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Abstract
A mini-library of star-shaped thermoresponsive polymers having six arms was prepared using a hexafunctional xanthate by reversible addition–fragmentation chain transfer (RAFT) polymerization. Star polymers with homopolymeric arms of poly(N-vinylcaprolactam) (PNVCL), copolymeric arms of poly(N-vinylcaprolactam-co-N-vinylpyrrolidone) (PNVCL-co-PNVP) and also arms of block copolymers of PNVCL-b-PVAc, (PNVCL-co-PNVP)-b-PVAc, and combinations of them changing the order of the block was achieved exploiting the R-RAFT synthetic methodology (or R-group approach), wherein the thiocarbonyl group is transferred to the polymeric chain end. Taking advantage of the RAFT benefits, the molecular weight of the star polymers was controlled (Mn = 11,880–153,400 g/mol) to yield star polymers of different sizes and lower critical solution temperature (LCST) values. Removing the xanthate group of the star polymers allowed for the introduction of specific functional groups at the ends of the star arms and resulted in an increase of the LCST values. Star PNVCL-b-PVAc diblock copolymers with PVAc contents of 5–26 mol % were prepared; the hydrophobic segment (PVAc) is located at the end of the star arms. Interestingly, when the PVAc content was 5–7 mol %, the hydrodynamic diameter (Dh) value of the aggregates formed in water was almost the same sa the Dh of the corresponding PNVCL star homopolymers. It is proposed that these star block copolymers self-assemble into single flowerlike micelles, showing great stability in aqueous solution. Star block copolymers with the PVAc hydrophobic block in the core of the star, such as PVAc-b-(PNVCL-co-PNVP), form micellar aggregates in aqueous solution with Dh values in the range from ~115 to 245 nm while maintaining a thermoresponsive behavior. Micellar aggregates of selected star polymers were used to encapsulate methotrexate (MTX) showing their potential in the temperature controlled release of this antineoplasic drug. The importance of the order in which each block constituent is introduced in the arms of the star polymers for their solution/aggregation behavior is demonstrated.
Highlights
Star-like polymers exhibit a polymer architecture that initially attracted attention due to its unique rheological properties [1]
We recently reported the preparation of star-shaped poly(N-vinylcaprolactam)-block
We recently reported the preparation of star-shaped poly(ethylhexylacrylate)-block-polyethylene glycol (PNVCL-b-PEHA-b-PEG) aggregates and its poly(N-vinylcaprolactam)-block-poly(ethylhexylacrylate)-block-polyethylene glycol application as nanocarriers of MTX [34]
Summary
Star-like polymers exhibit a polymer architecture that initially attracted attention due to its unique rheological properties [1]. Polymers 2018, 10, 20 promising is the use of reversible addition–fragmentation chain transfer (RAFT) polymerization by the core-first approach [17,18,19,20]. In this methodology, a thio-group containing chain transfer agent, called RAFT-agent, can be linked to the nucleus of the star (Z-group approach) or at the end of the arms of the star (R-group approach). The preparation of polymers using the R-group approach opens up a whole range of functionalization possibilities at the ends of the polymer chains through post-functionalization reactions [23,24]. The thiol functionality allows the stabilization of gold nanoparticles, and preparation of biopolymers conjugates, block copolymers, nano-objects, etc. [25]
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