R Dhamodharan

A novel method of preparing chitosan gels using in situ generated negatively-charged colloidal salts of a variety of metal ions is described. Their potential as scaffolds for tissue-engineering and as recoverable catalysts in aza-Michael addition is demonstrated here. Given their wide range of properties, they have broad scope for applications.

The polymerization of N-vinylcarbazole (NVK) and carbazole methacrylate (CMA) was carried out using controlled radical polymerization methods such as atom transfer radical polymerization (ATRP), single electron transfer (SET)-LRP, and single electron transfer initiation followed by reversible addition fragmentation chain transfer (SET-RAFT). Well-controlled polymerization with narrow molecular weight distribution (Mw/Mn) < 1.25 was achieved in the case of NVK by high-temperature ATRP while ambient temperature SET-RAFT polymerization was relatively slow and controlled. In the case of CMA, SET-RAFT is found to be more suitable for the ambient temperature polymerization. The polymerization rate followed first order kinetics with respect to monomer conversion and the molecular weight of the polymer increased linearly with conversion. The controlled nature of the polymerization is further demonstrated by the synthesis of diblock copolymers from PNVK and PCMA macroinitiators using a new flavanone-based methacrylate (FMA) as the second monomer. All the polymers exhibited fluorescence. The excimer bands in the homopolymers of PNVK and PCMA were very broad, which may be attributed to the carbazole-carbazole overlap interaction. The scanning electron microscopy analysis of the block copolymer reveals interesting morphological features. © 2010 Wiley Periodicals, Inc.

New, eco-friendly nanocomposite materials have been synthesized from natural rubber (NR) and magnetite nanoparticles for the first time. The poor oil resistance of natural rubber is exploited for the removal of oil spills. Towards this purpose, mildly epoxidized natural rubber (ENR)-magnetite nanoparticle (MN) nanocomposites are prepared and the absorption of petrol (gasoline) is studied. The extent of epoxidation is controlled in such a manner that the NR does not lose its elasticity while retaining to a significant degree its oil absorbing property. Epoxidation also serves as a means for binding sufficient quantity of MNs so that the composite can be recovered using a magnetic field. ENR with 5 mol% of epoxidation served as the best absorbent among all the absorbents studied as it was stable in petrol even after many days of immersion. It is observed that the ENR-MN nanocomposite absorbs 7 g of petrol per gram without any mass loss. The material was reused for several cycles without much loss in the capacity. The petrol uptake of ENR-MN is greater than that of butyl rubber which is the most commercially used rubber for oil spill removal. Porous rubber was also synthesized for the first time as oil uptake is facilitated not only by the hydrophobicity but also by the capillary absorption. Porous ENR absorbed a relatively larger amount of oil and exhibited the highest stability in oil. All the sorbents have quite high absorption capacities to be applied practically with a very low water uptake and a few of the absorbents could be satisfactorily reused. The model studies promise their potential use in the environmental field. © 2013 The Royal Society of Chemistry.

Polystyrene, poly(methylacrylate) and poly(methyl methacrylate) four and three-arm stars were synthesized by Reversible Addition Fragmentation chain-Transfer (RAFT) polymerization by using two new dithioester-derived chain transfer agents [CTA or R-S-(C = S)Z]), CTA-1 and CTA-2. CTA-1 is a four arm CTA while CTA-2 is a three-arm CTA. These were easily synthesized from commercially available reagents and were characterized by spectroscopic techniques such as 1H-NMR, 13C-NMR, IR and mass spectrometry. It is demonstrated that the two new CTAs enable the growth of arms away from the core (i.e., core first approach). An attempt has been made to study the effect of the structure of the R-group, which is present as the core in the CTA, on the polymerization, by analyzing the detailed kinetics. This study suggests that CTA-2, with a benzylic R group, enables the controlled star polymerization of styrene while CTA-1, with a R group similar in structure to the propagating radical derived from the polymerization of methyl acrylate (MA), enables the controlled polymerization of MA although to a lesser extent. This study also reveals that the temperature of free radical initiated RAFT (star) polymerization should be chosen in such a way that it is a compromise between reasonable rate of homolysis of the initiator and the CTA (R-group). © Taylor & Francis Group, LLC.

The polymerization of MMA, at ambient temperature, mediated by dansyl chloride is investigated using controlled radical polymerization methods. The solution ATRP results in reasonably controlled polymerization with PDI < 1.3. The SET-LRP polymerization is less controlled while SET-RAFT polymerization is controlled producing poly(methyl methacrylate) (PMMA) with the PDI < 1.3. In all the cases, the polymerization rate followed first order kinetics with respect to monomer conversion and the molecular weight of the polymer increased linearly with conversion. The R group in the CTAs do not appear to play a key role in controlling the propagation rate. SET-RAFT method appears to be a simpler tool to produce methacrylate polymers, under ambient conditions, in comparison with ATRP and SET-LRP. Fluorescent diblock copolymers, P(MMA-b-PhMA), were synthesized. These were highly fluorescent with two distinguishable emission signatures from the dansyl group and the phenanthren-1-yl methacrylate block. The fluorescence emission spectra reveal interesting features such as large red shift when compared to the small molecule. © 2012 Wiley Periodicals, Inc.

We have measured the fracture toughness, Gc, of an interface between polystyrene (PS) and poly (2-vinylpyridine) (PVP) reinforced with triblock copolymers (PVP-b-dPS-b-PVP) as a function of the areal chain density, Σ, of the copolymers at the interface. The failure mechanisms of the interface are studied by transmission electron microscopy and forward recoil spectrometry. For triblock copolymers with long PVP blocks (NPVP > NePVP, where NePVP is the entanglement polymerization index of PVP), a transition from chain scission at low Σ to crazing at high Σ* is observed. By comparing the areal chain density Σ* for the transition from chain scission to crazing for the triblock copolymers (Σ* = 0.015 chains/ nm2) to that for the diblock copolymers, PVP-dPS (Σ* = 0.03 chains/nm2), we show that most of the triblock copolymers form a "staple" structure at the interface with the dPS block making a loop on the PS side of the interface and the PVP ends anchoring the "staple" in the PVP side. As a result of the "staple" structure, the saturation areal chain density of the triblock copolymer (Σsat) at the interface is half of that for the diblock copolymer of similar molecular weight. For Σ< Σsat in the crazing regime, the fracture toughness of the interface is controlled by the areal joint density, Σcross, where Σcross is the number density of the copolymer excursions across the interface. For Σ> Σsat, the triblock copolymers appear to reinforce the craze fibrils at the crack tip better than the corresponding diblock copolymers, leading to an interface fracture toughness approaching that of the PS homopolymer itself. For a triblock copolymer with short PVP blocks NPVP < NePVP), there is a transition in the fracture mechanism from pull out of the PVP block to crazing with increasing Σ. Short triblock copolymers can form two chain conformations: one in which two PVP blocks anchor the copolymer on the homopolymer PVP side (staple structure) and one in which one PVP block anchors the copolymer on the PVP side (tail structure) of the interface. Comparison of Gc between the triblock copolymer and the corresponding diblock copolymer is made. The larger Gc values of the triblock copolymer reinforced interface in the crazing regime are observed as a result of enhancement in entanglements between the dPS loops of the triblock copolymer and the homopolymer PS.

The synthesis and characterization of a polymer with pendant groups carrying a cis-platinum complex by the chemical modification route is discussed. Diethyloxomalonate functionalized polystyrene (DPS) is synthesized by the Friedel-Crafts acylation of polystyrene with diethyloxomalonate in the presence of SnCl4, by a batch-wise addition process. The reaction of DPS with cis-diaquo(trans-l,2-diaminocyclohexane)-platinum(II) results in the formation of a cis-platinum chelated polymer (PtDPS). Proton NMR and IR spectroscopies are used to confirm the transformation. The polymer synthesized is further characterized using thermogravimetric analysis (TGA) as well as UV and fluorescence spectroscopic techniques. PtDPS thus synthesized is further modified into a water-soluble polymer by another polymer modification reaction resulting in the introduction of thiobarbituric acid moieties.

Controlled polymerization of styrene was carried out using ambient temperature single electron transfer initiation and propagation through the radical addition fragmentation chain transfer (SET-RAFT) method. SET-RAFT method was expected to have much wider applicability, especially for monomers that polymerize very rapidly by free/living radical polymerization. The typical RAFT polymerization required thermal/photochemical initiation and one of the end groups was required to be a fragment from the initiator. Under the SET-RAFT conditions, the ambient temperature initiation and the use of functional initiators would overcome some of the limitations of RAFT. SET-RAFT condition is expected to be very effective for monomers with a slower rate of initiation compared to the rate of propagation and those with a very high rate of propagation that require control.

Novel bifunctional initiators [1,1′-Bi-2-naphthol bis(2-bromo-2-methylpropionate); (R)-, (S)-, and racemic-] were synthesized from the esterification of 1,1′-bi-2-naphthol and used as initiators in atom transfer radical polymerization (ATRP) in conjunction with N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA), and copper (I) bromide or copper (I) chloride. The initiators synthesized were completely characterized by UV, FTIR, NMR, and Mass spectroscopies. A detailed investigation of the ATRP of methyl methacrylate (MMA) with the bifunctional initiators (BBiBN) along with CuBr or CuCl/PMDETA catalyst system in anisole was carried out at 30°. Thus, MMA polymerization is shown to proceed with first-order kinetics, with predicted molecular weight, and narrow polydispersity indices. The ATRP of glycidyl methacrylate (GMA) and tert-butyl acrylate (tBA) were also performed with BBiBN initiator in conjunction with CuBr/PMDETA catalyst system. The polymerization of GMA was carried out at 30°C, but tBA was polymerized at 60°C. Gel permeation chromatography (GPC), FTIR, NMR, UV spectroscopies, and TGA were used for the characterization of the polymers synthesized. © 2004 Wiley Periodicals, Inc.

Controlled radical polymerization of cyclohexyl methacrylate (CHMA), at ambient temperature, using various chain transfer agents (CTAs) is successfully demonstrated via single electron transfer-radical addition fragmentation chain transfer (SET-RAFT). Well-controlled polymerization with narrow molecular weight distribution (Mw/Mn) < 1.25 was achieved. The polymerization rate followed first-order kinetics with respect to monomer conversion, and the molecular weight of the polymer increased linearly up to high conversion. A novel, fluorescein-based initiator, a novel fluorescent CTA and two other CTAs comprising of butane thiol trithiocarbonate with cyano (CTA 1) and carboxylic acid (CTA 3) as the end group were synthesized and characterized. The polymerization is observed to be uncontrolled under SET and less controlled under atom transfer radical polymerization (ATRP) condition. CTA 2 and 3 produces better control in propagation compared with CTA 1, which may be attributed to the presence of R group that undergoes ready fragmentation to radicals, at ambient temperature. The poly(cyclohexyl methacrylate) [P(CHMA)] prepared through ATRP have higher fluorescence intensity compared with those from SET-RAFT, which may be attributed to the quenching of fluorescence by the trithiocarbonate and the long hydrocarbon chain. It is observed that block copolymers P(CHMA-b-t-BMA) produced from P(CHMA) macroinitiators synthesized via SET-RAFT result in lower polydispersity index in comparison with those synthesized via ATRP. © 2010 Wiley Periodicals, Inc.