An Alkenyl Boronate as a Monomer for Radical Polymerizations: Boron as a Guide for Chain Growth and as a Replaceable Side Chain for Post‐Polymerization Transformation

Abstract

The ability of isopropenyl boronate pinacol ester to serve as a monomer in radical polymerizations was established and exploited for the synthesis of polymers that are difficult to access using other polymerization techniques. Although the monomer exhibits an α‐methyl‐substituted unconjugated structure, which is usually unfavorable for radical propagation, both free and controlled radical polymerizations smoothly afford the corresponding polymers. A density‐functional‐theory‐based investigation revealed that the boron atom moderately stabilizes the radical species, which leads to the suppression of the degradative chain transfer to the α‐methyl groups, and thus guides the reaction towards the radical polymerization. The boronyl pendants, which are directly attached to the polymer backbone, can be replaced with ‐OH or ‐NH2 to yield poly(α‐methyl vinyl amine) or poly(α‐methyl vinyl alcohol), which has been inaccessible by conventional synthetic methods.

Stereoselective Diels-Alder Reactions of gem-Diborylalkenes: Toward the Synthesis of gem-Diboron-Based Polymers via ROMP

“…Our calculations indicate that the reaction favors the endo product 5c with Ts-energies of endo-BpinBdan-TS, which is 2 Kcal /mol less than that of exo-BpinBdan-TS (Figure 4E). 21,29,33 This is in a good agreement with the fact that the aromaticplanar Bdan group is less bulky than the Bpin group. 13,27,32,42 Therefore, it would appear that the diastereoselectivity in this case is driven primarily by sterics.…”

“…23,24 Moreover, the presence of the boron moiety on the polymer offers a great opportunity for postpolymerization modifications to achieve new functional groups that are difficult to achieve in regular polymerizations. 21,22 Although polymers which contain mono-boron units have been widely used, 24 their gem-diboryl analogs have not been investigated; thus, they can provide a new array of polymer properties to forge a wider diversity of gem-diboron-based polymers. Thus, gem-diborylnorbornenes hold great promise to serve as a monomer for ROMP 25 reactions that form polymers containing gemdiboryl units (Figure 6A).…”

“…In recent years, gem-diboryl compounds 2, 3' have been widely adopted as coupling partners in synthetic chemistry. 11,20 Despite the fact that organoborons 1, 1' (Figure 1A) have been applied in many fields including materials, polymers [21][22][23][24] I (Figure 1B), drugs, and in industry, gem-diboryl units 2, 3' have seldom been employed in these fields, e.g., polymer II (Figure 1C). To this end, we contend that a new paradigm of research is needed to complementarily propel this gem-diboryl class of compounds to reach the same application level as their mono-boron analogs.…”

α-Borylalkyl radicals: their distinctive reactivity in modern organic synthesis

“…Furthermore, the reaction results in good diastereoselectivity up to 19 : 81. 13 In 2019, Ouchi and Nishikawa 14 carried out the polymerisation of isopropenyl boronate pinacol ester (IPBpin) 24 by taking advantage of the stabilisation of radicals adjacent to the boron through the empty p-orbital 26. This moderate stabilisation of radicals allows a reaction towards the chain growth of the polymer and suppresses the chain transfer reaction, which leads to a greater chain length of the polymer.…”

“…Furthermore, they utilised the presence of boron in skeleton 25, which was transformed to alcohols 27 and amines 28 by the oxidation of pinacolatoboronate ester for the synthesis of poly(a-methyl vinyl alcohol) (PMVA) and poly(a-methyl vinyl amine) (PMVAm), respectively (Scheme 8). 14 2.1.3 DLP as a radical initiator. In 2015, Zard 15 and co-workers demonstrated a different reactivity between vinyl pinacolatoboronates (Bpin) and vinyl MIDA-boronates 29 toward radical chemistry.…”