Search

Small quantities of regio-defects in a regio-/stereoregular polymer weaken its tacticity and properties. This work clarified the origin of the regio-defect in the process of synthesizing poly(monothiocarbonate) through the copolymerization of propylene oxide (PO) and carbonyl sulfide (COS) catalyzed by a (salen)CrCl complex accompanied by bis(triphenylphosphoranylidene)ammonium chloride ([PPN]Cl). Quantitative characterization results from the MALDI-TOF-MS and 1H (13C) NMR spectroscopy suggested that the chain transfer reaction resulted in the regio-defect in the final copolymer, i.e., tail-to-tail (T-T) diad and dithiocarbonate (DTC) unit. The chain transferring to water in the reaction system led to the production of a (salen)Cr-OH intermediate, which initiated the copolymerization via either attacking PO first to result in formation of a T-T diad or first activating COS to produce mercapto (-SH) end-capped dormant chains via decarboxylation, thus generating a DTC unit in the final product through another chain transfer reaction and regrowth of the chain. The content of regio-defect in the final copolymer was directly related to the water content in the system. It is essential to reduce the regio-defect for an immortal COS/PO copolymerization reaction by eliminating trace amounts of water. We also demonstrated the application of α-OH, ω-OH poly(propylene monothiocarbonate) for synthesizing a well-defined ABA triblock copolymer, polystyrene-block-poly(propylene monothiocarbonate)-block-polystyrene (PS-b-PPMTC-b-PS), with a Mn of 10800 g/mol and a PDI of 1.08 via an atom transfer radical polymerization (ATRP) method. [ABSTRACT FROM AUTHOR]

It is a challenging topic to disconnect a linear polymer selectively at the mechanophore site by an external force in a "cold" fashion. In this work, the effect of the power output of ultrasonication on the selective cleavage at the centered urfuryl-maleimide Diels-Alder (DA) mechanophore of poly(methyl acrylate)s (DA-PMA-a and DA-PMA-b) were quantitatively investigated by comparative study on experimental and simulated chain scission kinetics as well as high-resolution 1H NMR spectroscopy (600 MHz). At low power output of the ultrasonication (2.10 W), DA-PMA-a with Mn of ca. 2Mlim (Mlim, below which no further chain scission was observed) presented a DI (degradation index)-t (sonication time) plot with a turnover point at ca. 1.0 and no clear variation of the molecular weight after the turnover, which met well with the calculated center cleavage mode. At 5.52 W, DA-PMA-a and a poly(methyl acrylate) that contained two centered ester bonds (ester-PMA) presented similar DI-t plots with turnover points less than 1.0 within same sonication times, while poly(methyl acrylate) with fully carbon-carbon chain (PMA) had a turnover at DI value of ca. 0.5. By way of contrast, high power output of the ultrasonication (5.52 W) caused a possible cleavage of ester bonds of DA-PMA-a, which would mask the selective cleavage at the DA site. High-resolution 1H NMR result of DA-PMA-b (115.8 kDa, Mn was slightly higher than 2Mlim) showed that DA conversions were up to 55% under 2.10 W and 38% under 5.52 W. The kinetics from GPC traces and 1H NMR results of DA-PMA-b as well as 1H NMR results of DA-PMA-c (68.4 kDa, Mn was slightly higher than Mlim) under sonication confirmed the observation that low power output favored selective chain scission at DA site. The turnover point in the DI-t plot might be used as characteristic parameter to gauge the selective chain scission at mechanophore site for single mechanophore-centered polymers. [ABSTRACT FROM AUTHOR]

Polyester synthesis from the alternating copolymerization of epoxides with cyclic anhydrides via a metal-free route remains a key challenge. This work reports the development of a highly active organocatalytic route for the copolymerization of a spectrum of epoxides and cyclic anhydrides. Fully alternating polyesters were synthesized by a variety of organic Lewis acid-base pairs including organoboranes and quaternary onium salts. The effect of the acidity, type, and size of Lewis pairs on the catalytic activity and selectivity of the copolymerization is presented. The undesirable transesterification and etherification were effectively suppressed even in the case of complete conversion of the cyclic anhydride. This could be ascribed to the formation of a unique tetracoordinate bond-carboxylate (or alkoxide) anion. The Lewis pairs are highly active, with a turnover frequency of 102 and 303 h-1 for the copolymerization of propylene oxide with maleic anhydride and phthalic anhydride, respectively, at 80 °C. Block polyester with narrow polydispersity of 1.05 was achieved via a sequential addition strategy. This work provides robust organocatalysts for the selective copolymerization of epoxides with cyclic anhydrides. [ABSTRACT FROM AUTHOR]