Radical initiated double ring-opening

Scheme 2.5 Unsaturated polyester formation with low volume shrinkage by radical initiated double ring-opening polymerization [23].

Apparently, the driving force for the ring opening is the relief of the strain in the spiro system and the formation of the stable carbonate double bond. The double ring opening is probably a concerted process from the initial radical addition product to the open-chain radical. Even though the spiro compound XI is an allyl monomer, it does copolymerize with a wide variety of comonomers. 

Lactide polymerization with the addition of tin octoate is proposed via the coordination—insertion mechanism, as shown in Figure 2.10 (Henton, et al., 2005). The tin catalyst initiates the ring-opening reaction by attacking the nearest double-bond oxygen of the lactide. The hydroxyl and nucleophilic species simultaneously react with the ring-opened radical and finally... 

The first route relies on the ROP of cyclic ketene acetals [1-3]. The electron-rich double bond is prone to react with radicals and electrophiles. Therefore, this class of monomers undergoes cationic and radical polymerization. For example, radical initiators react with the double bond to provide a new tertiary radical (Fig. 2). Two distinct mechanisms of polymerization can then take place direct vinyl polymerization or indirect ring opening of the cycle accompanied by the formation of a new radical, which is the propagating species (Fig. 2). The ester function is formed... 

Another approach for the ring expansion of epoxides uses low-valent iron complexes which open epoxides under reductive conditions, as reported by Hilt et al. [106]. The iron complexes are reduced and after coordination of the epoxide to the iron center an electron transfer initiates the radical-type ring opening of the epoxide. Under formal insertion of an alkene, regioselective formation of tetrahy-drofurans was observed (Scheme 9.46). The reaction is applicable to a broad range of acceptor-substituted alkenes bearing another double or triple bond system in conjugation with the inserted carbon-carbon double bond. 

Schematic representation of possible mechanistic pathways for olefin epoxidation by 3. The mechanisms described are, from right to left, concerted addition of oxygen to the double bond, reaction via a metallocylic intermediate, reaction via a ring-opened radical intermediate, and reaction proceeding via an initial electron-transfer step. ...

The initial step is the addition of a thiophenyl radical to the double bond. After fast ring opening of the cyclopropylcarbinyl radical, the critical event is the addition of the resulting radical to the olefin. If this intermolecular reaction is slow, trapping by a second equivalent of the thiophenyl radical leads to the formation of an undesired by-product. 

One of the important methods in the construction of five-membered rings is [3-f2] radical cycloaddition. This reaction is initiated by radical-induced opening of a three-membered ring followed by addition of ethylene to create a new radical and subsequent cyclization onto a double bond (Scheme 20). Addition of the initial ring-opened radical to the double bond can be carried out either inter- or intra-molecularly [27],... 

In contrast, here a bifunctional initiator is employed and the polymerization order of the two blocks is inverted In a first step, the styrene block is synthesized by atom transfer radical polymerization (ATRP) followed by the addition of lactide via the recently developed organocatalytic ring-opening polymerization, as depicted in Fig. 3.1 [4, 5]. This synthesis route reduces the involved steps and enables a simplified and time-efficient preparation of copolymers with different block compositions. Importantly, both polymerization techniques offer precise and robust control over the copolymer composition, which is an essential requirement to reliably target the double-gyroid s narrow location in phase space [6]. 

Summary A double-headed initiator was synthesized yielding two functional groups for the initiation of the nickel mediated ring-opening polymerization of y-benzyl-L-glutamate-N-carboxyanhydride and controlled radical polymerization of vinyl monomers via ATRP or NMP. Well-defined block copolymers combining polypeptides and synthetic polymers were obtained. 

The addition of the growing radical to the double bond is analogous to the first step of 1-5 addition, but apparently conformational restraints caused by the cyclobutane ring prevents normal cyclopropyl ring opening. Instead a hydrogen shift resulting in a resonance-stabilized radical takes place. The possibility of initial thermal isomerization followed by polymerization was excluded because monomer 13 when heated in the presence of DPPH did not isomerize or lymerize. 

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