical distribution of monomeric units in the arm chains were obtained [26].

An example of graft copolymer on the modified cellulose is hydroxypropyl cellulose-graft-poly(4-vinylpyridine), which is another macromolecule with potential use as a carrier in drug delivery system. Because of the amphiphilic nature, the copolymer forms micelles in aqueous medium which are sensitive to pH and temperature changes [27].

Conclusions

Recently, the increasing interest in the polymeric materials containing biocompatible and biodegradable sugar units has been observed. A variety and precise characterization of sugar derived structures are particularly highlighted in relation to so-called tailor made polymers, which can be prepared by using improved controlled polymerization methods (ROP, RAFT, NMP, ATRP). Saccharides with various number of hydroxyl groups, which are present in molecules have a particular meaning. The stereo- and regioselective modifications by incorporation of appropriate functional groups give opportunity to convert sugar units to both initiators and monomers. Furthermore, a special attention should be taken on numerous functions, which are served in living organisms by sugar compounds. In addition to the well-known energy storage and building functions, they also participate in the exchange of information between cells. In last time, it is known that sugars which cover the surface of cells are involved in processes related to the functioning of the immune system and thus affect the health of the body. Therefore, it is important to the perception of creating synthetic analogues of biopolymers as one of the directions of the development of the modern science of life and human health.

Literature
1. Anastas P.T., Warner J.: Green Chemistry. Theory and Practice, Oxford Univ. Press, Oxford 1998
2. Pearson S., Chen G., Stenzel M. H.: Engineered carbohydrate-based materials for biomedical applications polymers, surfaces, dendrimers, nanoparticles, and hydrogels. Chapter 1. Synthesis of glycopolymers, Wiley Interscience, New Jersey 2011, 2-104
3. Voit B., Appelhans D.: Glycopolymers of various architectures?more than mimicking nature. Macromol. Chem. Phys. 2010, 211, 727?735
4. Matyjaszewski K., Davis T.P.: Handbook of Radical Polymerization, Wiley Interscience, New York 2002
5. Braunecker W. A., Matyjaszewski K.: Controlled/living radical polymerization: Features, developments, and perspectives. Prog. Polym. Sci. 2007, 32, 93?146
6. Florjańczyk Z., Penczek S.: Chemia polimerów, Wydawnictwo Politechniki Warszawskiej, Warszawa 2001
7. Stenzel M. H., Davis T. P.: Star polymer synthesis using trithiocarbonate functional ?-cyclodextrin cores (reversible addition?fragmentation chaintransfer polymerization). J. Polym. Sci. Part A: Polym. Chem. 2002, 40, 
8. Haddleton D. M., Edmonds R., Heming A. M., Kelly E. J., Kukulj D.: Atom transfer polymerisation with glucose and cholesterol derived initiators. New J. Chem. 1999, 23, 477?479
9. Kakuchi T., Narumi A., Matsuda T., Miura Y., Sugimoto N., Satoh T., Kaga H.: Glycoconjugated polymer. 5. Synthesis and characterization of a seven-arm star polystyrene with a ?-cyclodextrin core based on TEMPO- mediated living radical polymerization. Macromolecules 2003, 36, 
10. 1Wiśniewski A., Madaj J.: Podstawy chemii cukrów, Wydawnictwo Uniwersytetu Gdańskiego, Gdańsk 2005
11. Ohno K., Wong B., Haddleton D.M.: Synthesis of Well-Defined Cyclodextrin- Core Star Polymers. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2206-2214
12. Stenzel-Rosenbaum M., Davis T., Fane A., Chen V.: Porous Polymer Films and Honeycomb Structures Made by the Self-Organization of Well-Defined Macromolecular Structures Created by Living Radical Polymerization Techniques. Angew. Chem. Int. Ed. 2001, 40, 3428-3432
13. Popielarski S. R., Mishra S., Davis M. E.: Structural effects of carbohydrate-containing polycations on gene delivery. 3. Cyclodextrin type and functionalization. Bioconjugate Chem. 2003, 14, 672?678
14. Xu F. J., Zhang Z. X., Ping Y., Li J., Kang E. T., Neoh K. G.: Star-shaped cationic polymers by atom transfer radical polymerization from ?-cyclodextrin cores for nonviral gene delivery. Biomacromolecules 2009, 10, 285?293
15. Badi N., Auvray L., Guegan P.: Synthesis of half-channels by the anionic polymerization of ethylene oxide initiated by modified cyclodextrin. Adv. Mater. 2009, 21,
16. Bisht K. S., Deng F., Gross R. A., Kaplan D. L., Swift G.: Ethyl glucoside as a multifunctional initiator for enzyme-catalyzed regioselective lactone ring-opening polymerization. J. Am. Chem. Soc. 1998, 120, 1363-1367
17. Vuorinen S., Heinämäki J., Antikainen O., Lahcini M., Repo T., Yliruus J.: Sugar end-capped poly-D,L-lactides as excipients in oral sustained release tablets. AAPS Pharm. Sci. Tech. 2009, 10, 2, 566-573
18. Suriano F., Coulembier O., Dubois P.: Synthesis of amphiphilic A3B mikto-arm copolymers from a sugar core: combination of hydrophobic PCL and hydrophilic glycopolymers for biocompatible nanovector preparation. J. Polym. Sci. Part A: Polym. Chem. 2010, 48,
19. Plamper F. A., Becker H., Lanzendörfer M., Patel M., Wittemann A., Ballauff M., Müller A. H. E.: Synthesis, characterization and behavior in aqueous solution of star-shaped poly(acrylic acid). Macromol. Chem. Phys. 2005, 206,
20. Wieberger F., Forman D. C., Neuber C., Gröschel A. H., Böhm M., Müller A. H. E., Schmidt H. W., Ober C. K.: Tailored star-shaped statistical teroligomers via ATRP for lithographic applications. J. Mater. Chem. 2012, 22, 73?79
21. Limer A. J., Rullaya A. K., San Miguel V., Peinado C., Keely S., Fitzpatrick E., Carrington S. D., Brayden D., Haddleton D. M.: Fluorescently tagged star polymers by living radical polymerisation for mucoadhesion and bioadhesion. React. Funct. Polym. 2006, 66, 51?64
22. Neugebauer D.: Synteza amfifilowych kopolimerów szczepionych metodą kontrolowanej kopolimeryzacji rodnikowej hydrofilowego makromonomeru poli(tlenku etylenu). Wiad. Chem. 2008, 62, 1-2, 49?66
23. Dong H., Xu Q., Li Y., Mo S., Cai S., Liu L.: The synthesis of biodegradable graft copolymer cellulose-graft-poly(l-lactide) and the study of its controlled drug release. Colloids Surf., B 2008, 66, 26?33
24. Lu D. R., Xiao C. M., Xu S. J., Ye Y. F.: Tailor-made starch-based conjugates containing well-defined poly(vinyl acetate) and its derivative poly(vinyl alcohol). Express Polym. Lett. 2011, 5, 6, 535?544
25. Bes L., Angot S., Limer A., Haddleton D. M.: Sugar-coated amphiphilic block copolymer micelles from living radical polymerization: recognition by immobilized lectins. Macromolecules 2003, 36,
26. Mielańczyk A., Biela T., Neugebauer D.: Polymers on the Odra River. Materiały konferencyjne Opole 2011, 56
27. Ma L., Kang H., Liu R., Huang Y.: Smart assembly behaviors of hydroxypropylcellulose- graft-poly(4-vinyl pyridine) copolymers in aqueous solution by thermo and pH stimuli. Langmuir 2010, 26, 23, 15

Translation into English by the Author

Anna MIELAŃCZYK ? M.Sc., graduated from the Faculty of Chemistry at the Silesian University of Technology (2010). She is a PhD student in the Department of Physical Chemistry and Technology of Polymers. Scientific interests: carbohydrate chemistry and polymer chemistry.
e-mail:

Dorota NEUGEBAUER ? Ph.D., (Eng.), graduated from the Faculty of Chemistry at the Silesian University of Technology (1992). In 1999 she received her Ph.D. degree in chemistry, and in 2008 she accomplished habilitation. Since, 2010 she is an associate professor at the Silesian University of Technology. Scientific interests: controlled polymerization methods, amphiphilic copolymers, molecular brushes.