This article describes a general procedure for synthesizing end-capped poly(phthalaldehydes) that depolymerize continuously from head-to-tail in response to a single reaction with a specific chemical or physical stimulus. The procedure (i) is reproducible and scalable over various batches of commercially available monomer, (ii) requires only 4 h to prepare the polymer, (iii) enables functionalized end-caps to be appended to both ends of the linear polymer, and (iv) provides control over the molecular weight of the polymer. This article also (i) provides spectroscopic and experimental evidence that depolymerization of endcapped poly(phthalaldehydes) is mediated by cleavage of the end-cap from the terminus of the polymer in response to a specific applied signal and (ii) shows that plastics made from poly(phthalaldehyde) containing a photoresponsive end-cap can be converted to monomers in the solid state.
Polymers that depolymerize continuously and completely from head-to-tail when a reaction-based detection unit is cleaved from the polymer provide both selective and amplified responses, a rare combination, to stimuli-responsive polymeric materials. This Viewpoint contextualizes this new class of depolymerizable polymers and outlines the key areas for growth and innovation.
Flow-focusing microfluidic techniques were used to provide access to core−shell microcapsules in which the shell is composed of end-capped poly(phthalaldehydes) that depolymerize completely from head-to-tail in response to fluoride. Microcapsules made from these depolymerizable polymers provide an amplified response to the applied chemical signal, where the rate of the response can be tuned both by varying the length of the polymer and the thickness of the shell wall. ■ INTRODUCTIONStimuli-responsive polymeric core−shell microcapsules are promising materials for controlled release applications. 1−8 Capsules of this type typically release their contents either through induced chemical changes in the polymers that compose the shell or via bulk structural changes to the shell wall. 1 These traditional triggered-release strategies provide responses that are nonamplified: i.e., a single membrane−signal interaction produces one small structural change in the shell wall, whereas release occurs only after many signals have reacted with and altered the shell wall. In contrast, a new release strategy has emerged where shell walls are formed from polymers that depolymerize continuously and completely from head-to-tail when an appropriate signal is detected by the polymer. 1,9 This depolymerization reaction provides an amplified response that, in theory, increases the sensitivity of the capsules as well as their rate of response once the appropriate signal is detected. Despite these favorable attributes, however, only one example of a stimuli-responsive core−shell polymeric microcapsule made from a head-to-tail depolymerizable polymer has been demonstrated to date, 10 with additional related examples demonstrated in responsive nanocapsules and micelles. 11,12 Further development of this depolymerizable shell wall concept has been hindered by the limited number of polymers that are capable of depolymerizing from head-to-tail in response to a specific signal, as well as by substantial challenges in fabricating core−shell microcapsules using polymers that are primed to depolymerize.In this article, we describe the use of flow-focusing microfluidic strategies for fabricating these types of depolymerizable core−shell microcapsules. 13−15 This technique is highly reproducible, readily forms capsules that contain aqueous interiors, is capable of generating capsules that can be suspended in an aqueous solution that differs from the solution within the capsule, and does not require synthetic manipulation of the polymer for incorporation into the shell wall. Moreover, the fabrication technique is exceptionally mild and thus enables the formation of core−shell microcapsules using sensitive depolymerizable polymers such as poly(phthalaldehyde) (PPHA) (Figure 1). 16,17 Herein we demonstrate these concepts by fabricating model stimuli-responsive core−shell microcapsules using PPHA that contains a fluoride-responsive endcap. The end-cap controls the stability and reactivity of the PPHA polymer and thus enables release of the ...
This Article describes the development of conditions for cleaving siliconÀoxygen bonds using catalytic quantities of fluoride at neutral pH in mixed organicÀaqueous solutions that contain buffer. A variety of silicon protecting groups can be removed under these conditions, which show tolerance for acid-and base-sensitive groups. A modified procedure also is presented using catalytic fluoride in anhydrous dimethyl sulfoxideÀmethanol, which generates primarily volatile silicon byproducts.
End-capped poly(4,5-dichlorophthalaldehyde) (PCl2PA), which is a new self-immolative CD(r) polymer with the unique capability of depolymerizing continuously and completely in the solid state when an end cap is cleaved from the polymer by reaction with a specific molecular signal, is described. End-capped poly(4,5-dichlorophthalaldehyde) is sufficiently stable to enable patterning of three-dimensional macroscopic polymeric materials by selective laser sintering. These unique materials are capable of 1) autonomously amplifying macroscopic changes in the material in response to specific molecular inputs, and 2) altering their responses depending on the identity of the applied signal. Thus, not only does end-capped PCl2PA provide new and unique capabilities compared to the small subset of existing CD(r) polymers, but it also provides access to a new class of stimuli-responsive materials.
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