Atom bifunctional initiation

Numerous examples of block copolymers formed in supercritical C02 via the bifunctional initiator approach have been reported [54], Perhaps the most common approach is to incorporate eROP with free-radical polymerization-the general scheme for this methodology is shown in Figure 13.3. Howdle et al. [55] was the first to report the synthesis of a block copolymer by the bifunctional initiator approach in supercritical C02 and showed the simultaneous eROP of e-caprolactone with controlled free radical polymerization of methyl methacrylate by atom transfer radical polymerization (ATRP)-at this time simultaneous eROP and ATRP had not been reported in any media. The bifunctional initiator incorporated both a primary hydroxyl group (as an initiation site for eROP of e-caprolactone) and a bromine moiety (for initiation of ATRP). Howdle showed that... 

Figure 13.3 Overview showing the use of bifunctional initiators for block-copolymer synthesis. Two examples using either reversible addition fragmentation chain transfer polymerization or atom transfer radical polymerization combined with eROP are shown.

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]. 

Reactions at ring atoms consist mainly of electrophilic attack at nitrogen and cycloadditions. Examples of the reaction of 2-substituted 1,3,4-oxadiazoles with bifunctional compounds at both ring nitrogen and at the substituent, leading to cyclic systems, are included in Section 4.06.5.2 irrespective of where the initial point of attack took place. A few examples of nucleophilic attack at unsubstituted carbon are described, the more common nucleophilic attack at substituted carbon being included under reactions of the appropriate substituent (Sections 4.06.7.1-7.5). 

Nilsson and coworkers42 showed that addition of trifluoroacetic acid to enamines in dry pentane at 0 °C results initially in protonation at the ft carbon and proposed that the proton is transferred to the nitrogen atom by a bifunctional catalysis according to equation 19. They also showed that appropriate exchange resins can be used to favour selective protonation of enamines42c. 

Thus, functionality is not an absolute property of a group, but always has to be considered in relation to the reaction partner. The chemical structure of the resulting macromolecules, moreover, will be decided not only by the functionality of the groups capable of polymerization, but also by the functionality of the molecules. The carbon-carbon double bond in vinyl chloride is bifunctional with respect to free radical polymerization initiators. However, the radicals can also attack already formed polymer, where, for example, a chlorine atom is abstracted with termination of a growing end and formation of a new polymer radical. The new polymer radical can in turn initiate vinyl chloride polymerization again ... 

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