Summary: The MADIX/RAFT mechanism, employing a xanthate as the reversible chain‐transfer agent, has been shown to facilitate the living radical polymerization of vinyl acetate in miniemulsion. Methyl (ethoxycarbonothioyl)sulfanyl acetate (MESA) successfully mediated the polymerization which was initiated with either of the water‐soluble initiators 2,2′‐azobis{2‐[1‐(2‐hydroxyethyl)‐2‐imidazolin‐2‐yl]propane} dihydrochloride (VA‐060) or 2,2′‐azobis[2‐(2‐dimidazolin‐2‐yl)propane] dihydrochloride (VA‐044). The polymerizations exhibit living characteristics, demonstrated by the evolution of molecular weight distributions. The formulation of the miniemulsion produced stable latexes with no coagulum.
The number‐average molecular weight and PDI as a function of monomer conversion for the RAFT miniemulsion polymerization of vinyl acetate. 相似文献
Summary: Poly(vinyl acetate) macroinitiators end‐capped by a Co(acac)2 complex (PVAc–Co(acac)2), prepared in bulk by cobalt‐mediated radical polymerization (CMRP), are used for the controlled radical polymerization of vinyl acetate in miniemulsion to give high‐molecular‐weight polymers and high monomer conversion. Stable poly(vinyl acetate) latexes with solid contents ranging from 25 to 30 wt.‐% are prepared within unusually short reaction times (∼1 h) at low temperatures (0–30 °C).
SEC chromatograms for the PVAc–Co(acac)2 macroinitiator and PVAc latex obtained under ultrasonication for 6 min at 0 °C (79% monomer conversion). 相似文献
Summary: Atom transfer radical polymerization (ATRP) and reversible addition‐fragmentation transfer (RAFT) polymerization of N‐methyl methacrylamide and methyl methacrylate were investigated in the presence of rare‐earth triflates known to enhance polymer isotacticity. Poly(N‐methyl methacrylamide) with controlled molecular weight, low polydispersity, and enhanced isotacticity was prepared by ATRP and RAFT in the presence of catalytic amounts of yttrium trifluoromethanesulfonate or ytterbium trifluoromethanesulfonate. The tacticity of poly(N‐methyl methacrylamide) depends on the Lewis acid concentration: well‐defined polymers with predominantly either syndiotactic, atactic, or isotactic triads were prepared by adjusting the concentration of the Lewis acid. Simultaneous control of molecular weights, polydispersities, and tacticities in the polymerization of methyl methacrylate was less successful.
Free radical propagation in the presence of a Lewis acid (LA) giving rise to chelate control. 相似文献
Trimethylsilyl halides (Me3SiY), in conjunction with zinc halides (ZnX2) (Y and X:I, Br, Cl), were employed to investigate the living cationic polymerization of isobutyl vinyl ether (IBVE) in toluene at ?15°C in the presence of p-methoxybenzaldehyde; with the aldehyde and IBVE monomer, Me3SiY yields an initiating species [Me3Si? O? CHC6H4(OMe) ? CH2CH(OiBu) ? Y] that triggers the IBVE polymerization via the activation of its carbon-halogen bond (C? Y) by ZnX2 into Cδ+…?Yδ?…?ZnX2. Living polymerizations occurred with the silyl iodide and bromide irrespective of the type of ZnX2, either when Y = X (Me3Sil/Znl2 and Me3SiBr/ZnBr2) or when Y ≠ X (Me3Sil/ZnBr2, Me3SiI/ZnCl2, and Me3SiBr/Znl2). With these five initiating systems, the number-average molecular weights (M?n) of the polymers increased in proportion to monomer conversion, and the molecular weight distributions (MWDs) of the polymers were narrow (M?w/M?n = 1.1?1.2). The Me3SiCl-based systems (Me3SiCl/ZnCl2 and Me3SiCl/Znl2), in (Me3SiCl/Znl2), in contrast, failed to give perfectly living polymerization; the M?n indeed increased with conversion, but the MWDs of the polymers were broader (M?w/M?n = 1.3?1.5). Thus, the living nature of the polymerizations with Me3SiY/Znx2 is primarily determined by the halogen Y in Me3SiY, which generates the terminal carbon-halogen bond (C? Y) that is activated by ZnX2 for the propagation via a species Cδ+…?Yδ?…?ZnX2. For Y?, not only the iodide but the bromide anion also is suited for living cationic polymerization. The virtual absence of the effects of X in ZnX2 implies that the halogen exchange between ZnX2 and Y from Me3 SiY at the growing end (Cλ+…?Yδ?…?ZnX2 ?Cδ+…?Xδ?…?ZnXY) is absent or negligible. 相似文献
Summary: Although controlled/living radical copolymerization has been extensively studied, the control of copolymer composition distribution receives little attention. In this paper, taking RAFT copolymerization as an example, we develop a mathematical model and simulate copolymerization systems with various reactivity ratios. It is demonstrated that through semi‐batch operations with programmed profiles of slow monomer feeding rate, precise control over copolymer composition distribution (uniform and designed gradient distributions) along polymer chain can be achieved. It is also found that the semi‐batch operations have lower rates of polymerization than their batch counterparts. The reason for this difference is analyzed, and the magnitude depends on the reactivity ratios and targeted copolymer composition. The improvement of the semi‐batch rate by distributing a part of the initiator amount to the monomer feeding tank is found to be minor.
Model‐based design and control over composition distribution of gradient copolymers implemented by semi‐batch operations. 相似文献