Synthesis of poly(2‐oxazoline)s with side chain methyl ester functionalities: Detailed understanding of living copolymerization behavior of methyl ester containing monomers with 2‐alkyl‐2‐oxazolines

Abstract

Poly(2-oxazoline)s with methyl ester functionalized side chains are interesting as they can undergo a direct amidation reaction or can be hydrolyzed to the carboxylic acid, making them versatile functional polymers for conjugation. In this work, detailed studies on the homo-and copolymerization kinetics of two methyl ester functionalized 2-oxazoline monomers with 2-methyl-2-oxazoline, 2-ethyl-2-oxazoline, and 2-npropyl-2-oxazoline are reported. The homopolymerization of the methyl ester functionalized monomers is found to be faster compared to the alkyl monomers, while copolymerization unexpectedly reveals that the methyl ester containing monomers significantly accelerate the polymerization. A computational study confirms that methyl ester groups increase the electrophilicity of the living chain end, even if they are not directly attached to the terminal residue. Moreover, the electrophilicity of the living chain end is found to be more important than the nucleophilicity of the monomer in determining the rate of propagation. However, the monomer nucleophilicity can be correlated with the different rates of incorporation when two monomers compete for the same chain end, that is, in copolymerizations.

Thermal Properties of Methyl Ester-Containing Poly(2-oxazoline)s

“…Series of homopolymers and copolymers of EtOx and nPropOx with MestOx and C3MestOx were prepared [37,38] with varying feed ratios of 10, 20, 30, 50, and 70 mol % MestOx or C3MestOx and a constant monomer to initiator ratio of [M]/[I] = 100, yielding four sets of copolymers. All polymers were terminated with piperidine instead of the widely used methanolic sodium hydroxide /cm at 5 mg¨mL´1 and were stirred at room temperature until all polymer was dissolved.…”

“…Dry solvents were obtained from a solvent purification system from J.C. Meyer, with aluminum oxide drying system and a nitrogen flow. MestOx [37], C3MestOx [38], and nPropOx [48] were prepared according to literature procedures. In this paper we describe the synthesis and thermal properties of methyl ester-containing PAOx.…”

“…Polymerizations were carried out under microwave irradiation at 140˝C for 20 min aiming for full conversion [37,38]. After cooling to room temperature, the polymerizations were quenched by the addition of piperidine.…”

“…Functional groups, often protected, can be introduced into PAOx by making use of a functional monomer or initiator during the living cationic ring opening polymerisation (CROP, Scheme 1) of 2-oxazoline monomers or by using a functional terminating agent, yielding well-defined PAOx with control over number and type of functionalities [15,[29][30][31][32][33][34][35][36][37][38]. Methyl esters are especially interesting, because they can undergo a direct amidation with a variety of amines to easily introduce other functional groups such as alcohols, hydrazide and amines [39][40][41][42].…”

“…In this paper we describe the synthesis and thermal properties of methyl ester-containing PAOx. Homopolymers and statistical copolymers, with near ideal random monomer distributions [38], of 2-ethyl-2-oxazoline (EtOx, 1) and 2-n-propyl-2-oxazoline (nPropOx, 2) with 2-methoxycarbonylethyl-2-oxazoline (MestOx, 3) and 2-methoxycarbonylpropyl-2-oxazoline (C3MestOx, 4) were prepared with 10, 20, 30, 50, and 70 mol % methyl ester content. The thermal solution and bulk properties of these polymers were determined by turbidimetry, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA), respectively.…”