Polymerization electron-deficient

In cationic polymerizations, electron-deficient initiators (mostly Bronsted or Lewis acids) react with electron-rich monomers. The active chain end (ACE) bears a positive charge with the active sites being either carbenium or 0x0-nium ions. Molecular weights are often limited by the inherent sensitivity to impurities, chain transfer, and rearrangement reactions. Suitable monomers for cationic polymerizations are vinyl monomers with electron-donating moieties or cyclic structures containing heteroatoms, while the latter case is termed cationic ROP. Eligible monomers include cyclic ethers, acetals, and amines as well as lactones and lactams (Scheme 3). 

Tertiary bismuthines appear to have a number of uses in synthetic organic chemistry (32), eg, they promote the formation of 1,1,2-trisubstituted cyclopropanes by the iateraction of electron-deficient olefins and dialkyl dibromomalonates (100). They have also been employed for the preparation of thin films (qv) of superconducting bismuth strontium calcium copper oxide (101), as cocatalysts for the polymerization of alkynes (102), as inhibitors of the flammabihty of epoxy resins (103), and for a number of other industrial purposes. 

A polymeric structure is exhibited by "beryllium dimethyl," which is actually [Be(CH3)2] (see the structure of (BeCl2) shown earlier), and LiCH3 exists as a tetramer, (LiCH3)4. The structure of the tet-ramer involves a tetrahedron of Li atoms with a methyl group residing above each face of the tetrahedron. An orbital on the CH3 group forms multicentered bonds to four Li atoms. There are numerous compounds for which the electron-deficient nature of the molecules leads to aggregation. 

M. Matsumi, K. Naka, and Y. Chujo, Extension of ir-conjugation length via the vacant p-orbital of the boron atom. Synthesis of novel electron deficient ir-conjugated systems by hydroboration polymerization and their blue light emitting, J. Am. Chem. Soc., 120 5112-5113, 1998. 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

Iodosylbenzene is sufficiendy reactive on its own to epoxidize electron-deficient olefins such as tetracyanoethylene (43). It is possible that coordinated monomeric iodosylbenzene is substantially more reactive than polymeric iodosylbenzene and that complexation of a monomeric form is sufficient to provide the requisite reactivity with normal olefins. 

It was already established that pure ethyl-" " and f-butyllithium exist as six- and fourfold polymers, respectively, in benzene solution. Apparently, C—Li bond cleavage takes place in this solvent leading to an exchange of alkyl groups between polymeric organo-lithium molecules when both compounds are present. The products are believed to be electron-deficient polymers of the type (EtLi) (f-BuLi) , wha-e m is a small number such as 4 or 6. ... 

Cationic polymerization of 4-membered imines (IUPAC azetidines) generally follows the same patterns as the aziridines [Matyjaszewski, 1984a,b Muhlbach and Schulz, 1988]. Imines are generally unreactive toward anionic polymerization presumably because of the instability of an amine anion (which would constitute the propagating species). The exception occurs with V-acylaziridines as a result of the electron deficiency of the nitrogen coupled with the highly strained 3-membered ring. 

Under optimized conditions regarding the choice of Br0nsted acid (mandelic acid 20), stoichiometry (1 1 ratio 9 and mandelic acid 20), solvent (the respective alcohol neat conditions), temperature (rt or 50°C), and catalyst loading (lmol% 9 and lmol% mandelic acid 20) electron-rich and electron-deficient styrene oxides underwent alcoholysis with simple aliphatic, stericaUy demanding as well as unsaturated and acid-labile alcohols. The completely regioselective (>99%) alcoholysis was reported to produce the corresponding P-aUcoxy alcohols 1-10 in moderate (41%) to good (89%) yields without noticeable decomposition or polymerization reactions of acid-labile substrates (Scheme 6.27). Notably, aU uncatalyzed reference experiments showed no conversion even after two weeks under otherwise identical conditions. 

The mechanism for the polymerization probably fits the general pattern of electrophilic attack by the initiator on the carbonyl oxygen atom for the initiation step, with propagation taking place by attack of the electron deficient C-4 of the last unit of the growing chain on the carbonyl group of a monomer molecule. Certain modifications of this general mechanism are apparently observed with specific catalysts, and the subject appears to need further study. 

The basic amino group of the 1-position in semicarbazide or thiosemi-carbazide may be used to react by a substitution reaction with activated halides [52], ethers [51], hydroxy [53], phenoxy [54], and amino groups [55] to yield substituted 1-semicarbazides or thiosemicarbazides. In addition, the amino group of the 1-position may add to electron-deficient double bonds [56]. Formaldehyde and other aldehydes may add to all the available free NH groups to give methylol, alkylol, or polymeric products under basic conditions [57]. Aldehydes or ketenes usually give semicarbazone derivatives, and these in turn are used analytically to identify the purity or structure of a known aldehyde [3]. 

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