Synthesis of star poly(N-isopropylacrylamide) by β-cyclodextrin core initiator via ATRP approach in water

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Abstract

Star poly(N-isopropylacrylamide) (PNIPAAm) based on a β-cyclodextrin (β-CD) core macroinitiator was synthesized by means of atomic transfer radical polymerization (ATRP) in water using copper(I)/2,2bipyridyl complex as a catalytic system at temperature above the lower critical solution temperature (LCST) of the PNIPAAm. The macroinitiator was prepared by the transesterification reaction of the (β-CD) with 2-bromopropionyl bromide. The LCST of the samples upshifts slightly when the absolute molecular mass of the star PNIPAAm increases. Over the phase transition, the solutions became bluish opalescent due to formation of a heterogeneous phase system consisting of collapsed polymer particles in water. Atomic force microscopy and dynamic light scattering analyses indicated two populations of self-assembled polymer structure: a larger population and a smaller population. The smaller size suggests to self-assembly of polymer micelles and the large one corresponds to aggregates of polymer micelles or star polymers coupled. Polydispersity of the star PNIPAAm ranged from 1.60 to 4.04 within 15 h of reaction, which was attributed to the collapse of the PNIPAAm chains at temperature above the LCST that causes a decrease of the polymer reactivity. This was also attributed to the star–star coupling that generates twice the value of the polydispersity for any time before 15 h of polymerization.

Introduction

In recent years, star polymers have attracted great attention due to their rheological properties. The main advantage of a star architecture is that it does not affect significantly the viscosity of the solution, compared with polymers of linear chains [1]. Several techniques have been explored for synthesis of architecture-controlled materials, such as, control living radical polymerizations (CLRP), click-chemistry [2], and TEMPO – mediated living radical polymerization [3]. Among the CLRP techniques, atom transfer radical polymerization (ATRP) can be highlighted [4], [5], [6]. The great interest on such a technique is that it provides a simple reaction route to many well-defined polymers with predetermined molecular mass. Furthermore, it makes easier the preparation of macroinitiators. There are several initiators that have been used on the synthesis of star polymers [7]. The chemical structure of such compounds can be either cyclic or aliphatic in which there can be many points of initiation, generally more than four points per molecule. β-cyclodextrin (β-CD) is an excellent candidate among molecules of cyclical structure that can be an efficient initiator for synthesis of star polymers via ATRP [8], [9], [10]. The β-CD is a cyclic oligosaccharide that contains seven units of α-(1-4)-linked D-glucopyranose organized in a conical structure. This is a chemical structure of specific configuration that exhibits a hydrophobic cavity in which shape- and size-appropriated organic molecules could be introduced in aqueous media [11].

The combination of β-CD and thermo-sensitive polymers, as poly(N-isopropylacrylamide) (PNIPAAm), can result in materials with potential applications on important areas of science and biotechnology, such as, biomedical, drug delivery, and tissue engineer. The main characteristic of the PNIPAAm is the transition from a hydrophilic to hydrophobic macromolecule as the temperature is raised above 30–35 °C [12], [13], [14]. In water, it exhibits reversible phase transition at lower critical solution temperature (LCST) with resulting dramatic shrinkage. This work aims at synthesizing star PNIPAAm polymers based on the β-CD macroinitiator by ATRP in water at above the LCST. Under this condition, the PNIPAAm chains collapse to a shrunken state that could affect, to some extent, the polydispersity of the star PNIPAAm. Studies on β-CD-initiated star PNIPAAm polymerization by ATRP at temperatures above the LCST for producing thermo-sensitive materials of controlled architecture have been hardly reported.

Section snippets

Materials

β-cyclodextrin 99.0% was purchased from Aldrich and dried at 100 °C under reduced pressure for further analysis. N-isopropylacrylamide (99.9% Acros) was recrystallized in hexane, dried under reduced pressure and stored at −15 °C. CuBr (99.9% Acros) was purified in acetic acid, ethanol and ether, and stored under reduced pressure. 2-bromopropionyl bromide (BiPBr) (97.0% Aldrich), 2.2 bipyridine (99.9% Acros), and 1-methyl-2-pirrolidone (NMP) anhydrous (99.5% Aldrich) were used as received by the

Spectroscopic analyses for macroinitiator characterization

FTIR spectra were recorded on a Bomem model MB 100. The samples were introduced as tablets of KBr, and a total of 128 scans were run for resolution of 4 cm−1.

Hydrogen nuclear magnetic resonance (1H NMR) spectra were recorded on a Varian Spectrometer, model Mercury Plus BB 300 MHz, by applying a frequency of 300.059 MHz for nucleus of 1H at room temperature. CDCl3 solutions consisting of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (TSP-d4), as the reference line, were prepared to acquire

Macroinitiator characterization

The reaction between the β-CD and the bromide acid (BiPBr) in anhydrous media occurred quantitatively via a transesterification mechanism. Fig. 2 shows the FTIR spectrum of (a) pure β-CD and (b) (Br)20-functionalized β-CD. The signals at 3365 cm−1 and 1157 cm−1 in the spectrum of pure β-CD were attributed to stretching frequency of hydroxyl groups (νOsingle bondH) and deformation of Osingle bondC groups, respectively. The signals at 1409 cm−1 and 1332 cm−1 in the same spectrum were corresponded to Osingle bondH deformation of

Conclusions

The reaction of β-CD with 2-bromopropionyl bromide allowed the direct creation of ATRP macroinitiator in the NMP solvent. The substitution degree of hydroxyl groups of the β-CD was 20. The ATRP technique was an excellent tool for the synthesis of β-CD-PNIPAAm with controlled structure in a water using Cu/Bpy as catalytic system. Over the polymerization, there was formation of a large number of star–star-coupled structures that affected the control of the process and that generated polymer

Acknowledgments

The authors acknowledge the CNPq-Brazil, CAPES-Brazil, and Fundação Araucária/PR-Brazil for the financial support. The authors also acknowledge the cooperation between UFRGS-Brazil and CERMAV-France, and the UEM/COMCAP-Brazil for the instrumental support.

References (21)

  • K. Inoue

    Prog. Polym. Sci.

    (2000)
  • D.V. Palaskar et al.

    React. Funct. Polym.

    (2010)
  • M.R. Mauricio et al.

    Mater. Sci. Eng.: C

    (2009)
  • A.V. Vivek et al.

    React. Funct. Polym.

    (2008)
  • H.F. Gao et al.

    Prog. Polym. Sci.

    (2009)
  • Z. Li et al.

    Polymer

    (2009)
  • J. Li et al.

    Tetrahedron Lett.

    (2005)
  • J. Li et al.

    Carbohyd. Polym.

    (2010)
  • T. Qu et al.

    J. Colloids Interface Sci.

    (2009)
  • U. Mansfeld et al.

    Polym. Chem.

    (2010)
There are more references available in the full text version of this article.

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