Temperature-responsive polymers amide

The common feature of thermo-responsive potymers are the coexistence of hydrophobic (e.g., methyl, ethyl, propyl) and hydrophilic (e.g., amide, carboxyl) groups in one macromolecular network. The polymers with an LCST are mostly used in drug delivery systems. The hydrophobic and the hydrophilic moieties in the molecular chain of a temperature-responsive polymer define its LCST. Hydrophilic monomers make the LCST increase and even disappear, and hydrophobic ones cause the LCST to decrease. Thus, a proper ratio of hydrophobic or hydrophilic moieties can be incorporated in order to get a suitable LCST. The adjustment of LCST to approximately body temperature is essential especially in the case of drug delivery applications [15, 18-20]. The transition temperature of the polymer can be modified using certain additives such as surfactant, salt concentration, or co-solvents. Surfactants act as amphiphiles when added into the pol5mieric solution modifies its hydrophilic-hydrophobic balance and further its transition temperature [21]. 

Trimellitic anhydride (TMA) is an important raw material for high temperature resistant polymers such as polyesterimide, poly-amideimide etc. The cyclic anhydride group in the TMA molecule offers a reaction site for imide formation and the carboxyl group for synthesis of ester or amide linkages. The polycondensation of TMA with glycol and/or diamine results in the formation of semiladder structures in the polymer, which is responsible for high temperature resistance. 

Of course, the temperature-responsive behavior of poly(acrylamide)s and poly(vinyl amide)s is not limited to the exact structures of PNIPAM and PVCL, and also analogous polymer structures have been reported to undergo temperature-induced phase separation upon heating in aqueous solution, such as poly(A-cyclopropylacrylamide) (Kuramoto and Shishido, 1998), poly(A( A-diethylacrylamide) (Lessard et al, 2001), poly(A-vinyl pip-eridone) (leong et al, 2011) and various substituted poly(A-vinyl pyrroli-done)s (Yan et al, 2010). 

However, the development of in vivo applications for PNIPAM is limited by its non-biodegradability and the presence of amide moieties that reduce its biocompatibility. For this reason, other thermo-responsive polymers have been investigated in recent years. Poly(N-vinylcaprolactam) is a promising alternative. This polymer has a LCST between 35 and 38°C, again close to the temperature of the human body, and is characterized by high biocompatibility and low toxicity (Konak et al, 2007 Medeiros et al, 2010 Shtanko et al, 2003 Yanul et al, 2001). Additionally, amphiphilic copolymers such as Pluronics and Tetronics have been developed, based on copolymers of polyethylene oxide and polypropylene oxide. These copolymer systems exhibit a solution-gel transition at close to human body temperature that permits their application as injectable implants (Samchenko et ai,2011). 

---