No abstract

Orthogonal, sequential “click” reactions were implemented to yield novel polymeric substrates with the ability to record holographic data. The base-catalyzed thiol–acrylate Michael “click” reaction was implemented to yield a writable, stage 1 polymeric substrate with glass transition temperatures (T g) ranging from 0 to −26 °C and rubbery storage moduli (E′) from 11.1 to 0.3 MPa. The loosely cross-linked matrix also contained a novel high refractive index monomer 9-(2,3-bis(allyloxy)propyl)-9H-carbazole (BAPC) that did not participate in the thiol–Michael reaction but allowed for large index gradients to be developed within the network upon subsequent exposure to coherent laser beams and initiation of the radical-mediated thiol–ene reaction. The holographic gratings were recorded with 96% diffraction efficiency and ca. 2.4 cm/mJ of light sensitivity in 2 s under a 405 nm exposure with an intensity of 20 mW/cm2. Subsequent to pattern formation, via a thiol–allyl radical “click” photopolymerization initiated by flood illumination of the sample, holographic materials with high T g, high modulus, diffraction efficiency as high as 82%, and refractive index modulation of 0.004 were obtained. Graded rainbow holograms that displayed colors from blue to red at a single viewing angle were readily formed through this new technique.

We report a dispersion polymerization method based on thiol−Michael addition reactions for the preparation of cross-linked, narrow dispersity microparticles with well-defined, tunable physicochemical properties. Polymerization between pentaerythritol tetra(3-mercaptopropionate) (PETMP) and trimethylolpropane triacrylate in methanol was chosen as a model system, with the addition of triethylamine as a catalyst and polyvinylpyrrolidone as a stabilizer. The formation of microparticles took place within seconds at ambient conditions, as a result of a polymerization driven phase transition from dissolved monomers to precipitated polymers. The particle size was found to be affected by the amount of catalyst, the monomer concentration, and the monomer/polymer solubility in the reaction media. Monodispersity was achieved within a range of particle diameters from 1.6 to 4.3 μm, as determined both by scanning electron microscopy and dynamic light scattering. The reaction kinetics were studied by Fourier transform infrared spectroscopy by analyzing aliquots withdrawn from the reaction system at various reaction time points. Nearly quantitative conversions were achieved within 6 h for stoichiometric systems and 1 h for off-stoichiometric systems, both initiated with triethylamine. By utilizing photolabile bases as the reaction catalyst, phototriggered formation of the microparticles was demonstrated with ultraviolet irradiation. Monodisperse particles were formed with hexylamine and 1,1,3,3-tetramethylguanidine, both with 2-(2-nitrophenyl)propyloxycarbonyl as the UV-labile photocage. Furthermore, as a demonstration of the versatility of this method, microparticles were prepared from copolymerizations between PETMP and four types of diacrylates with varied backbone structures. With increased backbone rigidity, the microparticle glass transition temperature increased from −36 to 8 °C. This method provides a platform for the realization of the nearly ideal step-growth networks in microscale, with highly tunable backbone structures, robust thermal transitions, and intrinsic functionalization capacity.

We present a composite material composed of dual polymer networks uniquely formed from a single reaction type and catalyst but involving monomers with dramatically different reactivities. This powerful new approach to creating polymer networks produces two narrow glass transition, homogeneous networks sequentially from a single reaction but with all monomers present and uniformly mixed prior to any polymerization. These materials exhibit a triple shape memory effect based on the dual polymer networks, which were both formed using the thiol−Michael addition reaction. Two multifunctional thiol monomers (i.e., mercaptoacetate (MA) and mercaptopropionate (MP)) and two multifunctional vinyls (i.e., vinyl sulfone (V) and acrylate (A)) were polymerized in situ using a nucleophilic initiator. The MA-V polymer network (T g = 55 °C) was generated first associated with the higher functional group reactivities followed by the formation of the MP-A network (T g = 10 °C) which was confirmed by FT-IR, SEM, DMA, and a separately prepared composite polymer consisting of MA-V particles embedded in an MP-A matrix. The triple shape memory effect was characterized using DMA, and it was demonstrated that the shapes could be programmed either by a one-step (single temperature) or a two-step method (two different temperatures). This material was able to hold its transitional shape for an extended time period (>1 h) at intermediate temperature (20 °C) between its two T g s, mainly due to narrow transitions of two separate networks. This new approach to obtain dual polymer networks with distinct transitions and characteristics is simple and robust, thus enabling applications in areas such as triple shape memory polymers, biomedical materials, and composites.

We introduce a new paradigm in microparticles, where “click” chemistry enables the fabrication of functional monodisperse microspheres from step-growth polymerization at ambient conditions.

An efficient visible-light-sensitive photobase generator for thiol-Michael addition reactions was synthesized and evaluated. This highly reactive catalyst was designed by protecting a strong base (tetramethyl guanidine, TMG) with a visible-light-responsive group which was a coumarin derivative. The coumarin-coupled TMG was shown to exhibit extraordinary catalytic activity toward initiation of the thiol-Michael reaction, including thiol-Michael addition-based polymerization, upon visible-light irradiation, leading to a stoichiometric reaction of both thiol and vinyl functional groups. Owing to its features, this visible-light photobase generator enables homogeneous network formation in thiol-Michael polymerizations and also has the potential to be exploited in other visible-light-induced, base-catalyzed thiol-click processes such as thiol-isocynate and thiol-epoxy network-forming reactions.

Synthetic polymer approaches generally lack the ability to control the primary sequence, with sequence control referred to as the holy grail. Two click chemistry reactions were now combined to form nucleobase-containing sequence-controlled polymers in simple polymerization reactions. Two distinct approaches are used to form these click nucleic acid (CNA) polymers. These approaches employ thiol-ene and thiol-Michael reactions to form homopolymers of a single nucleobase (e.g., poly(A)n ) or homopolymers of specific repeating nucleobase sequences (e.g., poly(ATC)n). Furthermore, the incorporation of monofunctional thiol-terminated polymers into the polymerization system enables the preparation of multiblock copolymers in a single reaction vessel; the length of the diblock copolymer can be tuned by the stoichiometric ratio and/or the monomer functionality. These polymers are also used for organogel formation where complementary CNA-based polymers form reversible crosslinks.

Holographic photopolymers capable of high refractive index modulation (Δn) on the order of 10 are integral for the fabrication of functional holographic optical elements that are useful in a myriad of optical applications. In particular, to address the deficiency of suitable high refractive index writing monomers for use in two-stage holographic formulations, here we report a novel high refractive index writing monomer, 1,3-bis(phenylthio)-2-propyl acrylate (BPTPA), simultaneously possessing enhanced solubility in a low refractive index (n = 1.47) urethane matrix. When examined in comparison to a widely used high refractive index monomer, 2,4,6-tribromophenyl acrylate, BPTPA exhibited superior solubility in a stage 1 urethane matrix of approximately 50% with a 20% higher refractive index increase per unit amount of the writing monomer for stage 2 polymerizations. Formulations with 60 wt % loading of BPTPA exhibit a peak-to-mean holographic Δn ≈ 0.029 without obvious deficiencies in transparency, color, or scatter. To the best of our knowledge, this value is the highest reported in the peer-reviewed literature for a transmission hologram. The capabilities and versatility of BPTPA-based formulations are demonstrated at varying length scales via demonstrative refractive index gradient structure examples including direct laser write, projection mask lithography of a 1″ diameter Fresnel lens, and ∼100% diffraction efficiency volume transmission holograms with a 1 μm fringe spacing in 11 μm thick samples.