Monomer 1,1-disubstituted

Polymers of vinylidene monomers (1,1-disubstituted ethylenes) have lower Tg s than the conesponding vinyl polymers. Polyisobutene and polypropylene comprise such a pair and so do poly(vinylidene chloride) and poly(vinyl chloride). Symmetrical disubstituled polymers have lower Tg s than ihe monosubstituled macromolecules because no conformation is an appreciably lower energy form than any other (cf. the discussion of polyisobutene in Section 4..1). 

DPXC ndDPXD. The economic pressure to control dimer costs has had an important effect on what is in use today (ca 1997). Attaching substituents to the ring positions of a [2.2]paracyclophane does not proceed with isomeric exclusivity. Indeed, isomeric purity in the dimer is not an essential requirement for the obtaining of isomeric purity, eg, monosubstituted monomer, in the pyrolysis. Any mixture of the four possible heteronucleady disubstituted dichloro[2.2]paracyclophanes, will, after all, if pyrolyzed produce the same monomer molecule, chloro- -xyljIene [10366-09-3] (16) (Fig. 4). 

Researchers at Du Pont used hydroquinone asymmetrically substituted with chloro, methyl, or phenyl substituents and swivel or nonlinear bent substituted phenyl molecules such as 3,4- or 4,4 -disubstituted diphenyl ether, sulfide, or ketone monomers. Eor example,... 

More recently, the copolymerization of ethyl cyanoacrylate with other 1,1 disubstituted electron deficient monomers and the effect of the monomers on adhesive properties have been studied. Monomers, such as diethyl methylene-malonate (DEMM), 8, were prepared [6,7]. Their homopolymers and copolymers... 

KJPAC recommendations suggest that polymers derived from I, I-disubstituted monomers CXY=CH (or CH,=CXY) be drawn as lb rather than as la. However, formula la follows logically from the traditional way of writing the mechanism of radical addition (e.g. Scheme 1.1). rtccausc of our focus on mechanism, the style la has been adopted throughout this book. 

Alkyl radicals, when considered in relation to heteroatom-centered radicals (e.g. r-butoxy, benzoyloxy), show a high degree of chcmo- and rcgiospecificity in their reactions. A discussion of the factors influencing the rate and rcgiospecificity of addition appears in Section 2.3. Significant amounts of head addition arc observed only when addition to the tail-position is sterically inhibited as it is in a,p-disubstituted monomers. For example, with p-alkylacrylates, cyclohexyl... 

The following discussion is limited to polymers of mono- or 1,1-disubstituted monomers. Other factors become important in describing the types of stereochemical isomerism possible for polymers formed from other monomers (e.g. 1,2-disubstituted monomers).1... 

Most monomers have an asymmetric substitution pattern and the two ends of the double bond are distinct. For mono- and 1,1-disubstituted monomers (Section 4,3.1) it is usual to call the less substituted end "the tail" and the more substituted end "the head". Thus the terminology evolved for two modes of addition head and tail and for the three types of linkages hcad-to-tail, hcad-to-hcad and tail-to-ta.il. For 1,2-di-, tri- and tetrasubstituted monomers definitions of head and tail are necessarily more arbitrary. The term "head" has been used for that end with the most substituents, the largest substituents or the best radical stabilizing substituent (Scheme 4.4). 

For mono- and 1,1-disubstituted monomers, steric, polar, resonance, and bond-strength terms (see Section 2.3) usually combine to favor a preponderance of tail addition Le. an almost completely isoregic structure. Fiowever, the occurrence of... 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

The monomer 2 can be regarded as a 2,4-disubstituted-1,3-dioxolane, and the thermodynamic parameters for the polymerization may be compared with those for the polymerization of 1,3-dioxolane in methylene chloride21. ... 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

Complexity in the manganese and rhenium pentarbonyl halides substitution arises from the fact that these decompose in inert solvents to form the halogen-bridged dimers [M(CO)4X]2. Both monomers and dimers react with phosphines, arsines, pyridine, aniline etc. to give the disubstituted compounds M(CO)3XL2. Thus three final products or any mixture of them may be obtained, depending upon the precise reaction conditions... 

The alternate head-to-head, tail-to-tail structures XII for polymers of disubstituted monomers... 

The 2 1 species are known as cuprates and are the most common synthetic reagents. Disubstituted Cu(I) species have the 3c 10 electronic configuration and would be expected to have linear geometry. The Cu is a center of high electron density and nucleophilicity, and in solution, lithium dimethylcuprate exists as a dimer [LiCu(CH3)2]2.3 The compound is often represented as four methyl groups attached to a tetrahedral cluster of lithium and copper atoms. However, in the presence of Lil, the compound seems to be a monomer of composition (CH3)2CuLi.4... 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

The iso- and syndiotactic isomerism in the insertion polymerization of dienes (for 1,2 polymerization of generic dienes and for cis-1,4 polymerization of 4-monosubstituted or of 1,4-disubstituted monomers) would be determined, according to the polymerization scheme proposed by Porri and co-workers,181 182 by the relative orientations of the two ligands (diene monomer and allyl terminal of the growing chain) in the preinsertion catalytic intermediates. 

Scheme 12. In the Ru-catalyzed conversion of disubstituted styrenyl ethers to chromenes the presence of ethylene is required for reaction efficiency as well as high yield of monomer formation...

In contrast to the reaction carried under an Ar atm, when (S)-49 was treated with 10 mol% la under an atmosphere of ethylene (22°C, CH2C12,24 h), (S)-41 was obtained in 81% isolated yield and >98% ee (Scheme 12). As expected, the use of ethylene atmosphere proved to be necessary for preferential monomer formation (10% of the derived dimer was also generated). These results indicate that the presence of ethylene is imperative for efficient metal-catalyzed chromene formation as well for processes involving disubstituted styrenyl ethers (25-30% yield of dimer 50 under argon). 

It should be noted that there are special features of the 9-substituted anthracenes that may introduce complications. The materials dimerize also in solution, yielding exclusively the centrosymmetric dimers, which are analogous to 87. Further, the few 9,9 -disubstituted dianthracenes that have been prepared are very unstable. Thus the head-to-head approach of two monomers is apparently an energetically unfavorable process. This is not so for the head-to-tail approach, and it is therefore not surprising that structures in which there are overlapped monomers related by centers of symmetry yield topochemical dimers. 

We also note that some 2,2-disubstituted oxiranes have toxicological significance, as exemplified by 2,2-dimethyloxirane (2-methyl-l, 2-epoxypropane, 10.43, R = Me). This compound is the toxic metabolite of 2-methyl-prop-1-ene (isobutene), a gaseous alkene widely used as a monomer in the industrial production of adhesives, plastics, and other polymers. Interestingly, detoxification of this epoxide catalyzed by liver epoxide hydrolase was high in the human, intermediate in the rat, and low in the mouse [125], These activities were inversely correlated with the epoxide levels measured in vitro in liver tissue of these species. 

In the oxidation of anionized 1,3-dicarbonyl compounds (Table 8, numbers 1-7) at potentials between 0.6 and 1.4 V (see) and in the presence of butadiene, mainly the additive dimer (24) is obtained in the presence of ethyl vinyl ether chiefly the disubstituted monomers (28) or (29) arise. 

Attempts to directly iodinate quinoxaline failed, and the synthesis of 2,3-diphenyl-5,8-dibromoquinoxaline is somewhat more involved (Scheme 9) [61]. Starting from ort/zo-phenylenediamine, reaction with SOCI2 gives benzothia-diazole in high yield. Bromination in HBr furnishes 4,7-dibromobenzothiadi-azole, which can be alkynylated or directly reduced [62]. Reduction of the dibromide with sodium borohydride leaves the halide substituents unmolested but opens the ring to furnish l,4-dibromo-2,3-diaminobenzene. Reaction of this intermediate with a 1,2-dione furnishes a 2,3-disubstituted 5,8-dibromo-quinoxaline. Pd-catalyzed alkynylation finishes the sequence off and removal of the TMS groups yields the desired 5,8-diethynylquinoxaline monomers (Table 9, entries 13,14). 

Whereas the cationic polymerization of furfurylidene acetone 3a engenders crosslinked structures (25), the use of anionic initiators results in linear structures (26). However, the propagation is preceded by an isomerization of the active species which eliminates the steric hindrance to propagation arising from the 1,2-disubstitution in the monomer structure. A proton shift from the 4- to the 2-position places the negative charge at the extremity of the monomer unit and the incoming monomer can add onto this anion without major restrictions. The polymer structure thus obtained is ... 

Good evidence for the formation of dimeric 4,4, 4",4 "-tetrasulphophthalo-cyanine oxovanadiumtiv) species in HjO-DMF solutions has been obtained from e.s.r. spectra, and the metal centres are estimated to be 450 pm apart. In aqueous solution, although dimers are present, most of the oxovanadium(iv) complexes are polymerized, and in DMF solution substantial amounts of monomer exist. Stability constants for the formation of 1 1 complexes between pyridine and some oxovanadium(iv)-2,4-disubstituted deuteriopor-phyrin dimethyl esters have been determined in chloroform solution at 25 °C. Magnetic and spectroscopic evidence suggest the presence of an electronic interaction between centres in these compounds. 

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