Ravin Narain

Water-Assisted Atom Transfer Radical Polymerization of N-Isopropylacrylamide: Nature of Solvent and Temperature

We demonstrate here via the atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) at low temperature that the negative function of water in aqueous ATRP is significantly suppressed. By the addition of a small amount of water in a water-miscible organic solvent and maintaining low polymerization temperature, the ATRP of NIPAM is relatively fast and well controlled. We observed that the rate of the polymerization in pure organic solvent at a monomer concentration of 20 wt % is slow, and relatively low conversions were obtained. The low conversion of PNIPAM in pure alcoholic media (such as methanol, ethanol, and n-propanol) is attributed to the poor solubility of the resulting low molecular weight polymer in such solvents. The consequence is that the PNIPAM chains are aggregated, resulting in the inaccessibility of the embedded halide atom of the polymer chain ends by the copper catalyst. As expected, the ATRP of NIPAM in pure water was found to be fast and uncontrolled. These results have therefore prompted us to study the ATRP of NIPAM in aqueous-organic mixtures. Room temperature polymerization of NIPAM in mixed aqueous-organic solvent mixtures (organic:water = 4:1 or 3:1) revealed to be fast and uncontrolled. However, when the NIPAM polymerization was conducted at low temperature (0 degrees C) in such solvent systems, the polymerization turned out to be well-controlled as the molar masses progress linearly with conversion, and pseudo-first-order kinetic plots were obtained. Furthermore, monomodal GPC traces and narrow molecular weight distributions were obtained in all aqueous-organic solvent systems. Chain extension for aqueous ATRP of NIPAM revealed to proceed well at low temperature as compared to room temperature. Furthermore, we observe that the rates of the polymerization of NIPAM in different aqueous-organic mixtures follow the trend of polarity in the case of the polar aprotic solvents. However, in the case of polar protic solvent (such as methanol, ethanol, 1-propanol, and 2-propanol), the rate of the polymerization was found to increase with increasing solubility of the PNIPAM from methanol to 2-propanol.

Covalently stabilized temperature and pH responsive four-layer nanoparticles fabricated from surface ‘clickable’ shell cross-linked micelles

Alkynyl-terminated double hydrophilic ABC triblock copolymer, poly(oligo(ethylene glycol) monomethyl ether methacrylate)-b-poly(2-(dimethylamino) ethyl methacrylate)-b-poly(2-(diethylamino) ethyl methacrylate) (alkynyl-POEGMA-b-PDMA-b-PDEA), was synthesized via atom transfer radical polymerization (ATRP) by sequential monomer addition using propargyl 2-bromoisobutyrate (PgBiB) as the initiator. The obtained triblock copolymer dissolves molecularly in acidic media and self-assembles into alkynyl surface-functionalized three-layer “onion-like” micelles consisting of a PDEA core, a PDMA inner shell, and a POEGMA corona at alkaline pH. Selective cross-linking of the PDMA inner shell with 2-bis(2-iodoethoxy)ethane (BIEE) results in structurally stable and surface ‘clickable’ shell cross-linked (SCL) micelles with pH-responsive PDEA cores. This new kind of SCL micelles could be further surface functionalized or conjugated with other azido- terminated polymer chains, functional groups, or biomolecules via Click chemistry. Four layer nanoparticles (SCL-PNIPAM) which have pH-responsive PDEA cores and temperature responsive PNIPAM outer coronas were fabricated from surface ‘clickable’ shell cross-linked (SCL) micelles and azide-terminated poly(N-isopropylacrylamide) (PNIPAM-N3) using Click chemistry. These novel four layer nanoparticles might act as suitable nano-sized drug delivery vehicles for the encapsulation and release of hydrophobic drugs as a function of either temperature or pH of the environment

Synthesis and characterization of novel glycosurfaces by ATRP

The direct synthesis of well defined sugar methacrylate-based homopolymer brushes with high grafting densities based on D-gluconamidoethyl methacrylate (GAMA) and 2-lactobionamidoethyl methacrylate (LAMA) from functionalized gold substrates was carried out using surface-initiated atom transfer radical polymerization (ATRP). A good control of the polymerization leading to an increase in the dry film thickness with reaction time was achieved in mixed methanol/water solvent media. However, grafted glycopolymer films of low thicknesses, which did not increase further with longer polymerization times were synthesized in water, suggesting the premature termination of the polymerization in the aqueous solvent. Attenuated total reflectance (ATR)-FTIR spectroscopy confirmed the successful grafting of the glycopolymer brushes on the modified gold substrates, while atomic force microscopy (AFM) verified that the anchored film covered the substrate surface completely and homogeneously. The surface roughness found by AFM was below 1 nm suggesting the preparation of very smooth glycopolymer films. The grafting of the glycopolymer chains onto the gold substrates afforded an increase in the surface hydrophilicity as confirmed by contact angle measurements. The synthesized glycopolymer films exhibited strong binding interactions with specific lectins via the “glycocluster” effect.

Septic sera induces apoptosis and DNA fragmentation factor 40 activation in fibroblasts

Sepsis, the systemic response to infection, is the leading cause of death in the intensive care units worldwide. Septic patients can succumb through the development of early refractory hypotension or late multiple organ dysfunction. Misregulation of apoptosis during sepsis may contribute to cellular dysfunction and multiple organ dysfunction. Utilizing a tissue culture model which mimics the human disease, we demonstrate that the addition of sera derived from septic patients induces apoptosis in human fibroblast cells. Addition of septic sera to 2fTGH cells induced apoptosis by activating caspase 8, caspase 3 and DNA fragmentation factor 40 (DFF 40). Interestingly, the addition of septic sera to cells which lack STAT1 (U3A cells) did not activate DFF 40. U3A cells were also shown to be resistant to septic serum induced apoptosis. These data suggest that DFF 40 mediated apoptosis plays a significant role in mediating sepsis induced cellular dysfunction.

CHAPTER 3:Well-Defined Cationic Polymers for Nucleic Acid Delivery

A large number of cationic polymers has been prepared and studied for their gene delivery efficacies, since the failure of retro-virus vector-based gene therapy trials in the 2000s. The introduction of the living radical polymerization (LRP) approach has allowed the synthesis of tailored gene delivery vectors of known molecular weights, architectures and compositions for gene delivery applications. The term “gene delivery” refers to the delivery of both deoxyribonucleic acid (DNA) and small interfering ribonucleic acid (siRNA) in living cells and tissues. Although the cargo delivery site for the two nucleic acids is different, the basic components of cationic vectors exploited in the design of gene delivery vectors are essentially the same. For LRP, atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer polymerization (RAFT) have allowed the synthesis of cationic vectors of near precise dimensions, hence establishing structure–activity relationships between cationic vectors and their gene delivery profiles. This attribute of LRP has enabled researchers to pinpoint and overcome the hurdles associated with traditional cationic polymers for gene delivery applications. In this chapter a brief account of the types of cationic vectors prepared by LRP and their role in gene expression in vitro and in vivo is discussed.