The Relationship of Cycloaddition Reactions to Spontaneous Vinyl Polymerizations

Abstract

To Professor Rolf Huisgen with grateful appreciation for his contributions to organic polymer chemistry! The zwitterionic ± biradical tetramethylene proposed by Huisgen as the key intermediate in stepwise [2 2] cycloaddition reactions has been shown to be the crucial intermediate in spontaneous vinyl polymerizations as well. Predominantly biradical tetramethylenes initiate free-radical copolymerizations, while predominantly zwitterionic tetramethylenes initiate cationic or anionic homopolymerizations. Stepwise cycloaddition is viewed as a spontaneous polymerization lacking a propagation step. These tendencies could be correlated in the form of an organic chemists Periodic Table, which has recently been put on a quantitative basis.Huisgen also showed experimentally that [4 2] Woodward ± Hoffman-allowed cycloadditions are completely concerted. Spontaneous copolymerizations accompanying these cycloadditions, therefore, were ascribed to the s-trans diene form. This concept was given support by kinetics studies, as well as by exclusive cycloaddition from s-cis cyclopentadiene, and exclusive copolymerization from s-trans verbenene.Introduction. ± The great poet builds larger than he knows. This familiar saying also applies to great scientists. When Rolf Huisgen was studying the role of the tetramethylene intermediate in [2 2] cycloaddition reactions, surely the idea that this intermediate might play an important role in polymer chemistry never crossed his mind. Yet this has proven to be the case.During the 1960s and 1970s, polymer chemists encountered various unexpected and perplexing observations in vinyl polymerizations. A tendency of the monomer units to alternate in the copolymer chain was observed. This was the case when electron-rich monomers, such as styrene or vinyl ethers, were copolymerized with electron-poor monomers, such as methyl acrylate or acrylonitrile. As the donor and acceptor character of the co-monomers increased, the alternation tendency grew stronger and stronger. Eventually, a new phenomenon set in, namely spontaneous polymerization. At first, spontaneous copolymerization occurred by a radical mechanism to form strictly alternating copolymers. With still stronger donor-substituted monomers, homopolymers of the donor monomer were found, which could only be formed by a spontaneous cationic homopolymerization.Attempts by polymer chemists to understand these phenomena centered on the visible charge-transfer (CT) complexes formed by the two monomers. It was attractive to suppose that free-radical homopolymerization of this CT complex would lead to the observed strictly alternating copolymers. But how this explained the cationic homopolymers or the CT complex initiated polymerizations, was not made clear.

A paradigm for the mechanisms and products of spontaneous polymerizations

“…Cyclobutanes can retain reactivity, either reverting to zwitterion or undergoing metathesis, whereas the cyclohexenes do not react further. The intimate relationship between stepwise cycloaddition and spontaneous polymerization was elucidated in a recent article 2…”

Mechanism and Kinetics of the Spontaneous Thermal Copolymerization of Styrene/Maleic Anhydride. Experimental and Simulation Studies in the Presence of 4‐oxo‐TEMPO

“…These calculations seem to indicate that after the addition of even a small amount of MA the propagation mechanism drastically changes. The old literature ascribed these mechanistic changes to the intermediation of a charge transfer complex; more recent studies4, 5 discard this hypothesis and explain related experimental observations through the influence of a tetramethylene diradical which is formed by the reaction of S and MA molecules (see Scheme ). According to these authors this diradical acts as an intermediate in the propagation reaction and is responsible for the alternating character of the copolymer formed.…”

“…Various mechanisms have been proposed to explain the initiation process of the spontaneous copolymerization of S with electron acceptor monomers such as MA and acrylonitrile. The copolymerization of S with electron acceptor monomers has been the subject of extensive mechanistic discussions 4, 5…”