Physiologically, carbohydrates, including sugar molecules hence the sweet in the title, are involved in highly specific cellular events including cellular recognition (such as host-pathogen interactions), adhesion, cell growth regulation, mounting of immune response, cancer cell metastasis, fertilisation and inflammation. By exploiting one or more of these mechanisms, chemists have assembled a wide range of structurally diverse polymeric artificial carbohydrate polymers (glycopolymers) as effectors or inhibitors of cellular processes and analytical tools for investigating carbohydrate-protein binding events. Synthetic glycopolymers basically function as mimics of naturally occurring polysaccharides, thereby displaying anti-inflammatory, anti-coagulant and anti-tumour properties.
Multiple simultaneous interactions (multivalency) are commonly found in nature and exhibit properties that are qualitatively different from properties than that of monovalent interactions. The concept of 'multivalency', the simultaneous binding of multiple ligands on one entity to multiple receptors on another can result in a significantly increased affinity as compared to that for the binding of a single ligand. These interactions have several mechanistic and functional advantages over their monovalent counterparts. Among these are the ability to create conformal contact between large biological surfaces, the ability to produce graded responses with a single type of interaction, and the ability to increase the specificity of an interaction.
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Multivalent interactions can serve as the strategic basis for the design of pharmaceutical agents, including agents that could possibly deliver polyvalent antigens to promote/boost immune cells in order to reduce the incidence of cancer development.
Figure 1: Illustration of a monovalent and multivalent interaction between ligands and receptors.
Multivalent interactions are able to regulate the strength of an interaction through the number of ligand-receptor contacts for such as carbohydrate-binding proteins known as lectins. Modification (blocking or enhancement) of protein-carbohydrate interaction provides a powerful therapeutic strategy for the treatment of many human diseases. Studies have shown that carbohydrate ligands inherently exhibit low affinity for their protein receptors, lectins. Lectins typically exist in multimeric assemblies, a variety of polyvalent saccharide ligands have been prepared in the search for high affinity. Although the mechanism by which multivalent ligands act is still unclear it is increasingly accepted that the cluster glycoside effect relies on aggregation [ref].The cluster glycoside effect, represents the best strategy for overcoming the "weak binding" problem. The overall effect is the increase in binding affinity to an extent which exceeds the sum of the constituent binding events in sufficient strength to effect or inhibit a cellular or physiological response.(ref). Lectins serve as a useful probe in studying carbohydrates of cell surfaces, especially the lectin Concanavalin A (Con A) which is widely researched. Studies have revealed that the physiological responses of multivalent carbohydrate interactions with lectins include apoptosis of T cells, regulation of the T-cell receptors and cell cycling kinetics and activities of cytokine receptors. (ref).
Con A binds specifically to Î±-Mannose, Î±-Galactose structures found in sugars, glycoproteins and glycolipids. The lectin has been utilized in hormone receptor studies, mitogenic assays and for characterizing normal and malignant cells as cancer cells are readily aggregated by Con A while normal cells are not. Con A can also initiate cell division mainly by acting on T-lymphocytes.
Polymeric systems, where a collection of similar or different ligands can be covalently linked together in polymer chains, provides for a scaffold to elaborate the concept of multivalent interactions. Con A has shown increased affinity for a synthetic polymer of multiple mannose residues compared to its monosaccharide counterpart. Biessen and co workers showed that the affinity of human mannose receptor increased with a series of lysine-based cluster mannosides when the number of mannose residues per molecules was increased from two to six (ref), further highlighting the importance of multivalent interactions with lectins. Both Con A and galactins (galactose-binding lectin) cross-link cell surface receptors have been implicated in initiation of signal transduction.
Figure 2: Schematic representation of Con A clustering by multivalent ligand (glycopolymer)F:\Supriya LAB\Dissertation\con A cluster.jpg
Synthetic glycopolymers with biocompatible and biodegradable properties are used in tissue engineering and controlled drug release devices. Carbohydrate-based vaccines, such as tumour-associated carbohydrate antigens (e.g., sTn), are an active area of research.
Immune response to carbohydrates
It has been shown that antibodies that target tumour-related carbohydrate and glycopeptide antigens have the ability to eliminate circulating tumour cells. These antibodies can be acquired by passive immunization (i.e. immunization with the antibody itself), or by active immunization with a vaccine that contains the carbohydrate epitope. The antibodies can also be acquired naturally.
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Antibodies against tumour-associated carbohydrates can mediate elimination of tumour cells by complement-dependent cytotoxicity (CDC) and/or by antibody-dependent cellular cytotoxicity (ADCC) performed by natural killer (NK) cells and macrophages. The antibodies have also been shown to interfere with receptor-mediated signalling, adhesion, and metastasis.
Antibodies are produced by B-cells that have been activated with their cognate antigen. The B-lymphocytes carry membrane-bound Ig proteins that can recognize a wide variety of compounds. Carbohydrates can bind to receptors of B-lymphocytes and induce cross-linking of the Ig proteins, which will lead to activation of the B-cell and production of low affinity IgM antibodies.
Development of synthetic carbohydrate-based cancer vaccines
The biggest challenge of developing carbohydrate-based cancer vaccines by isolating carbohydrates from natural sources is that they are mostly conjugated to a protein carrier by reductive amination through the aldehyde group of the reducing end sugar. This can result in destruction of vital recognition elements resulting in a decrease or possibly complete loss of immunogenicity.
Synthetic carbohydrates offer an advantage in that they can be designed to incorporate a linker containing a functional group with unique reactivity for selective conjugation to a polymer backbone, in a manner that does not interfere with the antigenic epitope.
Engineering of synthetic glycopolymers
Synthetic glycopolymers containing pendant sugar moieties have been shown to interact multivalently with lectins in a similar manner to that of natural glycoproteins. This mimicry has caused significant interest in the fields of carbohydrate chemistry and glycobiology, and a number of different strategies have been employed to obtain the required multivalent carbohydrate ligands. Mantovani and co-workers reported the use of a novel click chemistry/living radical polymerisation strategy in order to prepare a linear glycopolymer library containing carbohydrates attached pendantly. (ref)
Radical polymerisation: General concepts
Radical polymerisation (RP) is a relatively simple polymerisation process and can be used to polymerise a wide range of monomers. Conventional RP is accounts for the production of over 50% of all commercial polymers including Low density polyethylene, poly(vinyl chloride), polystyrene and its copolymers (with acrylonitrile, butadiene, etc.), polyacrylates, polyacrylamides, poly(vinyl acetate), poly(vinyl alcohol) and fluorinated polymers. Radical polymerisation is used widely commercially especially in industrial and personal care sectors.
The active species, free radicals, of radical polymerisation are typically sp2 hybridised and exhibit poor stereoselectivity. However these free radicals can be stabilized via resonance and result in polymers of good regioselectivity and chemoselectivity as evidenced by the general high degree of head-to-tail structures in the chain and the formation of high MW polymers.
However, free radical polymerisations are problematic in that they offer low uniformity of the polymer chain, resulting in a high polymer polydispersity index with heterologous polymer chains with varying molecular weights i.e. polymers do not exhibit architecture and hence no pure block copolymers can be formed. These undesirable properties are attributed to the general kinetics of conventional radical polymerisation where high concentration of radical species favours the rate of termination over the rate of propagation.
Recently, new methods have been developed that allow control over radical polymerization to minimise monomer content and produce very uniform molecules. This control can be achieved by a variety of techniques including the use of atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization and nitroxide-mediated polymerisation (NMP) - collectively termed as 'controlled radical polymerisation'.
Controlled Radical Polymerisation (CRP)
CRP systems are fairly similar to conventional radical polymerisations in that polymeric radicals grow and terminate with similar rate constants. The main difference lies in the way in which the radicals (active species) are generated. Precise formation of different polymer architecture is attributed to the minimization of chain breakages and simultaneous growth of all chains which is the result of nearly instantaneous initiation. CRP incorporates a combination of fast initiation and absence of irreversible termination which serve to contrast with free RP, where polymerisation proceeds via slow initiation resulting in many short and unequal growing polymer chains.
Due to the controlled rate of growth of the polymer chains, CRP allows for the formation finely tuned polymers of a range of as shown in figure ___ ?
The heart of all CRPs lies with the establishment of a dynamic equilibrium between propagation radicals and various dormant species. Radicals are either reversibly trapped in a deactivation/activation process or they may be involved in a degenerative exchange 'reversible transfer' (figure?)
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In an ideal living polymerisation, there is no irreversible chain transfer for termination as all chains are initiated at the start of the reaction; chains grow at similar rates and survive polymerisation. As compared to conventional radical polymerisation where initiation is rapid with respect to propagation, the molecular weight distribution is narrow and chains can be extended by further adding monomer into the reaction.
Principles of atom transfer radical polymerization (ATRP)
The basic mechanism of ATRP involves a reversible switching between two oxidation states of a transition metal complex. The polymerisation active species is generated through a reversible redox in a transition metal complex (Mtn-Y/Ligand) undergoes a one-electron oxidation with simultaneous transfer of halogen atom, X, from a dormant species, R-X (figure 2). Polymer chains grow by the addition of monomer to the intermediate radicals in a similar fashion to conventional radical polymerisation.
Figure3 : Basic mechanism of ATRP
The figure above highlights that the equilibrium is predominantly shifted to the left or dormant species in order to minimise irreversible termination and transfer reactions which are favoured in the presence of high concentrations of radical species. Irreversible termination ideally is limited to only a few percent of the polymer chains. This small amount of termination actually serves to generate oxidised metal complexes, X-Mtn+1, as persistent radicals to reduce the stationary concentration of growing radicals hence minimizing the contribution of termination. Polymers formed from a successful ATRP generally exhibits uniform growth, ensuring a uniform distribution of molecular weight which accounts for a low polydispersity index (pdi). This is due to chain growth achieved by quick initiation and rapid but reversible deactivation. (ref)
The main disadvantage of ATRP is assuring the complete removal of the transition metal from the final polymer product. Depending on the metal catalyst used, for example, using copper(I)bromide can be tricky as it is strictly air sensitive and can be converted to copper(II)bromide causing termination of polymerisation, resulting in short and unequal polymer chains.
Cobalt catalysed chain transfer polymerisation (CCCTP)
In the case of metal-mediated (such as copper) living radical polymerisation (ATRP) the polymer chain carried a terminal halide atom which undergoes several activation-deactivation cycles and is susceptible to side reactions leading to loss of polymer chain end integrity. Thus it is challenging for polymer chemists to synthesize end functional polymers with high integrity. Hence CCCTP is a versatile technique employed to produce end functional polymers whilst maintain its fidelity and also exhibit controlled molecular weight. This is because CCCTP employs bis(boron difluorodimethylglyoximate) cobalt(II) (CoBF) which is catalyst in neutral aqeous media that allows for polymerisation of acidic monomers unsuitable for anionic polymerisation and also provides stability.
Reversible Addition-Fragmentation chain Transfer (RAFT) polymerisation
RAFT polymerization is a reversible deactivation radical polymerization and one of the more robust and versatile methods for providing living characteristics to radical polymerization.1-7
In recent years RAFT polymerization has emerged as a very attractive method for producing polymers whilst providing for control of molecular weight and molecular weight distributions with polydispersity indexes typically in the range of 1.03-1.25. Its main potential lies in its versatility towards the types of monomers it can polymerize, including styrenic, (meth)acrylamides, (meth)acrylates, acrylonitrile, vinyl acetates, vinyl formamide, vinyl chlorides as well as a range of other vinyl monomers. (ref)
This type of CRP is tolerant of unprotected functionality in monomer and solvent, therefore polymerisations can be carried out in aqueous or protic media. RAFT polymerisation is compatible with vast reaction conditions such as bulk, organic/aqueous, emulsions and suspensions. It is one of the most easy to implement and also inexpensive as compared to other CRP methods.
In RAFT polymerisation, reagents that are capable of reversibly deactivating propagating radicals are used so that the majority of living chains are maintained in a dormant form. The reaction conditions support a rapid equilibrium between the active and dormant species.
The sequence of addition-fragmentation equilibria is a key feature of RAFT polymerisation. Initiation and radical-radical termination occur as in conventional radical polymerisation. Addition of propagating radical (PnÎ‡) to a chain transfer agent followed by fragmentation of the intermediate radical gives rise of to a polymeric chain transfer agent and a new radical - all occur during the early stages of polymerisation.The key to the RAFT polymerization process is the chain transfer agent or also known as RAFT agent.
Z-group controls the reactivity of the
C-S double bond; influences the rate
of radical addition and fragmentation
RAFT agent - general structure
Free radical leaving group, R
(must be able to reinitiate polymerization)
Reaction of the radical (RÎ‡) with monomer forms a new propagating radical (PmÎ‡). A rapid equilibrium between the active propagating radicals (PnÎ‡ and PmÎ‡) and the dormant polymeric RAFT agent allows for equal probability for all chains to grow and production of narrow dispersity polymers. Radicals are neither formed nor destroyed in the chain equilibration process, hence once the equilibria are established; the rates of polymerisation should be similar to that of conventional radical polymerisation. Studies have shown that with some RAFT agents, RAFT polymerisation is half order in initiator and zero order in the RAFT agent over a wide range of initiator and RAFT agent concentrations.
Figure 3: The RAFT mechanismF:\Supriya LAB\Dissertation\RAFT MECHANISM SCHEME.jpg
Nitroxide Mediated Polymerisation (NMP)
Prior to the development of NMP, nitroxides were well known as inhibitors of polymerisation. Nitroxides are able to efficiently scavenge carbon-centred radicals by combining with them at near diffusion-controlled rates to form alkoxyamines. These unique properties of nitroxides lead to its utilization as a trapping agents to define initiation mechanisms. The exploitation of alkoxyamines as polymerisation initiators, NMP has grown in popularity for producing block and end-functional polymers as first described in by Solomon et al. With suitable selection of alkoxyamine and control of reaction conditions, NMP resulted in synthesis of narrow dispersity polymers.
The underlying principle behind nitroxides-mediated polymerisation is the utilization of the nitroxides group to efficiently cap the end of the growing polymer chains by a reversible termination reaction, ensuring equal growth of all polymer chains and suppressing unwanted termination reactions. Alkoxyamines serves a dual function in NMP reactions as an initiator and end-capping group. This gives full control over the concentration of initiating radicals in the reaction mixture. (ref)
Figure 4: NMP mechanismF:\Supriya LAB\Dissertation\Picture1.jpg
Figure 4 illustrates the NMP mechanism. The reaction begins with the haemolytic cleavage of the alkoxyamine upon heating to produce the initiating radical R3 Í˜ and the stabilized Í˜ONR1R2 that is present long enough for a monomer unit M to react with R3 Í˜ before recombination occurs. NMP reactions were actually carried out at temperatures over 100áµ’C but recently have also been carried out at temperatures below this in aqueous media.
The alkoxyamine group can withstand reaction conditions of other controlled-polymerisations such as RAFT and ATRP which allows for the synthesis of block co-polymers by combining NMP with other techniques (ref)
Ring opening polymerisation (ROP)
ROP is not a part of controlled living polymerisation but is a chain polymerisation technique that offers access to a wider range of cyclic monomers which could not be polymerised by other techniques. It is possible to control the molecular weight of polymers made via ring opening polymerisation due to its dependence on conversion and the ratio of monomer to initiator concentration. It is important to mention ROP as a polymerisation technique as it can be used to prepare polymers to later form biodegradable glycopolymers.
However, compared to controlled living polymerisation this technique requires time and effort to produce a well defined polymer. Due to the sensitivity of reactions it is necessary to ensure that the reactants are not in any contact with water or air to avoid deactivation of ionic initiators, long reactions times and low temperatures. Hence this is not a popular method of polymerisation to eventually lead to glycopolymer formation.
Each CRP technique has its limitation and advantages and some maybe more suitable to polymerise different monomers or exhibit different conditions to optimize the polymerisation process and is summarized in table ___
Most monomers with activated double bonds
No vinyl acetate
Mostly all monomers
Styrenes with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
Large temperature range (-30 to 150áµ’C)
Less reactive monomers require elevated temperatures
Elevated temperature <120áµ’C for TEMPO
Some tolerance to oxygen
Sensitive to oxygen
Sensitive to oxygen
Either SN, E, or radical chemistry for transformations
Radical chemistry for transformations
Radical chemistry for transformations
Transition metal catalyst should be removed
May produce many new chains
NMP maybe accelerated with acyl counpounds
Inexpensive and available
RAFT polymerisations can be carried out at temperatures which are typical for any radical polymerisation which is around 60-110áµ’C. ATRP is typically performed at temperatures ranging from 20-120áµ’C. All three systems RAFT, ATRP and NMR are sensitive to oxygen but in particular ATRP as it employs the use of metal catalyst, usually Copper, and oxygen is rapidly scavenged by metals than by growing radicals. It is possible to perform ATRP in the presence of Cu(0) or Fe(0) even with limited amount of oxygen and inhibitor present in the system which highlights the influence of choice of metal catalyst used on the sensitive of ATRP to oxygen.
In ATRP a halogen is reversibly transferred to transition metal making it exchangeable with other halogens which controls the structure and reactivity of end groups and allows for the formation of block copolymers. NMP and ATRP can be initiated by conventional radical initiators such as AIBN in the presence of nitroxides or metal halides with higher oxidation states. Additional additives need to be used in NMP unless reaction rate acceleration is required whereas RAFT depends on a radical source and ATRP on a catalyst. The radical initiators in RAFT may result in the production of too many new chains and reduce chain end functionality if used in larger amounts. In ATRP the metal catalyst should ideally be removed or recycled as even a minute amount of residual catalyst can effect polymer stability and mechanical properties.
ATRP works optimally with low molar mass polymers with special functionalities and also can be used to prepare block copolymers that are not easily synthesized via other techniques. RAFT works efficiently with less reactive monomers and high molecular weight polymers. NMP works optimally with systems that require absence of metals and other elements such as Sulfur.
Part 2: CLICK chemistry
Click chemistry is a concept that was introduced by K. Barry Sharpless in 2001 which incorporates the generation of substances both quickly and reliably by joining small units together via simple chemistry. Click chemistry itself is not a single reaction but a restricted group of chemical transformations which should ideally fulfill a list of criteria as defined by Barry Sharpless. The basic criteria is that the reaction is wide in scope, easy to perform, uses only readily available reagents, insensitive to oxygen and water and relies on simple and efficient processes - i.e. distillation or crystallisation, for the purification of the final products.
Table 1: Ideal 'Click' chemistry reaction criteria
The reactions should ideally:
The process should ideally:
give very high chemical yields
generate only inoffensive by-products
be physiologically stable
exhibit a large thermodynamic driving force > 84 kJ/mol to favour a reaction with a single reaction product.
high atom economy
have simple reaction conditions
use readily available starting materials and reagents
use no solvent or use a solvent that is benign or easily removed (preferably water)
provide simple product isolation by non-chromatographic methods (crystallisation or distillation)
Several types of reaction have been identified that fulfil these criteria, thermodynamically-favoured reactions that lead specifically to one product, such as nucleophilic ring opening reactions of epoxides and aziridines, non-aldol type carbonyl reactions, i.e. formation of hydrazones and heterocycles, additions to carbon-carbon multiple bonds, such oxidative formation of epoxides and Michael Additions, and cycloaddition reactions.
Given that a number of reactions could possibly fit the basic click criteria, it is the 1,3-dipolar cycloaddition of alkynes to azides to form 1,4-disubsituted-1,2,3-triazoles that serves as the 'perfect' example of a click reaction. This reaction uses Copper(I) as a catalyst, does not require protecting groups and in many instances the product formed does not require purification. The azide and alkyne functional groups are largely inert towards biological molecules and aqueous environments and at ambient temperature only react with each other when an appropriate copper catalyst is introduced. Hence the cycloaddition can be used for target guided synthesis and activity based protein profiling. (ref) The triazole has similarities to the ubiquitous amide moiety found in nature, but unlike amides, is not susceptible to cleavage and almost impossible to oxidize or reduce.
The starting materials for the reaction such as monosubstituted alkynes and organic azides are commercially available and many others can easily be synthesized encompassing a wide variety of functional groups, and their cycloaddition reaction selectively gives 1,2,3-triazoles.
Non CuI Huisgen 1,3-Dipolar Cycloaddition of alkynes to azides requires elevated temperatures and produces mixtures of the two regioisomers when using asymmetric alkynes, hence fails as a true click reaction. A refinement made by Sharpless et al introduced a copper-catalyst which allowed for the reaction to occur even at room temperature. Also this copper-catalyzed reaction allows the synthesis of the 1,4-disubstituted regioisomers specifically, better complying with the definition of click chemistry. The incorporation of the CuAAC process allows sugar monomers to be prepared with ease by the introduction of azide functionality. This reaction has now become the focused click reaction prototype.
Figures 5 & 6: Mechanism of the Copper-Catalyzed Azide-Alkyne Cycloaddition (CuAAC)
F:\Supriya LAB\Dissertation\huisgen cycled mechanism.jpg
The copper (I)-catalyzed cycloaddition between the azide and the alkyne proceeds through a stepwise mechanism as illustrated in figure 6. The mechanism starts by coordination of the alkyne (B) to copper (I) (A). This displaces one ligand and re-coordination gives the copper acetylide (C). This is then followed by the replacement of one of the ligands by the azide (D) and the copper complex binds to the nitrogen adjacent to the carbon, forming intermediate (E). The terminal nitrogen on the complex (E) attacks on C-2 in the acetylide to form a six-membered Copper (III) intermediate (F). Finally, rearrangement of the copper (III) intermediate results in a five-membered complex (G) which is then proteolysed to give complex (H). (ref)
Applications of click chemistry
The Huisgen 1,3-cycloaddition reaction catalysed by copper(I) is now highly recognized as one that is very efficient and also stereoselective tagged with exceptional functional group compatibility. The properties of the reaction allow for precisely synthesized complex material including bioconjugates, dendrimers, therapeutics, functionalized polymers, affinity chromatogram supports and also sugar derivatives.
Introduce non-pot route
A decade ago formation of glycopolymer was challenging with limited attempts at reacting a functional polymeric backbone with a carbohydrate. The reason being the difficulty of introducing sufficiently reactive pendant groups onto the polmer backbone to reach with carbohydrate. The modification of poly(vinylalchohol) with 4-nitrophenyl carbonate groups to yield a reactive polymer backbone was one successful attempt at forming glycopolymers. The reactive nitrophenyl carbonate groups were transformed with glucosamine into glycopolymers and were then investigated for their interaction with a commonly used lectin, Concanavalin A (Con A).
The one pot route to clickable scaffolds(ref)
The one pot route in synthesizing clickable scaffolds is useful in forming functional polymers which can then be 'clicked' to carbohydrates, or more specifically, sugar molecules to form the desired glycopolymer. This process is the basic concept in possible bulk formation of a carbohydrate -based cancer vaccine.
Figure 7: Illustration of a polymeric system with clickable functional unitsThe advantage in using the one-pot route one is that libraries of different materials can be generated from one single polymeric precursor. The functional polymers produces will exhibit the same macromolecular characteristics which include the degree of polymerisation, polydispersity index and also polymeric architecture. Another benefit is that some molecules that exhibit functional groups that are incompatible with the conditions used to polymerise them. These conditions can possible poison the polymerisation catalyst i.e. the catalyst is actually be incorporated onto the polymer backbone. F:\Supriya LAB\Dissertation\polymer and clicklable scaffold.jpg
One route in synthesising glycopolymers involves the one step synthesis of a trimethylsilyl protected (TMS) propargyl methacrylate monomer. The monomer is polymerized via ATRP which is then followed by the deprotection which generates a reactive propargyl unit, proving a 'clickable' scaffold onto which a variety of functional sugar azides can react. This route of glycopolymer synthesis was optimized to reduce the number of steps required to synthesize sugar azides. This reaction makes it possible to generate a library of polymers all containing the same macromolecular characteristics, the only variant is that the different sugars maybe chosen to be 'clicked' onto the polymer scaffold.
Using ATRP and click chemistry as the route for glycopolymer synthesis, Mantovani and co-workers reported a successful conjugation of glycopolymers to bovine serum albumin (BSA), a glycopolymer mimic. This was shown to induce immunological behaviour via interaction with mannose binding lectin. Glycopolymers formed via this route have successfully proven to show biological activity.
Figure 8: Glycopolymer synthesis via clickable sugars (sugar azide) route
Adapted from: Slavin Stacy, Burns James, Haddleton David M. Synthesis of glycopolymers via click reactions, European Polymer Journal
Haddleton and co-workers reported that a protected maleimide (PM) initiators enables convenient conjugation to available thiol residues present in proteins after deprotection. Hence glycopolymers formed via the clickable sugar route are suitable for instigating interactions with mannose-binding lectin while eliminating the effects of chain length or architecture. Well-defined maleimide end functionalized glycopolymers can be formed with the combination of ATRP and CuAAC. This most striking feature of this combination is that is it simple, efficient and a wide range of glycopolymers can be synthesized.
Another route to glycopolymer synthesis (figure 9) is the production of a sugar methacrylate monomer via CuAAC click reaction. This is followed by the polymerisation of this sugar monomer via ATRP but without the need to protect the hydroxyl group present on the sugar molecular, typically required in glycopolymer synthesis. The maleimide end group is then deprotected in the final step simply carried out in a vacuum oven at 80áµ’C. This eliminates the use of unnecessary organic solvents and purification steps.
Figure 9: Glycopolymer synthesis via a sugar monomer
F:\Supriya LAB\Dissertation\CLICK SUGAR REACTION 2.jpg
Another route (figure 10) for synthesizing glycopolymers investigated was via a one-pot process with simultaneous CuAAC and living radical polymerisation however the mechanism is not yet fully understood. However it was demonstrated that it is possible to both living polymerisation and CuAAC reactions occurring in the same reaction mixture. Tweaking reaction conditions such as solvents used temperature and concentrate of catalyst proved to influence the rate of each process. The ability to tweak reaction conditions is important as any un-clicked alkyne groups may actually undergo side reactions in situ and lead to poor control over polymerisation. Also the one-pot system eliminates the use of sugar functional methacrylate monomers, reducing the number of synthetic steps involved in glycopolymer synthesis.
In the past five years thiol based click reactions have gained more attention due to the commercial availability of a wide variety of thiols. Thiol based click reactions are versatile and can actually be used to tailor make macromolecular architectures such as complex dendritic polymers. Thiol reactions with various functional groups such as alkenes, alkynes, para-fluoro phenyl and halides are used to make glycopolymers. These reactions are very efficient and render high yields of glycopolymer under defined conditions.
UV-light and a photo initiator can be used to provide a radical source for thiol-ene click reactions. Other thiol click reactions are either catalysed by base or are nucleophilic in natures and proceed via ambient conditions. Preparation of thiol-sugars are much more complex than sugar-azides, however, due to the commercial availability of thiol-sugars, the number of synthetic steps to prepare glycopolymers is reduced. Polymers with alkene, alkyne, para-fluoro and halide pendant groups can easily be obtained and undergo thiol-click chemistry to produce glycopolymers. (ref page 436)
There are different combinations of click reactions and polymerisation techniques with different sugars (saccharides) to synthesize glycopolymers and are tabulated below.
Acrylate or Methacrylate
Acrylate or Methacrylate
Acrylate or Methacrylate
The combination of CuAAC and ATRP works particularly well as clickable functionality into polymers can be incorporated with ease to polymers prepared by ATRP. ATRP method of polymerisation works very well with Copper(i) click chemistry. The copper(i) catalyst typically used in ATRP is also used to catalyse the CuAAC click reaction and both processes are typically performed with similar Nitrogen based ligands. This combination to form tailored glycopolymers has allowed for one-pot synthesis and clicks functionalization of polymers (ref) and has resulted in a vast variety of new polymeric materials for biological application and macromolecular engineering. (ref)
On the other hand combination of RAFT and CuAAC also works well considering the wide variety of monomers RAFT polymerisation can accommodate. RAFT process does not require the use of a metal catalyst which can prove difficult to remove to obtain a pure polymer.