Poly(2-cyclopropyl-2-oxazoline): from rate acceleration by Cyclopropyl to Thermoresponsive properties

The synthesis and microwave-assisted living cationic ring-opening polymerization of 2-cyclopropyl-2-oxazoline is reported revealing the fastest polymerization for an aliphatic substituted 2-oxazoline to date, which is ascribed to the electron withdrawing effect of the cyclopropyl group. The poly(2-cyclopropyl-2-oxazoline) (pCPropOx) represents an alternative thermo-responsive poly(2-oxazoline) with a reversible critical temperature close to body temperature. The cloud point (CP) of the obtained pCPropOx in aqueous solution was evaluated in detail by turbidimetry, dynamic light scattering (DLS) and viscosity measurements. pCPropOx is amorphous with a significantly higher glass transition temperature (T(g) similar to 80 degrees C) compared to the amorphous poly(2-n-propyl-2-oxazoline) (pnPropOx) (T(g) similar to 40 degrees C), while poly(2-isopropyl-2-oxazoline) piPropOx is semicrystalline. In addition, a pCPropOx comb polymer was prepared by methacrylic acid end-capping of the living cationic species followed by RAFT polymerization of the macromonomer. The polymer architecture does not influence the concentration dependence of the CP, however, both the CP and T(g) of the comb polymer are lower due to the increased number of hydrophobic end groups

Efficient cationic ring-opening polymerization of diverse cyclic imino ethers: unexpected copolymerization behavior

The recently developed fast microwave-assisted cationic ring-opening polymerization procedure for 2-oxazolines seems to be ideally suited for slower polymerizing cyclic imino ether monomers. In this study we report the effect of the cyclic imino ether structure on the polymerization rate under exactly the same microwave-assisted conditions revealing that indeed less reactive cyclic imino ethers, including 2-oxazines as well as 4- and 5-substituted 2-oxazolines, can be polymerized to at least 50% conversion for the slowest monomer, namely 5-methyl-2-butyl-2-oxazoline, within 10 h. In addition, the copolymerization behavior of 4-ethyl-2-butyl-2-oxazoline with 2-methyl-2-oxazoline and 2-phenyl-2-oxazoline unexpectedly revealed faster incorporation of the less reactive 4-ethy1-2-buty1-2-oxazoline monomer compared to 2-phenyl-2-oxazoline due to the increased bulk of the latter monomer amplifying the sterical hindrance for polymerization onto the 4-ethyl-2-butyl-2-oxazolinium propagating species

Poly(cyclic imino ether)s beyond 2-substituted-2-oxazolines

2-Oxazolines (2-OZO) are 5-membered cyclic imino ethers whose cationic ring-opening polymerization (CROP) mechanism and resulting polymer properties are extensively studied. However, also 6- and 7-membered cyclic imino ethers can be polymerized via CROP. Together with the much less studied 4- and 5-substituted mainchain chiral poly(2-oxazoline)s (P-2-OZO), these compounds are interesting monomers to enhance the versatility of (co) poly(cyclic imino ether) s. To emphasize the potential of such alternative cyclic imino ether monomers, we provide an overview on the polymerizations of 2-oxazine (2-OZI) and chiral 4- and 5-substituted 2-OZO as well as of selected properties of the resulting polymers. In addition, the hydrolysis of these polymers into the corresponding poly(alkylene imine) s will be addressed

Thermoresponsive poly(2-oxazine)s

The monomers 2-methyl-2-oxazine (MeOZI), 2-ethyl-2-oxazine (EtOZI), and 2-n-propyl-2-oxazine (nPropOZI) were synthesized and polymerized via the living cationic ring-opening polymerization (CROP) under microwave-assisted conditions. pEtOZI and pnPropOZI were found to be thermoresponsive, exhibiting LCST behavior in water and their cloud point temperatures (TCP) are lower than for poly(2-oxazoline)s with similar side chains. However, comparison of poly(2-oxazine) and poly(2-oxazoline)s isomers reveals that poly(2-oxazine)s are more water soluble, indicating that the side chain has a stronger impact on polymer solubility than the main chain. In conclusion, variations of both the side chains and the main chains of the poly(cyclic imino ether)s resulted in a series of distinct homopolymers with tunable TCP