Mechanochemistry Of Mechanophore-centered Polvmer In A Dilute Solution:Responsive Selectivity And Enhanced Activation

Polymer mechanochemistry is a crossed research area covering polymer chemistry, polymer physics and advanced materials. One important research task of this area is to develop new mechanophores. Such chemical moiety is a force-sensitive molecular unit that possesses mechanically labile bonds, when incorporated within the mid-point of the mechanically stressed polymer chains, mechanophores will selectively undergo chemical transformations. So, it is an important basic research topic to build scientific evaluation methods for the responsive selectivity of the mechanophore, however, such valuation reports on the selectivity of the mechanophore are rare. In this work, two kinds of mechanoresponsive behaviors in the polymer dilute solution system are studied: one is that the mechanophore of Diels-Alder (DA) adduct was incorporated in the mid-point of polymer chain, and then the cleavage selectivity of this mechanophore was explored under various sonication conditions. The mathematical simulation results were well accorded with the scission kinetic datas from GPC results, and this work may offer a general research method for evaluating site-specific response of mechanophores. The other is that the mechanophore of spiropyran was placed in the micelles, and its mechanically activation was explored under sonication. Of interest, the effect of the enhanced activation within micelles has been verified, and this system will push forward the building of complicated mechanochemistry system and the development of mechanoresponse within nanoscale.1. The site-specific mechanoresponse (cleavage response) of polymers and the related judgment criterion. It is a challenging topic to disconnect a linear polymer selectively at the mid-point by external force in a "cold" fashion. Herein, three kinds of poly (methyl acrylate) (PMA) were synthesized, one with urfuryl-maleimide Diels-Alder (DA) adduct in the mid-point (PMA1), and the other two are its controls (PMA2 and PMA3), with nearly the same MWs and MWDs under ultrasound irradiation, by comparing the results of mathematical simulation and the GPC results of scission kinetic, it was observed that only PMA1 underwent a clear center cleavage model while its controls PMA2 and PMA3 showed common Gaussian cleavage model at the low ultrasonic energy output (2.1 W). At the high ultrasonic energy output of 5.5 W, the degradation rates of PMAl, PMA2 and PMA3 seemed to be the same, and they presented similar kinetic model. This result highlights that the choice of energy output of a sonicator is a key gauge for evaluating the selective cleavage at the mechanophore position in the polymer, for different kinds of mechanophores, the ultrasonic energy should be selected optimally instead of a similar energy reported by mostly related research work. Moreover, the choice of ultrasonic energy was verified to satisfy a fact that the turning point in the plot of degaradation index (DI) vs. time from a positive slope to zero (DI=1) should be obtained.2. Enhanced mechanically activation witin micelles. The mechanical activation of macromolecular assemblies in dilute solution was first designed and explored in this work. SP-[poly (tert-butyl acrylate-b-N-isopropylacrylamide)]2 [SP-(t-BA88-b-NIPAM62)2, P1] with one spiropyrane (SP) moiety at the mid-point of chain was synthesized by single-electron transfer living radical polymerization (SET-LRP) combined with an atom transfer radical polymerization (ATRP) process. Spherical P1 micelles with different sizes were fabricated in the THF/water mixed solvent. With increasing the water content in the THF/water mixed solvent, the sizes of spherical PI micelles decreased, when the water content reached a critical value of 37.5 v%, further increasing the water content would lead to the precipitation of P1. The mechanical response of P1 micelles in THF/water (30 v%) mixed solvent was a 5-fold increase as compared to that of P1 in pure THF. A serial of control experiments were carried out to confirm the contribution of micellization of P1 in THF/water mixed solvent to the enchanced SP activation and we think there would be two reasons for this phenomenon, one is based on the fact of the increase of the dielectric constant of the medium around the SP in the core, and the other is that SP-centered PtBA block would entangle and partially swell in the micellar core.In summary, a criterion for judging the selectivity of the mechanophore in the mid- point of polymer chain in a dilut solution is established, which makes it significant to guide the development of new mechanophores with highly selectivity; and then the mechanical activation response of mechanophore within nano-sized micelles was first explored here, which expanded the polymer mechanochemistry to the region of nanoscale. Of importance, the aggregation of the macromolecular assemblies would enhance the mechanophores mechanically activation, which was first verified here. Moreover, this new enhanced mechanophore activation model demonstrated here is valuable as a probe to detect stress activation within nano-sized particles and to design multiple-responsive materials. In addition, a serial of polymers incorporated with N-ethyl spiropyrane were synthesized and their related mechanical activation were also explored here.