|In 1980’s a new class of material named as dendrimer became popular both in the field of polymer science and engineering. Dendrimer is an example of symmetric, highly branched three dimensional globular nano-object. It possess several interesting physical and chemical properties like low solution and melt-viscosity, lower intermolecular chain entanglement, large number of end groups placed at the molecular periphery, relatively high solubility with respect to their linear counterpart. In order to get this perfectly branched structure, one has to go through the tedious multistep synthetic approach, repetitive chromatographic purification and protection-deprotection strategies in every step; all of which limits the large scale production and thus commercialization.
On the other hand, hyperbranched polymer, a highly branched analogue of dendritic polymer with few defects in their branching architecture, which can be prepared in a single step, show similar physical and chemical properties as that of dendrimer. Polymerization of AB2 monomer is one of the well established method to generate hyperbranched polymer which upon polymerization, generates plenty of ‘B ’groups at the periphery along with a single ‘A’ group as a focal point in the resulting hyperbranched polymer as shown in Figure 1. From the structural point of view, hyperbranched polymers consist of three distinctly different compartments such as periphery, interior and a (single) focal point.
During the past decade our lab have developed a novel melt trans-etherification process to generate polyethers and have utilized to access to a wide variety of hyperbranched structures. One of the challenges we addressed is to selectively functionalize the periphery of the hyperbranched polymer during the polymerization process. Polycondensation of ‘AB2’ monomer is not sufficient enough to generate a wide variety of hyperbranched polymer as the periphery of hyperbranched polymer is limited to the ‘B’ functional group unless it could be modified via ‘post-polymerization modifications’.
Copolymerization of ‘AB2’ monomer with stoichiometric amount of ‘A-R’ monomer should result in hyperbranched polymer decorated with ‘R’ groups in the periphery that can be prepared in a single step. One of the prerequisite in the ‘AB2+A-R’ approach is that the comonomer ‘A-R’ should have silent ‘R’ group which does not interfere during the polymerization.
During the copolymerization process with stoichiometric amount of ‘A-R’ monomer, ‘AB2’ monomer having one equivalent excess of ‘B’ can react with the ‘A’ group from ‘A-R’ monomer eventually generating the hyperbranched structure with peripheral ‘R’ groups. By appropriately choosing the ‘R’ group, one can access a wide class of hyperbranched polymer with the required functionality. Further by having a reactive ‘R’ group that is not participating in polymerization can act as a handle for post-polymerization modifications. For instance, copolymerization of 1-(6-Hydroxyhexyloxy)-3,5-bis(methoxymethyl)-2,4,6-trimethylbenzene (Hydroxy as ‘A’ and methoxy as ‘B’) and 6-bromo-1-hexanol where ‘OH’ and ‘-(CH2)6Br’ is ‘A’ and ‘R’ functional groups respectively, generates hyperbranched polymer with peripheral alkyl bromide functional groups as shown in Figure 2. The peripheral alkylbromides has been quantitatively transformed to quaternary ammonium or pyridinium salts using trimethyl amine or pyridine respectively. Thus by the post polymerization modification, we have transformed a hydrophobic hyperbranched polymer to a water soluble cationic hyperbranched polymer by simple and efficient post-polymerization modification. In a slightly different objective we
Another problem that I have addressed is the difficulty associated with the aforementioned copolymerization approach. In spite of the fact that stoichiometric amounts of ‘A-R’ type monomer was taken in ‘AB2 + A-R’ approach, the extent of peripheral functionalization i.e. the incorporation of ‘R’ group is relatively lower. Further the molecular weight of the hyperbranched polymer obtained is also not high. One of the reasons we adopted ‘AB2 + A-R’ approach is to provide a functional handle for the subsequent post-polymerization modification. We modified the ‘AB2’ type monomer with a functionalizable handle to circumvent the lower amount of incorporation of the ‘A-R’ type monomer in ‘AB2 + A-R’ approach.
Of all the readily functionalizable handles, click chemistry found to be a very useful tool for the post-polymerization modifications as the reactions conditions are mild, no side product, high selectivity, easy purification, etc. Another advantage of this reaction is that, we can incorporate any type of functional group starting from a single clickable parent hyperbranched polymer. In this particular project, I have
Earlier design of the ‘AB2’ type monomer in our group, to prepare hyperbranched polymer via melt transetherification process, involved benzylic methoxy groups as ‘B’ in ‘AB2’ monomer leading to a hyperbranched polymer with peripheral methoxy groups. Transetherification under melt-conditions is an equilibrium reaction which was driven towards the hyperbranched polymer by continuous removal of methanol from the system as a volatile alcohol. In the new design of ‘AB2’ monomer; we have used benzylic allyloxy groups as ‘B’ in ‘AB2’ monomer, where in polymerization is driven by the continuous removal of allyl alcohol (instead of methanol as in the previous case), generates hyperbranched polymer with peripheral allyloxy group containing hyperbranched polymer. The allyloxy groups can be subsequently functionalized with a variety of thiol, we prepared a hydrocarbon-soluble octadecyl-derivative, amphiphilic systems using 2-mercaptoethanol and chiral amino acid (N-benzoyl cystine) hyperbranched structures by using thiol-ene click reactions (Figure 3). Polymers prepared from the parent hyperbranched polymer have significantly different physical properties like glass transition temperature (Tg), melting point (Tm) etc; thus considering the versatility of functionalization, parent polymer could be envisioned as a clickable hyperscaffold. More interestingly by functionalizing cystine derivative, we have demonstrated the possibility of biconjugation of the hyperbranched polymer. In summary, the limitations of ‘AB2+A-R’ copolymerization approach (low molecular weight
Molecular weight and molecular weight distribution are very important parameters that influence the physical property and thus the application of the polymeric materials. As predicted by Flory, hyperbranched polymers are inherently polydisperse in nature and it tends to infinity when the percent of conversion is very high. Experimentally observed value of polydispersity is also significantly higher compared to their linear analogues. Control of the molecular weight and polydispersity of hyperbranched polymer by using a suitable amount of reactive multifunctional core has been demonstrated in this project. We have substantiated by using very little amount of ‘B3’ core along with ‘AB2’ monomer; wherein ‘B’ in ‘B3’ are more reactive than ‘B’ in ‘AB2’ monomer, regulate the molecular weight and polydispersity of the resulting hyperbranched polymer. As the ratio of core to monomer increases the molecular weight and polydispersity reduces in nearly linear fashion. In a slightly different objective, the core and periphery are functionalized with two different fluorophore by using orthogonal click reactions and demonstrated the possibility of energy transfer from periphery to the core of the hyperbranched polymer.
In this section of my thesis, the self-assembly behavior of a periodically grafted amphiphilic copolymer has been studied. Polymer was synthesized via melt transesterification approach where hexaethylene glycol monomethyl ether (HEG) containing diester monomers are reacted with alkylyne diol monomers with varying carbon spacer (C12 and
Another interesting problem, I approached is to functionalize the interior part of the hyperbranched polymer. In the case of dendrimer, as it is a step-wise synthesis, internal functionalization could be accomplished with the order of monomer addition i.e. by putting the internal functional group containing monomer first followed by other monomer not having those functional groups, whereas it is a bit challenging task for hyperbranched polymers especially when dealing with polycondensation of AB2 monomers, as it is a single step polymerization process. For a hyperbranched polymer in the polycondensation of ‘AB2’ monomer, the internal functional group should reside in between of the ‘A’ and ‘B’ functional group wherein the internal functional groups are silent during the process of polymerization. In order to do so, we have designed and synthesized a new AB2 monomer (a in Figure: 4). Here decanol is the volatile condensate that was removed during the transetherification reactions leading to a hyperbranched polymer having allyl group as the internal functional group and decyloxy as the peripheral functional group (b in Figure: 4). As a post-polymerization modification, the interior allyl groups were modified by thiol-ene click reaction with variety of thiol derivatives. In one example, the inherent hydrophobic nature of the parent hyperbranched polymer which is enhanced by the decyl chain at the molecular periphery, is converted to a alkaline water soluble hyperbranched polymer by the click reaction with mercapto succinic acid (d in Figure: 4) or mercapto propionic acid (c in Figure: 4) to the internal allyl groups, generating a novel amphiphilic hypersystem. This kind of amphiphilic systems are very interesting to study for their self-assembly behavior, in this particular case, the modified hyperbranched polymer adopts as a large spherical aggregates in alkaline water evidenced by FESEM (Figure: 4) and AFM images. Further investigation is being carried out to understand the exact nature of these aggregates. As the hyperbranched polymer contained ‘-S-‘ group in the interior, we utilized this as the scaffold for scavenging heavy metal ions like Hg2+ from aqueous solutions to the chloroform solution containing polymer. This hyperbranched polymer could trap Hg2+ ions even when present in ppm level of contamination.