Thermal decomposition of dithioesters, e.g., cumyl dithiobenzoate (CDB), poly(methyl methacrylate) (PMMA) end-capped with dithioester, and 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate (EPDB), and the consequent effect on reversible addition−fragmentation chain transfer polymerization were investigated. The former two dithioesters underwent thermal decomposition at 120 °C. The thermal decomposition yielded unsaturated compound and dithiobenzoic acid, leading to some loss of living character of the polymerization, such as retarded reaction rate and broadened molecular weight distribution. Nevertheless, thermal decomposition of EPDB, a model compound for PMMA dithioester, does not yield unsaturated product despite the resemblance of the chemical structures. Thermogravimetric analysis shows that PMMA dithioesters are more thermally unstable than the other two.
Mikto three-arm, ABC-type star copolymers of styrene, isoprene, and 1,3-cyclohexadiene were synthesized by anionic polymerization using 1,3-bis(1-phenylvinyl)benzene (MDDPE) as linking precursor. A polystyrene macromonomer was prepared by monoaddition of polystyryllithium toward MDDPE. The resulting macromonomer was coupled with polyisoprenyllithium through the remaining double bond in MDDPE moiety, forming a diblock copolymer of styrene and isoprene. The diblock copolymer possessed a living center of diphenylethyllithium, which subsequently initiated the polymerization of a small amount of isoprene ("seeding") and 1,3-cyclohexadiene. It was found that, without the "seeding" process, diphenylethyllithium was not able to initiate the polymerization of 1,3-cyclohexadiene. The resulting macromonomer and star polymers were characterized by gel permeation chromatography, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and 1H NMR spectroscopy. In addition, linear ABC-type triblock copolymers of the corresponding monomers with different sequences were also synthesized. Microphase separation of the linear and star copolymers was investigated by differential scanning calorimetry and transmission electron microscopy.
Well-defined comb-on-comb copolymers of styrene, isoprene, and α-methyl-styrene are prepared through cascade grafting-onto methods. The polymer main chain is prepared by nitroxide-mediated radical polymerization while the branches are prepared by anionic polymerization. The grafting-onto approach employs the coupling chemistry of macromolecular anions, such as polystyryllithium, polyisoprenyllithium, or poly(α-methylstyryl)lithium, toward either benzyl chloride or epoxy ring on precursor backbones. Thus a series of ABA-, ABB-, and ABC-type comb-on-comb copolymers are prepared and characterized by gel permeation chromatography equipped with a multi-angle laser light scattering detector and a viscometer. Unusual U-shaped dependences of radius of gyration, Rg, on molecular weight are observed for comb-on-comb products, which are attributable to delayed elution of the densely grafted copolymers from GPC columns. The result also shows that the comb-on-comb copolymers exhibit morphologies from hard sphere to cylindrical rod, depending on the length ratio of the main chain to the branches.
This chapter summarizes the synthesis of sequence-controlled polymers by chain growth polymerization routes. Living chain growth polymerization is a robust and versatile synthesis method that allows control over the polymer chain microstructural properties. However, control over the monomer sequences in chain polymerization is not precise to the monomeric level due to the statistical nature of this system. We highlight the various innovative approaches demonstrated thus far to achieve higher order control for different chain growth polymerization systems, such as anionic, cationic, ring-opening, ring-opening metathesis, and controlled radical polymerizations.
Amphiphilic dendrimer-like polymers are expected to be promising candidates as micro containers or carriers due to their much larger size in relative to regular dendrimers. In the present study, we synthesize amphiphilic dendrimer-like copolymers possessing interior poly(styrene) segments and peripheral poly(ethylene oxide) (PEO) segments through iterative anionic polymerization, hydrosilylation, coupling, followed by olefin cross-metathesis with a PEO macromonomer bearing acrylic terminus (denoted as PEGMEA480). The molecular weight of the resulting dendrimer-like copolymer, G3-g-PEO2900, is as large as 2.16 × 106, with number of outer PEO arms up to 2900, and the average diameter 32.8 nm in solution. The behavior of the dendritic product as unimolecular micelles is investigated, using dynamic light scattering (DLS) and pyrene probing, together with the thermal responsiveness arising from the low critical solution temperature (LCST) of PEO segments. The amphiphilic dendrimer-like copolymers are used as nanoreactors for the nucleophilic displacement of benzyl halide by KSCN and the hydrolysis reaction of benzyl halide, in which significant increases in the reaction yields are observed. More interestingly, the nanoreactor can be regulated activation-and-deactivation due to thermoresponsiveness of peripheral PEO segments, and the dendritic nanoreactor is recyclable for at least 7 times.
Inspired by the gene editing process, chain editing of synthetic polymers, including functionality “knock-out”, “knock-in” and replacement, was performed through cross metathesis and thiol-Michael addition.
Polymer segment reorganization is capable of changing the chemical structure of the backbone, inserting a functionality into or removing a functionality from the chain, and replacing a specific moiety at the precise site of the chain. It is useful for the preparation of polymer samples with orthogonal structures of segments and functionalities, which is significant for the study of structure–property relationships. In the present work, we developed a strategy for iterative replacement of functional groups in the middle point of the polymer main chain. The strategy involves the "cutting-off" of the polymer precursor by a molecular "scissor", a small molecular olefinic compound, through cross-metathesis (CM) reaction in the presence of Grubbs catalyst, followed by recombination of the cleaved products by a (functional) "binder", a dithiol agent, via thiol-Michael addition reaction. The polymer precursors were prepared through atom transfer radical polymerization (ATRP) using an olefin-containing dibromide initiator to introduce an olefinic moiety in the middle of the chain. The key point of the present work is the use of the itaconic derivative (Z)-O,O′-(but-2-ene-1,4-diyl) 4-dimethyl bis(2-methylenesuccinate) (BES), an itaconic derivative containing one olefinic and two vinylic double bonds, as the molecular "scissor". Owing to the high selectivity of the double bonds in CM and the thiol-Michael reaction, iterative replacement of functionalities at the middle of the main chain was achieved for the first time. The protocol can be useful for the preparation of arrays of polymer samples with orthogonal functionalities and building blocks, which is significant for the study of the structure–property relationship of polymers.
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