Thermoresponsive star-shaped poly(2-isopropyl-2-oxazolines) based on octa-tert-butylcalix[8]arene

Abstract

The cationic polymerization of 2 oxazolines is a simple and suitable approach to the incorporation of polyethyleneimine into copolymers of various struc tures. After the biocompatibility of poly(2 alkyl 2 oxazolines) was discovered [1], the interest in poly mers of this type increased appreciably. In addition, aqueous solutions of a number of poly(2 alkyl 2 oxazolines) possess lower critical solution tempera tures, a circumstance that makes it possible to create new types of thermoresponsive materials on their basis [2].Poly(2 isopropyl 2 oxazoline) is one of the best studied thermosensitive polymers. In aqueous solu tions below the LCST (~40°С), the amide groups of this polymer are heavily hydrated; as a result, macro molecules turn out to be hydrophilized. Above the LCST, the degree of hydration decreases and intramo lecular hydrogen bonds form between amide frag ments. Owing to transition to the globular conforma tion, macromolecules become hydrophobic and the polymer loses the ability to dissolve in water [3].The incorporation of fragments able to efficiently bind inorganic ions and organic compounds into poly mers is a new strategy to designing supramolecular colloid structures and thus allows modeling of pro cesses occurring in living nature [4]. It was shown that the transition from linear to star shaped macromole cules significantly improves the capability of polymers for molecular recognition [5]. As is known [6], calix arenes are substances that can form complexes with low molecular mass compounds and metal ions that are capable of molecular recognition and formation of supramolecular structures. In our opinion, star shaped polymers with the central calixarene core and thermoresponsive poly(2 isopropyl 2 oxazoline) arms may be used as molecular containers for low molecu lar mass compounds and metal ions; moreover, they serve as elements of thermoresponsive colloid systems.There are two main approaches to the synthesis of star shaped macromolecules. In the first case, arms are synthesized separately and then are bound by poly functional crosslinking agents. In the second case, arms grow on a multicentered initiator. This variant was found to be efficient for the synthesis of star shaped polymers based on calixarenes, especially when living ionic or controlled radical polymerization is used. For example, initiators based on calix[8]arene and tetraethylresorcinarene were employed to imple ment the living cationic polymerization of isobutylene [7,8] and 2 methyloxazoline [9], respectively. Con trolled radical polymerization, primarily atom trans fer radical polymerization (ATRP), is also in wide use for the synthesis of star shaped polymers with calix(resorcinarene)arene cores and arms that are polystyrene [10,11], polyacrylate [11,12], or poly methacrylate [13] moieties. Therefore, the approach based on the use of the macrocyclic polyfunctional initiator for the cationic polymerization of 2 isopro pyl 2 oxazoline was selected for the synthesis of star shaped polymers with octa tert butylcalix[8]arene cores...

Influence of arm length and number on star-shaped poly(2-isopropyl-2-oxazoline) aggregation in aqueous solutions near cloud point

“…It was shown earlier that star-shaped polyoxazolines with calix [8]arene core could be synthesized by core-first approach using the omega-bromocarboxylic acid t-butylcalix [8]arene octa ester [14]. In the present paper the same approach was used for the synthesis of four-arm star samples with the similar structure, keeping in mind the results that we published previously [14] as well as the likely higher steric hindrances in the case of calix [4]arene moiety vs.…”

“…The polymer behavior above cloud point temperature T cp was studied using stars with 20 methacrylic homo-(degree of polymerization N arm = 97) or copolymer (N arm = 96 and 166) as arms [12]. The investigation of eight-arm poly(2-isopropyl-2-oxazolines) (PiPrOx 8 ) showed that decrease in arm length leads to a shift in T cp to higher temperatures, whereas the temperature range of phase transition increases from 0.5 to 10 °С [14]. A c c e p t e d M a n u s c r i p t 3 Furthermore, the aggregate shape factor, that is the ratio of radius of gyration to hydrodynamic radius, increased at passage from homopolymer to copolymer arms from 0.68 to ≥ 1, which was identified as formation of looser structure.…”

Deuterium Isotope Effect on Solution Behavior of Thermoresponsive Star-Shaped Poly(2-isopropyl-2-oxazoline)

“…[16] The PiPrOx 8 sample was prepared with a feed ratio of monomer to initiator of 1:20, CH 3 NO 2 as solvent, temperature of 70 C, and time of polymerization of 120 h. The polydispersity index of the arms (after hydrolysis) was 1.41 versus polyethylene oxide standards according to the gel-permeation chromatography data. The molecular characteristics of the PiPrOx 8 sample under investigation were determined in chloroform solution by static and dynamic light scattering, sedimentationdiffusion analysis, and viscometry.…”

“…[15] In our previous studies, the behavior of PiPrOx 8 in H 2 O solution was investigated. [15][16][17] The molar-mass dependence of T cp was determined, and it was shown that T cp increases with decreasing solution concentration. The lower critical solution temperature for PiPrOx 8 was about 10 C lower than for linear analogues.…”

Influence of arm length on aqueous solution behavior of thermosensitive poly(2-isopropyl-2-oxazoline) stars

“…For series of six-and 13-arm PEtOx stars, T CP reduces with the growth of molar mass [10]. While N arm growth from 44 to 59 decreases phase transition temperature by 3°C for six-arm samples, the 13-arm stars exhibit T CP growth by 9-10°C with arm shortening from 17 to 11 monomer units.…”