Cascade polymerizations: recent developments in the formation of polymer repeat units by cascade reactions

Abstract

Traditionally, most polymerizations rely on simple reactions such as alkene addition, ring-opening, and condensation because they are robust, highly efficient, and selective. These reactions, however, generally only yield a single new C-C or C-O bond during each propagation step. In recent years, novel macromolecules have been prepared with propagation steps that involve cascade reactions, enabling various combinations of bond making and breaking steps to form more complex repeat units. These polymerizations are often challenging, given the requirements for high conversion and selectivity in controlled polymerizations, yet they provide polymers with unique chemical structures and significantly broaden the scope of how polymers can be made. In this perspective, we summarize the recent developments in cascade polymerizations, primarily focusing on single-component cascades (rather than multi-component polymerizations). Polymerization performance, monomer scope, and mechanisms are discussed for polymerizations utilizing radical, ionic, and metathesis-based mechanisms.

Fig. 1. Examples of cyclic (A) exomethylene and (B) vinyl monomers undergoing nonselective cascade polymerization with free radical polymerization conditions or primarily rROP with ATRP conditions. Labels under the polymers = polymer label: M_n (Đ) degree of polymerization (DP). Abbreviations: DTBP = di-t-butyl peroxide. bpy = 2,2′-bipyridine. EBB = ethyl 2-bromobutyrate. BPO = benzoyl peroxide. Me₆TREN = tris[2-(dimethylamino)ethyl]amine. EBiB = ethyl 2-bromoisobutyrate.

Fig. 2. (A) Mechanism of the organocatalyzed photoredox cascade polymerization of vinyl cyclopropanes. (B) The linear vs. cyclic repeat unit content could be tuned by changing monomer concentration, LED source, and temperature. Representative examples shown. Labels in part B = conditions: M_n in kDa (Đ) DP. Abbreviations: PC = photocatalyst (N,N-di(2-naphthyl)dihydrophenazine). ISC = inter system crossing. DBMM = diethyl 2-bromo-2-methylmalonate.

Fig. 3. (A) The cascade polymerization of macrocyclic monomers (ring size shown in blue). (B) Mechanism of the RO/fragmentation cascade. Labels under the polymer structures = polymer label: M_n (Đ) DP. Abbreviations: AIBN = azobisisobutyronitrile. CTA = chain transfer agent (structure shown in solid box).

Fig. 4. (A) Ring-opening/ring-closing/fragmentation cascade polymerization of macrocyclic monomers (ring sizes shown in blue). (B) Mechanism of the cascade sequence. The structure of CTA2 is shown in the solid box. Labels under the polymer structures = polymer label: M_n (Đ) DP. Abbreviations: DMF = dimethylformamide.

Fig. 5. Examples of cascade polymerizations with (A) anionic and (B) cationic mechanisms. Labels under the polymer structures = polymer label: M_n (Đ) DP. Abbreviations: E = electrophile. DMSO = dimethylsulfoxide. DCM = dichloromethane.

Fig. 6. (A) Ionic step-growth cascade polymerization of diselenide monomers. (B) Mechanism of the ionic cascade sequence. Labels under the polymer structures = polymer label: M_n (Đ) DP.

Fig. 7. (A) Ru catalysts used in metathesis-based cascade polymerizations. The primary cascade sequences in metathesis-based cascade polymerizations are based on (B) RC/RO, (C) RO/RC, and (D) various combinations of RC and MS. Labels under the polymer structures = polymer label: M_n (Đ) DP. Abbreviations: Ts = tosyl. TBDMS = t-butyldimethylsilyl. TES = triethylsilyl. Ms = mesyl. THF = tetrahydrofuran. 3,5-Cl₂Py = 3,5-dichloropyridine.

Fig. 8. Cascade polymerizations of (A) challenging, (B) sugar-derived, and (C) furfuryl alcohol-derived monomers. (C) Degradation conditions and products. Labels under the polymer structures = polymer label: M_n (Đ) DP. For P27, the repeat unit ratio is also shown. Abbreviations: TFA = trifluoroacetic acid. RT = room temperature. TIPS = triisopropylsilyl.

Fig. 9. (A) Cascade polymerization of biscycloalkene monomers. (B) One-shot multiple-olefin metathesis polymerization (MOMP) combining RC, RO, and cross-metathesis reactions to achieve alternating polymers. Labels under the polymer structures = polymer label: M_n (Đ) DP.

Fig. 10. M&M polymerizations consisting of (A) RC/MS/RC, (B) RC/MS/RC/MS/RC, and (C) RC/MS/MS/RC cascade reactions. Labels under the polymer structures = polymer label: M_n (Đ) DP.

Fig. 11. (A) β-selective M&M polymerization of tetrayne monomers. (B) Mechanism of β-addition. After the 6-membered ring formation, the cascade proceeds as shown in Fig. 7D. Labels under the polymer structures = polymer label: M_n (Đ) DP, the % of β repeat units is also shown. Abbreviations: TMS = trimethylsilyl.