New developments in bond‐forming initiation: Lewis acids accelerate spontaneous initiation of copolymerizations of olefins and dienes

Abstract

Lewis acids have long been known to influence the free radical propagation of donor‐acceptor olefin monomer pairs, changing random copolymers into strictly alternating ones, by complexing to the lone pair electrons of the acceptor monomer, and enhancing its electron‐poor character. This work reports that this same interaction enhances the rates of the spontaneous initiation in these systems. In agreement with earlier literature, we found that styrene‐acrylonitrile copolymerizations proceed readily in the presence of ZnCl2 at 40° where no copolymerization occurs in the absence of the Lewis acid. p‐Methoxystyrene and isobutyl vinyl ether with AN gave more complicated results, due to competitive complexation of oxygen lone pairs with ZnCl2. Reaction of more electrophilic olefins with these donor monomers gave steadily increasing rates and a shift in product from spontaneous copolymer to inverse Diels‐Alder adduct (in the case of ester substituents) to cyclobutane. As for substituted diene monomers as the donor component, reactions of isoprene and 2,3‐dimethylbutadiene with AN are accelerated in the presence of ZnCl2 and the Lewis acid affects the competition between the [4+2] cycloaddition and the copolymerization. All of these results are interpreted in the context of formation of Lewis‐acid complexed initiating diradicals by bond‐formation.

Endophytic Actinobacteria from the Brazilian Medicinal Plant Lychnophora ericoides Mart. and the Biological Potential of Their Secondary Metabolites

“…DNA sequencing was performed by the company Genotyping Biotechnology LTDA, and the identified sequences were compared to the EzTaxon-e (http://eztaxon-e.ezbiocloud.net/ezt_identify) [76], GenBank (http:// blast.ncbi.nlm.nih.gov/Blast.cgi) [77], and CBS (http:// www.cbs.knaw.nl/Collections) databases. Sequences of each isolated species were analyzed with multiple alignments using BioEdit v7.1.3 [78]. Partial sequence data of 16S rRNA genes were deposited in the GenBank nucleotide sequence data libraries.…”

Organic and polymer chemistry of electrophilic tri‐ and tetrasubstituted ethylenes

“…We wanted to see if Lewis acids also accelerated initiation by increasing the polarity of the tetramethylene intermediate. The addition of zinc chloride to mixtures of several p ‐substituted styrenes with acrylonitrile led to accelerated spontaneous copolymerization 100, 101. A sterically hindered base had to be added in many cases to inhibit the cationic homopolymerization of the donor monomer (Scheme ).…”