The synthesis of linear polyethylenimine (L-PEI) is mostly performed via the acidic or basic hydrolysis of poly(2-alkyl-2-oxazoline)s (PAOx). Besides full hydrolysis leading to L-PEI, the partial hydrolysis of the PAOx sidechains results in PAOx-PEI copolymers having secondary amine groups in the polymer backbone. The secondary amine groups of L-PEI and PAOx-PEI can act as charge carriers for the complexation of DNA for gene therapy. Furthermore, they are also excellent chemical moieties for post-polymerization modification reactions providing straightforward access to new PAOx (co)polymers based on a variety of PAOx, including the commercially available poly(2-ethyl-2-oxazoline) (PEtOx). Within this review, we will discuss the acidic and basic (partial) hydrolysis of PAOx as well as the corresponding mechanisms. In addition, an overview of the recent literature on the post-polymerization modification of the fully hydrolyzed L-PEI and the partially hydrolyzed PAOx-PEI is provided.

Boronic acid-functionalized (co)polymers have gained increasing attention in the field of responsive polymers and polymeric materials due to their unique characteristics and responsiveness towards both changes in pH and sugar concentrations. This makes these (co)polymers excellently suited for various applications including responsive membranes, drug delivery applications and sensor materials. Unfortunately, boronic acid-based polymer research is also notorious for its challenging monomer synthesis and polymerization and its overall difficult polymer purification and manipulation. In light of this, many research groups have focused their attention on the optimization of various polymerization techniques in order to expand the field of BA-research including previously unexplored monomers and polymerization techniques. In this paper, a new post-polymerization modification methodology was developed allowing for the synthesis of novel boronic acid-decorated poly(2-alkyl-2-oxazoline) (PAOx) copolymers, utilizing the recently published PAOx methyl ester reaction platform. The developed synthetic pathway provides a straightforward method for the introduction of pH- and glucose-responsiveness, adding this to the already wide variety of possible responsive PAOx-based systems. The synthesized BA-decorated PAOx are based on the thermoresponsive poly(2-n-propyl-2-oxazoline) (PnPropOx). This introduces a pH and glucose dependence on both cloud and clearance point temperatures of the copolymer in aqueous and pH-buffered conditions, yielding a triply-responsive (co)polymer that highlights the wide variety of obtainable properties using this pathway.

Poly(2-alkyl/aryl-2-oxazoline)s (PAOx) are a class of cytocompatible polymers that have received growing interest over the past decade. This growing interest can mainly be attributed to their greater chemical versatility compared to other cytocompatible polymers, such as poly(ethylene glycol) (PEG), that originate from the diverse set of monomers compatible with the cationic ring-opening polymerization. Within this contribution we focus on the modification of methyl ester functional PAOx, on which we recently reported the quantitative conversion of methyl ester side chain bearing PAOx via direct amidation for a number of amines. While this approach is robust, it can be challenging to introduce certain amines, due to their high cost, poor solubility or additional functionalities. Here we evaluated three alternative amidation approaches of methyl ester side chain bearing PAOx with tyramine in terms of selectivity and quantitative conversion to the respective secondary amides. The amidation proceeded successfully via DMTMM coupling, PFP-activation and TBD catalysis, although only the latter 2 methods selectively yielded the desired product. In summary, we present valuable alternatives to our earlier reported method, contributing further to the biomedical application potential of PAOx.