42987-AC7
Studies on Mechanochemical Activation in Polymer Molecules
Recently there has been a growing interest in the field of mechanochemistry and a desire to comprehend the parameters that affect mechanochemical reactions. Determining the rate of mechanochemical reactions and the role of molecular weight on the rate has not yet been investigated, but could lead to the design of optimized stress-responsive materials. While, the photolytic and thermal properties of spiropyrans (SPs) have been well studied, here we describe in detail the mechanokinetics of the ultrasound-induced 6pi electrocyclic ring opening of a SP mechanophore and investigate the effect of molecular weight on the mechanochemical reaction using a series of SP-linked poly(methyl acrylate)s 1 (PMA-SP-PMA). We also compare the photolytic and mechanochemical ring-opening reactions using UV spectroscopy. Gaining a fundamental understanding of factors such as rate will aid in the rational development of new mechanophores and stress-responsive materials. Mechanical activation has been shown to cleave the weak spiro carbon-oxygen bond, resulting in formation of the planar merocyanine, but the details of this reaction have not yet been examined. A proposed kinetic scheme was defined for the rate of the ultrasound-induced mechanochemical activation of PMA-SP-PMA and several potential competing side-reactions were evaluated. In addition to ring-opening, degradation of the open form upon sonication would affect the observed kinetics. In addition to decomposition, chain scission could potentially occur before, simultaneously with, or after activation of the SP. To investigate these possibilities, a spiropyran bis-functionalized with a-bromo-a-methyl-propionyloxy groups was used to syntehsize PMA-SP-PMA 1 via single electron transfer living radical polymerization (SET-LRP). A series of molecular weights was synthesized, ranging from low molecular weight 10 kDa to high molecular weight 400 kDa polymer. With a few key experiments, the potential side-reactions were shown not to influence the observed rate of mechanochemical ring-opening. To exclude SP degradation (SP* _ decomposition pdt), a solution of the polymer was irradiated with UV light to shift it to the open form and sonicated. Monitoring the UV absorbance over time illustrated that the UV maximum remained constant, indicating that the SP does not decompose under the sonication conditions and that decomposition does not influence the reaction rate (see Supporting Information). The reverse ring closing process from the open MC to the closed SP form (SP*_ SP) was also analyzed. The sample was irradiated at 254 nm to shift the SP to the open MC form and the reversion reaction was monitored by UV spectroscopy at 8 ĄC in the dark, the conditions used for sonication. Since kf >> kr, for most molecular weights, kr (0.0037 min-1) can be excluded from the rate equation. At the lowest active molecular weight of 43.5 kDa (kf = 0.025 min-1), the reverse reaction will have a small affect on the observed rate of ring-opening. Several control experiments were performed to ensure the SP ring-opening was due to mechanical stress. A SP functionalized with both polymer chains on the same side of the sprio bond and end-functionalized PMA were sonicated, and no increase in the UV absorbance due to ring-opening of the SP was observed (see Supporting Information). Additionally, 10 kDa PMA-SP-PMA with a molecular weight below the threshold value for chain scission was sonicated, and an increase in the UV spectrum at 554 nm was absent. In a final control experiment, a sonicated solution in which complete chain scission had occurred, was resubmitted to ultrasound and no significant increase in the UV absorbance was seen (see Supporting Information). Once scission occurs, the SP is no longer located near the center of the polymer backbone and therefore cannot be activated by mechanical stress. In conclusion, we have investigated the mechnokinetics of the ultrasound-induced ring-opening of SP. An increase in reaction rate is observed with increasing molecular weight and a kinetic scheme, in which activation occurs prior to or simultaneously with chain scission, is proposed. An understanding of ultrasound-induced mechanokinetics will allow for future exploration of the different factors influencing mechanochemical reactivity, such as polymer architecture, sonication conditions, and optimal linker connectivities.
