Thiol-Michael Addition Polymerization Reactions

This thesis is focused on understanding and implementation of the thiol-Michael addition reaction for controlling polymer structures and properties. The thiol-Michael addition `click' reaction has numerous advantageous characteristics for polymerization reactions, such as having a high reaction rate and yield under mild reaction conditions without the formation of byproducts, starting from a large selection of easily accessible monomers. Firstly, control of the onset of the thiol-Michael addition reaction was achieved by development of two different initiator systems, i.e., a latent initiator (nucleophilic initiator and acid inhibitor) and a photoinitiator (sensitizer, base-tetraphenylborate complex and radical inhibitor). Both initiator systems provided excellent control of the onset of the reaction and provided facile methods to form crosslinked polymer networks. Secondly, selectivities of various functional groups toward the thiol-Michael addition reaction were assessed, and it was demonstrated that multiple functional group combinations present over 99% selectivity of one functional group over the other. This concept of kinetically selective thiol-Michael addition reactions are implemented to control polymer structural evolution, and eventually led to dendrimer synthesis and multiphase network polymer formation. In dendrimer synthesis, a 5th generation dendrimer or dendritic-linear polymer conjugate were synthesized in less than a half day, and in the case of dendritic-linear polymer conjugate, the reactions were all in one-pot with just one purification step for the whole procedure. Multiphase network polymer formation was demonstrated by sequential, selective thiol-Michael addition reactions of multicomponent monomer mixtures, and it was shown that the material exhibits two distinct glass transition temperatures (10 and 55 °C). Thus, triple shape memory behavior was achieved with a stable intermediate shape. Lastly, spatiotemporal control of both the thiol-ene reaction and thiol-Michael addition reaction were utilized to form a material with patterned functionalities and material properties, starting from a mixture of multifunctional thiols and an excess of multifunctional acrylates. This approach is based on the difference in the thiol-vinyl reaction mechanisms between the radical pathway and the anionic pathway, in which the former consumes more acrylates via homopolymerization and the latter consumes both thiols and acrylates in a stoichiometric rati

Click chemistry and its unique benefits in composite formulation

Although proposed only 15 years ago, “click” chemistry has already become a powerful tool in many synthetic/preparatic schemes in polymer science. The simplicity and robustness of “click” chemistries render them ideal candidates in engineering composite materials with well-defined physiochemical properties for vast range of applications. Herein we present the advances in the implementation of thiol-X “click” chemistries in various composite materials, exemplified by dental restoration materials, shape memory programmable composites/laminates and latex composite materials. In the first example, we developed a series of thiol-ene monomers in which esters are absent so that they are hydrolytically stable in basic conditions. We strengthened these thiol-ene polymers with functionalized silica nanoparticles (up to 65 wt% loading). Compared with the commercialized polymethacrylate systems, these polymeric composites show exceptional stability in both chemical and mechanical properties. Second example involves a two-stage thiol-isocyanate-methacrylate network polymer used for fabricating of programmable surface patterns and geometric shapes, which are induced by external strain and are erasable or permanently fixable. Moreover, the narrow and highly controllable glass transitions were also utilized for constructing layer-by-layer multiple shape memory laminates. Further, composites with multiple Tg’s were also prepared by polymerization induced in situ phase transition, and polymeric microparticle-filled thiol-Michael materials. In final example, several novel latex materials were developed by the thiol-Michael addition miniemulsion polymerization. Inherently functionalized latex films were shown to undergo facile further surface functionalizations, as well as the dual-cure latex materials by a second stage photo-polymerization of excess acrylates after the first stage off-stoichiometric thiol-acrylate Michael addition polymerization

Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies

A poly(methyl methacrylate) (PMMA) with an unsaturated chain end (PMMA–Y) was used as a macroinitiator in the polymerizations of several monomers to generate block copolymers via addition–fragmentation chain transfer (AFCT). PMMA–Y also worked as a macromonomer to generate branched polymers via propagation. A kinetic study revealed that the occurrence of AFCT and propagation significantly depends on temperature in the styrene polymerization; namely, while propagation was predominant below 60 °C as previously reported, AFCT was predominant at elevated temperatures such as 120 °C as newly revealed in the present work. This new kinetic finding opened up an efficient synthesis of block copolymers of PMMA with polystyrene at an elevated temperature. AFCT was also predominant over propagation in the polymerizations of acrylonitrile and acrylates. Thus, block copolymers of PMMA with polyacrylonitrile and functional polyacrylates were successfully obtained. The polymerization was controlled using iodine transfer polymerization (ITP) for styrene and reversible complexation mediated polymerization (RCMP) for the other monomers. PMMA–Y with different molecular weights were also tested. This approach to obtain block copolymers is practically attractive for the ease of operation.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Reversible complexation mediated living radical polymerization using tetraalkylammonium chloride catalysts

This work reports the first use of organic chloride salts as catalysts for reversible complexation mediated living radical polymerization. Owing to the strong halogen-bond forming ability of Cl−, the studied four tetraalkylammonium chloride catalysts (R4N+Cl−) successfully control the polymerizations of methyl methacrylate, yielding polymers with low dispersities up to high monomer conversion (>90%). Benzyldodecyldimethylammonium chloride is further exploited to other methacrylates and yields low-dispersity block copolymers. The advantages of the chloride salt catalysts are wide monomer scope, good livingness, accessibility to block copolymers, and good solubility in organic media. Because of the good solubility, the use of the chloride salt catalysts can prevent agglomeration of catalysts on reactor walls in organic media, which is an industrially attractive feature. Among halide anions, chloride anion is the most abundant and least expensive halide anion, and therefore, the use of the chloride salt catalysts may lower the cost of the polymerization.National Research Foundation (NRF)Submitted/Accepted versionThis work was partly supported by National Research Foundation (NRF) Investigatorship in Singapore (NRF-NRFI05-2019-0001)

Synthesis of core-crosslinked star polymers via organocatalyzed living radical polymerization

Core-crosslinked star polymers were prepared using organocatalyzed living radical polymerization via a “grafting-through” approach. A PBA homopolymer, an amphiphilic PMMA–PPEGA block copolymer, and a hard-soft PMMA–PBA block copolymer were synthesized as macroinitiators, where PBA is poly(butyl acrylate), PMMA is poly(methyl methacrylate), and PPEGA is poly(poly(ethylene glycol) methyl ether acrylate). The macroinitiators were utilized in the polymerizations of crosslinkable divinyl monomers, generating core-crosslinked star polymers in 40–80% yields. The PMMA–PBA block copolymer macroinitiator was synthesized from a PMMA with an unsaturated chain end (PMMA–Y) via an addition-fragmentation chain transfer method. The ease of the handling of PMMA–Y is an advantage of the use of PMMA–Y. One-pot synthesis of a PBA star was also successful, giving a star in a relatively high yield (73%). The one-pot synthesis offers a practical approach for synthesizing a core-crosslinked star. The present approach is free from metals and odorous compounds, which is an attractive feature of the present approach.National Research Foundation (NRF)Accepted versionThis work was partly supported by National Research Foundation (NRF) Investigatorship in Singapore (NRF-NRFI05-2019-0001)

Use of poly(methyl methacrylate) with an unsaturated chain end as a macroinitiator precursor in organocatalyzed living radical block polymerization

Poly(methyl methacrylate) (PMMA) with an unsaturated chain end (PMMA–Y) was transformed into PMMA–iodide (PMMA–I) in situ in the organocatalyzed living radical polymerization of butyl acrylate; the generated PMMA–I acted as a macroinitiator to successfully generate a PMMA–PBA–I block copolymer, where PBA is poly(butyl acrylate). The use of PMMA–Y overcomes the drawback of the direct use of the isolated PMMA–I, i.e. its general lack of long-term stability upon storage, and thus significantly improves the ease of operation. This method is highly suitable for practical use. A quantitative mechanistic study on the transformation of PMMA–Y into PMMA–I was also conducted.MOE (Min. of Education, S’pore

Triple Shape Memory Materials Incorporating Two Distinct Polymer Networks Formed by Selective Thiol–Michael Addition Reactions

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 <i>in situ</i> using a nucleophilic initiator. The MA-V polymer network (<i>T</i>_g = 55 °C) was generated first associated with the higher functional group reactivities followed by the formation of the MP-A network (<i>T</i>_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 <i>T</i>_gs, 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

Facile and Efficient Synthesis of Dendrimers and One-Pot Preparation of Dendritic–Linear Polymer Conjugates via a Single Chemistry: Utilization of Kinetically Selective Thiol–Michael Addition Reactions

A kinetically selective thiol–Michael addition “click” reaction was employed for facile and efficient synthesis of dendrimers as a first example of using solely a single chemistry without protection/deprotection reactions in dendrimer synthesis. First, a wide range of thiols and vinyls were assessed for their reaction selectivity, and several combinations demonstrated superior selectivity. This result led to a design of new monomers A*A₂ (vinyl) and B*B₂ (thiol) that both have three functional groups with one much more reactive and two much less reactive moieties. Starting from a multifunctional core thiol, a fifth-generation dendrimer with 96 peripheral functional groups was synthesized in less than a half day by sequentially reacting A*A₂ and B*B₂ monomers all under thiol–Michael addition reaction conditions. Furthermore, a one-pot dendritic–linear polymer conjugation was demonstrated by a convergent synthesis approach starting from an alkyne-terminated dendron synthesis that was subsequently coupled with azide-terminated poly­(ethylene glycol), all in one pot with just a single, final purification step needed for the entire procedure. The results herein will provide a new, robust, and efficient methodology for synthesis of dendrimers as well as other complex polymer architectures such as dendritic–linear polymer conjugates, heterofunctional dendrimers, and dendronized surfaces

Programmable Mechanically Assisted Geometric Deformations of Glassy Two-Stage Reactive Polymeric Materials

Thiol-isocyanate-methacrylate two-stage reactive network polymers were developed and used for fabrication of well-defined surface patterns as well as functional geometric shapes to demonstrate a new methodology for processing of “smart materials”. The dynamic stage I networks were synthesized in base-catalyzed thiol-isocyanate cross-linking reactions to yield tough, glassy materials at ambient conditions. Methacrylate-rich stage I networks, incorporating photoinitiator and photoabsorber, were irradiated with UV light to generate stage II networks with intricate property gradients. Upon directional straining and subsequent temperature-dependent stress relief of the predefined gradient regions, the desired surface or bulk geometric transformations were achieved. Depending on the gradient extent in conjunction with photoorthogonal initiators, the introduced deformations were shown to be easily erasable by heat or permanently fixable by bulk polymerization