Applications Of Tetramethylguanidine-Promoted Efficient Esterification In Synthesis Of Polymers

Recently, functional polymers have been drawn much attention due to their particular properties and wide applications in catalysts, separation, bionics, medicine and optoelectronic fields. Up to now, there are mainly two methods for the synthesis of functional polymers, which are (1) the functional modification of present polymers and (2) polymerization of functional monomers. However, no matter which method was used, the chemical reaction chosen during the preparation of functional polymers must have excellent tolerance to the functional groups. Otherwise, the protection and de-protection processes for the functional groups are always needed. As we know, there are a few reactions that can match these characters.1,1,3,3-Tetramethylguanidine (TMG) as a strong organic base has been widely used in organic synthesis. It was reported that the TMG promoted esterification reaction of carboxylic acids with methyl iodide shows high tolerance to several of functional groups, such as hydroxyl group, although the reaction efficiency was unsatisfactory. Undoubtedly, it is much desired if the efficiency of the TMG promoted esterification reaction can be improved, which may provide a potential application of the reaction in the preparation of functional polymers. Therefore, the main works in this paper were as follows:1. A Facile and highly efficient strategy for esterification of carboxylic acids with halogenated compounds have been achieved successfully using TMG as a promoter at room temperature. The effects of solvents, water, structures of carboxylic acids, and halogenated compounds on the TMG promoted esterifications have been investigated in detail. The results indicate that a wide variety of carboxylic acids can be activated at room temperature by TMG and then react with halogenated compounds to form carboxylate esters in excellent yields in most common organic solvents, even in the presence of water or under solvent-free condition. Polar solvents such as dimethyl sulfoxide (DMSO) and N, N-dimethyl formamide (DMF) were faverable for the reaction. The existence of water can decrease the reaction rate, but have no influence on the reaction efficiency. The structures of carboxylic acids have little effect on the esterification reaction. And the structures of the halogenated compounds have a significant effect on the reaction. Both primary and secondary halogenated compounds showed high reactivity, however, no reactions could be observed when tertiary halogenated compounds were used. Moreover, a possible mechanism is proposed to be a bimolecular nucleophilic substitution (SN2) reaction via the investigation of the reaction kinetics. To be noted, the purification of product is very simple and convenient, due to both the high efficiency of the reaction and the easy removal of the by-product water-soluble TMG salts. Furthermore, the promoter TMG which is commercially available can be recovered via washing with sodium hydroxide solution and vacuum distillation.2. Esterification reactions of poly(methacrylic acid)(PMAA) and poly(acrylic acid)(PAA) have been investigated with a wide variety of halogenated compounds using TMG as a promoter. The influences of solvent, reaction condition and the structures of halogenated compounds on the esterification reaction were examined. The results demonstrate that poly(meth)acrylates with excellent degree of esterification can be obtained at room temperature. It was found that polar solvents, such as DMSO and DMF, are favorable for the esterification and high degree of esterification can be achieved in a short time. Moreover, esterification reaction of PMAA has been successfully performed in an aqueous solution of DMSO, indicating that the solvent is not necessary to be dried for this reaction. Primary and secondary halogenated compounds can successfully react with PMAA or PAA, while tertiary halogenated compounds fail to react. The functional groups, such as carbon-carbon double bond and carbon-carbon triple bond, can be conveniently introduced via the method. In addition, combining this esterification reaction with atom transfer radical polymerization (ATRP), macromonomers were conveniently prepared by reaction of halogen-capped polymers with methacrylic acid under mild condition.3. A novel one pot strategy by combination of the in situ TMG promoted esterification of acrylic acid with halogenated compounds and reversible addition fragment chain transfer (RAFT) polymerization has been developed for preparation of polyacrylates. Through this novel strategy, polyacrylates with pendant functional groups can be directly synthesized from acrylic acids and halogenated compounds, getting rid of the preparation and purification procedures of acrylates. The results suggest that polyacrylates with different and controllable structures can be conveniently achieved via the simple choice of the halogenated compounds used during preparation process. The linear homopolyacrylates were obtained when primary and secondary mono-halogenated compounds were used, while the linear copolymers could be prepared when tertiary halogenated compounds were used. Moreover, the hyperbranched polymers with high branched ratio were obtained by using di-halogenated compounds based on this one pot method. And the branched ratio can be adjusted via the control of monomer conversion and the ratio of monomer to RAFT agent.4. The TMG promoted efficient esterification reaction of carboxylic acid with halogenated compound has been developed to synthesize bio-based functional polyesters from10-undecenoic acid, which is abundantly available and derived from ricin oil. First, two di-carboxylic acids derivated from10-undecenoic acid and variety of functional di-halogented compounds were designed and synthesized. Then, the TMG promoted polyesterifications of the di-acid monomers with the di-halogenated compounds monomers were successfully performed at room temperature. The results demonstrate that the bio-based polyesters with high molecular weight can be efficiently obtained. In comparison with the conventional polyesterification, this approach exhibits many advantages such as high efficiency and very mild reaction conditions. Moreover, a more important point is that the polymerization shows a high tolerance to variety of functional groups, such as alkenyl, alkynyl, nitro, epoxy, hydroxyl, and bromoisobutyrate. Thus, it can be used as promising precursors for designing and preparing novel and useful functional polymer.5. One polytrithiocarbonate (Mn,GPC=7500g mol-1, PDI=1.86) was designed and prepared via the TMG promoted esterification reaction of2,2’-(thiocarbonylbis(sulfanediyl))bis(2-methylpropanoic acid) with1,4-bis(bromomethyl)benzene at room temperature. Then, using the polytrithiocarbonates as chain transfer agents, RAFT polymerization of styrene was performed. Through this strategy, multiblock polystyrene (PS) with trithiocarbonate group as linker was obtained. Moreover, the photodegradability of multiblock PS film (about150μm) was investigated under UV irradiation at room temperature in an air atmosphere. It was found that the trithiocarbonate groups in the multiblock PS could be degraded under the irradiation of UV light. And the molecular weight was changed from27900g mol-1to7900g mol-1when the polymer was irradiated for745h under UV light. On the contrary, the conventional PS prepared via conventional radical polymerization showed no obvious change in the molecular weight under the same conditions. These results demonstrated that the incorporation of trithiocarbonates in the backbone of PS gives the polymer with photodegradability under UV irradiation. At the same time, the thermal studies reveal that the introduction of trithiocarbonates has slight influence on the glass transform temperature and thermal stability.