Amphiphilic polymers with carboxyl groups

Linear amphiphilic polymers with carboxyl groups were also available via sequential cationic polymerization of a fimctional vinyl ether and an alkyl derivative [6]. The sequential living cationic polymerization of diethyl 2-(vinyloxy)ethylmalonate [VOEM CHf=CHOCH2CH2CH(COOCH2CH3)2]... 

Poly(A-vinylamides) are another class of synthetic thermo-responsive polymers. Two of the most common polymers of this class are poly(Al-vinylpyrrolidone) (PVP) and poly(A-vinylcaprolactam) (PVCL) (Figure 1.3). PVCL is a water-soluble nonionic amphiphilic polymer with its basic unit comprising a seven-membered cyclic amide with a polar hydrophilic carboxyl group and an amide group connected directly to a hydrophobic vinyl chain. 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

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]. 

Polyacrylic acid and polyacrylamide are typical water-soluble polymers that can be functionalized by the addition of hydrophobic sidechains that turn them into amphiphilic materials. The usual processes would include esterification and amida-tion of the carboxyl groups. Similar results can be obtained by preparing copolymers of acrylic and methacryhc acid with the desired preformed esters. A third alternative is the formation of polyacrylate or polymethacrylate esters followed by controlled saponification or hydrolysis to a desired degree of free carboxyl groups. The derivitization of acrylics is also an essentially random process, so that the exact characteristics of the final product may be somewhat variable. Close process control can, again, ensure a functionally reproducible product. 

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