Initiation mechanism of t-BuOK on anionic polymerization of n-HMA in THF.

A series of well-defined diblock copolymers containing PMMA, PEMA and PBMA blocks were synthesized with narrow MWDs and high yields via anionic polymerization, while t-BuOK was selected as initiator in THF at 0 °C. .

In order to break through the bottleneck of anionic polymerization, polar monomers such as methyl methacrylate (MMA), ethyl methacrylate, butyl methacrylate, and hexyl methacrylate are subjected to anionic polymerization at room temperature in tetrahydrofuran (THF) using potassium tert-butoxide (t-BuOK) as the initiator. The polymerization of alkyl methacrylates is studied by multidetector gel permeation chromatography, proton nuclear magnetic resonance ( 1 H-NMR) and 13 C-NMR spectroscopy, and dynamic laser light scattering. It is found that t-BuOK can initiate the living anionic polymerization of polar alkyl methacrylate, and the polymerization conversion almost reaches up to 100%. t-BuOK exists into two kinds of agglomerates, whose hydrodynamic volumes are 10 and 80 nm, respectively. t-BuOK in THF is similar to emulsion and has a critical active species concentration of about 0.0265 mol L −1 and does not depend on how much t-BuOK is added. After the initiation of the polymerization, the large agglomerates of a loose and less regular structure that have occupied the main part of t-BuOK are greatly reduced, but they do not continue to decrease until they disappear according to the equilibrium theory. Similarly, the active chain after initiation also will not aggregate again. Furthermore, t-BuOK also has an active species with smaller average vibration size between cation and anion pairs, which can only initiate the polymerization of MMA with rather slow rate but cannot initiate other alkyl methacrylates. At last, because t-BuOK can make the dormant species caused by side reactions to be revived, the anionic polymerization of MMA could obtain a high yield.

It still remains a concern to break through the bottlenecks of anionic polymerization of polar monomers, such as side reactions, low conversion and low temperature (–78°C). In this work, potassium tert‐butoxide (t‐BuOK) was chosen to initiate the anionic polymerization of 2‐ethylhexyl methacrylate (EHMA) in tetrahydrofuran. The conversions were above 99% at 0 or 30°C, and above 95% at 60°C without side reaction inhibitors. The high conversions implied t‐BuOK could suppress the side reactions. A series of block copolymers of EHMA, n‐hexyl methacrylate (HMA) and methyl methacrylate (MMA) were further synthesized at 0°C, and the conversions were all above 99%. The GPC and 1H NMR results confirmed the successful synthesis of the block copolymers. The molecular size of monomer and the state of t‐BuOK (free ion pairs or aggregates) remarkably affected the polymerization rates and the molecular structures of the products. The DMA results indicated that the glass transition temperatures of PEHMA or PHMA block and PMMA block were 20°C and 60°C, respectively, which deviated from –2°C and 105°C of homopolymer, respectively, due to the partial compatibility of the blocks. This work explored a route of the anionic polymerization of polar monomers at room temperature.

Using potassium tert‐butoxide (t‐BuOK) as initiator, anionic polymerization and copolymerization of the methacrylate monomers are carried out in tetrahydrofuran at 0 °C or above. The polymers are characterized by gel permeation chromatography (GPC), proton nuclear magnetic resonance (1H‐NMR), Fourier transform infrared (FTIR), and dynamic mechanical analyzer (DMA). There is a critical concentration of the active species in t‐BuOK, above which no matter how much more t‐BuOK is added, it has no initiation activity. Among the active species of t‐BuOK, the main part possesses large average vibration distance between anion–cation pairs, which can initiate all methacrylate monomers in this study. While another part of the active species possesses the small average vibration distance, which can only initiate the polymerization of methyl methacrylate at slow rate. It provides an supporting evidence for the “channel idea” proposed previously. Finally, poly(methyl methacrylate)‐b‐poly(lauryl methacrylate)‐b‐poly(methyl methacrylate) (PMMA‐b‐PLMA‐b‐PMMA) three‐block copolymer is synthesized. This study paves the way for the synthesis of thermoplastic elastomers with full polar monomers and full saturated chains by anionic polymerization.

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