Reversible down-regulation and up-regulation of catalytic activity of poly(N-isopropylacrylamide)-anchored gold nanoparticles

Regulating catalytic activity plays an important role in further optimizing and developing multifunctional catalysts with high selectivity and high activity. Reversible dual regulation of catalytic activity has always been a challenging task. Here, we prepared poly(N-isopropylacrylamide)-anchored gold nanoparticles (AuNP@CDs-Azo-PNIPAM) through host-guest interaction of cyclodextrin capped gold nanoparticles (AuNP@CDs) and azobenzene-terminated poly(N-isopropylacrylamide) (Azo-PNIPAM). Azo-PNIPAM as thermal and light responsive ligand allows reversible dual regulation of catalytic activity. When the temperature is higher than the lowest critical solution temperature (LCST), the PNIPAM chain shrinks rapidly, increasing the steric hindrance around AuNPs and reducing the catalytic activity. Under ultraviolet light irradiation, cis-azobenzene disassembles from cyclodextrin and the number of surface active sites of AuNPs increases, which improves the catalytic activity. The reaction rate of UV irradiation is almost 1.3 times that of visible light irradiation. This work provides a simple and effective strategy for the construction of reversible catalysts.

Aggregation of Gold Nanoparticles in Presence of the Thermoresponsive Cationic Diblock Copolymer PNIPAAM48-b-PAMPTMA6

The adsorption of the thermoresponsive positively charged copolymer poly(N-isopropylacrylamide)-block-poly(3-acrylamidopropyl)trimethylammonium chloride, PNIPAAM48-b-PAMPTMA6(+), onto negatively charged gold nanoparticles can provide stability to the nanoparticles and make the emerging structure tunable by temperature. In this work, we characterize the nanocomposite formed by gold nanoparticles and copolymer chains and study the influence of the copolymer on the expected aggregation process that undergoes those nanoparticles at high ionic strength. We also determine the lower critical solution temperature (LCST) of the copolymer (around 42 °C) and evaluate the influence of the temperature on the nanocomposite. For those purposes, we use dynamic light scattering, UV-vis spectroscopy and transmission electron microscopy. At the working PNIPAAM48-b-PAMPTMA6(+) concentration, we observe the existence of copolymer structures that trap the gold nanoparticles and avoid the formation of nanoparticles aggregates. Finally, we discuss how these structures can be useful in catalysis and nanoparticles recovery.

Precise Synthesis and Thermoresponsive Property of Poly(ethyl glycidyl ether) and Its Block and Statistic Copolymers with Poly(glycidol)

In this paper, we describe a comprehensive study of the thermoresponsive properties of statistic copolymers and multiblock copolymers synthesized by poly(glycidol)s (PG) and poly(ethyl glycidyl ether) (PEGE) with different copolymerization methods. These copolymers were first synthesized by ring-opening polymerization (ROP), which was initiated by tert-butylbenzyl alcohol (tBBA) and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylidenamino]-2Λ5,4Λ5-catenadi(phosphazene) (t-Bu-P4) as the catalyst, and then the inherent protective groups were removed to obtain the copolymers without any specific chain end groups. The thermoresponsive property of the statistic copolymer PGx-stat-PEGEy was compared with the diblock copolymer PGx-b-PEGEy, and the triblock copolymers were compared with the pentablock copolymers. Among them, PG-stat-PEGE, PG-b-PEGE-b-PG-b-PEGE-b-PG, and PEGE-b-PG-b-PEGE-b-PG-b-PEGE, and even the specific ratio of PEGE-b-PG-b-PEGE, exhibited LCST-type phase transitions in water, which were characterized by cloud point (Tcp). Although the ratio of x to y affected the value of the Tcp of PGx-stat-PEGEy, we found that the disorder of the copolymer has a decisive effect on the phase-transition behavior. The phase-transition behaviors of PG-b-PEGE, part of PEGE-b-PG-b-PEGE, and PG-b-PEGE-b-PG copolymers in water present a two-stage phase transition, that is, firstly LCST-type and then the upper critical solution temperature (UCST)-like phase transition. In addition, we have extended the research on the thermoresponsive properties of EGE homopolymers without specific α-chain ends.

Polimeros termoconmutables

En años recientes, una de las áreas de los polímeros más ampliamente investigadas, por las importantes aplicaciones tecnológicas a que pueden dar lugar, es la síntesis de macromoléculas conmutables o sensibles a cambios en el medio ambiente tales como; por ejemplo, temperatura, pH, luz, y campo eléctrico o magnético. Este tipo de polímeros, conmutables o sensibles, tienen alto potencial para aplicaciones en biomateriales, en microsistemas, sensores y catálisis, entre otros. Entre los polímeros conmutables que más se han investigado se encuentran los polímeros termosensibles Se pueden obtener polímeros con respuesta térmica usando los homopolímeros y copolímeros de N-isopropilacrilamida, N-vinilcaprolactama, y vinilmetiléter. Sin embargo, en años recientes, se descubrió que también algunos polímeros de 2-oxazolinas tienen la propiedad de tener sensibilidad térmica, tales como, poli(isopropil-2-oxazolina), poli(n-propil-2-oxazolina), poli(etil-2-oxazolina) y poli(ciclopropil-2-oxazolina). Los polímeros termosensibles presentan, en solución acuosa, una transición conformacional bien definida, donde las propiedades físicas cambian drásticamente en un estrecho rango de temperatura. Esta temperatura es denominada en inglés “Low critical solution temperature” (LCST). Por ejemplo, los polímeros de N-isopropilacrilamida, cuando están disueltos en agua, pasan a los 32 grados centígrados de un estado hidrofílico a un estado hidrofóbico y precipitan o se dispersan en el medio acuoso y este fenómeno es reversible, ya que, al disminuir la temperatura por debajo de los 32°C, se disuelven nuevamente, pudiendo servir, entonces, como un “switch” o un interruptor macromolecular. Esta temperatura LCST se puede disminuir o aumentar al copolimerizar la N-isopropilacrilamida con monómeros hidrofóbicos o hidrofílicos, respectivamente, o mediante la adición de surfactantes o sales inorgánicas. Se pueden obtener temperaturas de transición cercanas a la temperatura corporal humana, abriendo la posibilidad para aplicaciones tecnológicas muy interesantes. La transición conformacional LCST se puede utilizar en términos prácticos, por ejemplo, para elaborar hidrogeles termosensibles que puedan servir como sistemas de liberación controlada de medicamentos y fertilizantes, en ingeniería de tejidos o en válvulas y sensores para microfluidos. En el Perú, se debe promover la investigación básica y aplicada en polímeros y, en particular, en este tipo de polímeros conmutables que tienen un potencial de alto valor agregado. Se deben formar, en nuestro país, recursos humanos especializados en los polímeros tradicionales, pero también en los polímeros funcionalizados o con propiedades especiales.