Functional Hyperbranched Polyethers Via Melt-Transetherification Polymerization

Abstract

Dendrimers are highly branched macromolecules which are prepared by a stepwise procedure. The presence of a well-defined core, discrete generations and a large number of terminal groups in dendrimers make them structurally very interesting and potentially useful for a wide variety of applications.1 Hyperbranched polymers,2 on the other hand, do not possess a unique core or discrete generations and they contain a large number of statistically distributed defects. Despite the presence of structural imperfections, studies have indicated that hyperbranched polymers capture many of the essential features of dendrimers, such as adoption of a compact conformation and the presence of a large number of readily accessible terminal functional groups. The first chapter of this thesis provides a brief introduction to hyperbranched polymers, with an emphasis on different methods for synthesizing them, followed by a discussion of the various approaches to control their molecular structural features, such as molecular weight, polydispersity, degree of branching, branching density, terminal end-groups, etc. One of the main objectives of the present study is to develop a simple synthetic strategy to generate peripherally functionalized (or functionalizable) hyperbranched polymers (HBP) that could potentially exhibit core-shell type behavior; in other words, polymers that carry segments of distinctly different solubility preferences within the core-region and the peripheral shell. To this end, in chapter 2 we describe the use of the melt-transetherification process,3 using an AB2 monomer along with a mono-functional A-R type comonomer, to directly generate core-shell type hyperbranched structures in a single step.4 Given that an AB2 monomer carries one equivalent excess of B functionality, copolymerization with an A-R type molecule bearing a single A functional group, readily permits the decoration of the periphery of the hyperbranched structures with these R-units. Thus, hyperbranched polyethers having polyethylene glycol (PEG) segments at their molecular periphery were prepared by a simple procedure wherein an AB2 type monomer was melt-polycondensed with an A-R type monomer, namely heptaethylene glycol monomethyl ether (HPEG). The presence of a large number of PEG units at the termini rendered a lower critical solution temperature (LCST) to these copolymers, above which they precipitated out of an aqueous solution.5 In an effort to understand the effect of various molecular structural parameters on their LCST, the length of the hydrophobic spacer segment within the hyperbranched core and the extent of PEGylation, were varied. Increase in the size and hydrophobicity of the hyper-core resulted in a continuous lowering of its LCST, while an increase in the level of PEGylation, increases the LCST, for a given size of the hyper-core. Additionally, linear analogues that incorporates pendant PEG segments were also prepared and comparison of their LCST with that of the hyperbranched polymer clearly revealed that the hyperbranched topology leads to a substantial increase in the LCST, highlighting the importance of the peripheral placement of the PEG units as shown in figure 1.5 This observation also provided an indirect evidence for the development of core-shell type topology in these peripherally functionalized hyperbranched structures. Figure 1. Transmittance of a 0.4 wt % aqueous solution of the linear and hyperbranched polymers as a function of temperature, measured at 600 nm. Such core-shell type HBPs could be also exploited both as unimolecular micelles and reverse micelles by suitably modifying the nature of the AB2 and A-R type monomers4. In the third chapter, the preparation and dye-encapsulation properties of unimolecular micelles as well as reverse micelles based on core-shell HBPs have been presented. In case of micelle forming polymers, an AB2 monomer carrying a decamethylene spacer was used along with heptaethylene glycol monomethyl ether (HPEG) as the A-R type comonomer. One the other hand, for the preparation of reverse micelle forming polymers, an AB2 monomer containing an oligo(oxyethylene) spacer was used along with cetyl alcohol as the A-R type comonomer as shown in scheme 1. The former was readily soluble in water while the latter was soluble in hydrocarbon solvents, like hexane. NMR spectral studies confirmed that both the approaches generated highly branched structures wherein ca. 65-70 % of the terminal B groups were capped by the A-R comonomer. scheme1. Synthesis of the unimolecular micelle and reverse micelle forming polymers using a one step AB2 + A-R type copolymerization. (REFER PDF FILE) One of the approaches commonly used to demonstrate core-shell behavior is to examine the ability of such polymers to encapsulate appropriate dyes from a suitable medium. In the case of the micelle-forming polymer, an aqueous solution of the polymer (6 μM) was sonicated in the presence of excess pyrene for varying periods of time. From the UV-visible spectra (Figure 2) of the aqueous solution (after filtration), it is evident that the saturation uptake is attained in about 7 h. Similar studies were also carried out for reverse-micelle forming polymers in hexane, using methyl orange as the dye. These dye-uptake studies, in conjunction with dynamic light scattering, unequivocally confirmed the formation of unimolecular micelles/reverse micelles. Figure 2. Absorbance as a function of sonication time for micelle-forming polymers (A), and absorbance as a function of the amount of solid dye taken, for reverse micelle-forming polymers (B). (REFER PDF FILE) Another novel approach to generate core-shell systems, using A2 + B3 + A-R type terpolymerization, was also explored in an effort to simplify the synthesis even further. However, dye-uptake measurements revealed that the polymers prepared via the AB2 + A-R approach exhibited a significantly larger uptake compared to those prepared via the A2 + B3 + A-R approach. This suggests that the AB2 + A-R approach generates hyperbranched polymers with better defined core-shell topology when compared to polymers prepared via the A2 + B3 + A-R approach, which is in accordance with previous studies6 that suggest that A2 + B3 approach yields polymers with significantly lower branching levels and consequently less compact structures. In chapter 4, different strategies for functionalization of the core-region and periphery of core-shell type hyperbranched polymers (HBP) using the “click” reaction7 have been explored. For achieving peripheral functionalization, an AB2 + A-R1 + A-R2 type copolymerization approach was used (as depicted in scheme 2), where the A-R1 is heptaethylene glycol monomethyl ether (HPEG-M) and A-R2 is tetraethylene glycol monopropargyl ether (TEG-P). A very small mole-fraction of the propargyl containing monomer, TEG-P was used to ensure that the water-solubility of the core-shell type HBP is minimally unaffected. Scheme 2. Preparation of a hyperbranched polyether having a few percent of propargyl groups at the molecular periphery and further click reaction to place fluorophores at the periphery. Similarly, to incorporate propargyl groups in the core region, a new propargyl group bearing B2-type monomer was designed and utilized in an AB2 + A2 + B2 + A-R1 type copolymerization, such that the total mole-fraction of B2 + A2 is small and their mole-ratio is 1:1 (Scheme 3). Further, using a combination of both the above approaches, namely AB2 + A2 + B2 + A-R1 + A-R2, hyperbranched structures that incorporate propargyl groups both at the periphery and within the core were synthesized. Since the AB2 monomer carries a C-6 alkylene spacer and the periphery is PEGylated, all the derivatized polymers form core-shell type structures in aqueous solutions. In order to ascertain and probe the location of the propargyl groups in these HBP’s, a fluorescent azide, namely azidomethyl pyrene, was quantitatively clicked onto these polymers and their fluorescence properties were examined in solvents of different polarities. Fluorescence spectra in water was unable to differentiate between the fluorophores present at different locations suggesting that the tethered pyrene at the end of a flexible oligoethylene oxide unit is probably tucked within the core-region because of its intrinsic hydrophobic nature. Scheme 3. Preparation of a hyperbranched polyether bearing a few percent of the propargyl groups within the core and further click reaction to place fluorophores in the core-region. The conventional melt-transetherification polymerization proceeds by continuous removal of methanol as volatile by product.3 The fifth chapter describes the design and development of a new AB2 monomer that carries two propargyloxy benzyl groups and one hydroxyl group, which underwent melt-transetherification condensation by exclusion of propargyl alcohol (instead of methanol) to generate a hyperbranched polyether containing numerous propargyl ether groups located on their molecular periphery as shown in scheme 4. These propargyl groups were readily “clickable” under very mild conditions with a variety of azides using the copper (I) catalyzed Huisgen type dipolar cycloaddition, popularly known as click reaction,7 to generate a range of functionalized hyperbranched polymers. The simplicity of the monomer synthesis, the solvent-free melt polymerization process and the mild conditions under which quantitative peripheral derivatization is achievable, makes this process ideally suited for the generation of hyperscaffolds onto which a wide range of functionalities could be placed. This turned out to be a rather remarkable extension of the melt transetherification polymerization that permitted the direct generation of peripherally clickable hyperbranched scaffold that, in principle, could be used to generate a wide range of interesting structures. Scheme 4. Synthesis of the hyperbranched polyether with clickable surface in a single step. (For structural formula pl refer pdf file)