Polymerization of 1,2-Disubstituted Ethylenes

With some exceptions, 1,2-disubstituted ethylenes containing substituents larger than fluorine such as maleic anhydride, stilbene, and 1,2-dichloroethylene exhibit little or no tendency 

Polymers from 1,2-disubstituted ethylenes (XXXVI) possess 1,3-interactions, but the steric strain is not as severe as in XXXV. Both XXXV and XXXVI possess the same number of 1,3-interactions but the distribution of the interactions is different. For XXXV, pairs of 1,3-carbons each have a pair of 1,3-interactions. No pair of 1,3-carbons in XXXVI has more than 

The low tendency of 1,2-disubstituted ethylenes to polymerize is due to kinetic considerations superimposed on the thermodynamic factor. The approach of the propagating radical to a monomer molecule is sterically hindered. The propagation step is extremely slow because of steric interactions between the P-substituent of the propagating species and the two substituents of the incoming monomer molecule  

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Complications arising from other types of isomerism. Positional and geometrical isomerism, also described in Sec. 1.6, will be excluded for simplicity. In actual polymers these are not always so easily ignored. Polymerization of 1,2-disubstituted ethylenes. Since these introduce two different asymmetric carbons into the polymer backbone (second substituent Y), they have the potential to display ditacticity. Our attention to these is limited to the illustration of some terminology which is derived from carbohydrate nomenclature (structures [IX]-[XII]) ... 

The polymerization of 1,2-disubstituted ethylenes, RCH=CHR, such as 2-pentene (R = — CH3, R = —C2H5), presents a different situation. Polymerization yields a polymer structure II in which there are two different stereocenters in each repeating unit. Several possibilities of ditacticity exist that involve different combinations of tacticity for the two stereocenters. Various stereoregular structures can be defined as shown in Fig. 8-2. Diisotactic structures occur when placement at each of the two stereocenters is isotactic. 

A more complicated picture emerges when the polymerization of 1,2-disubstituted ethylenes (CHR=CHR ) is considered because now each carbon atom in the chain becomes a chiral center. The resulting ditactic structures are illustrated in Figure 6.1(d,ed). Two isotactic structures are obtained, the erythro, in which all the carbon atoms have the same configuration, and the threo, in which the configuration alternates. Only one disyndiotactic structure is possible. The differences arise from the stereochemistry of the starting material if the monomer is cis-substimted the threo form is obtained, whereas a trans monomer leads to the erythro structure. 

Ditactic polymers possess two stereoisomeric centers per constitutional monomeric unit, and tritactic polymers possess three. Ditactic polymers may be formed by the polymerization of 1,2-disubstituted ethylene derivatives, as, for example, with pentene-2 ... 

Recent our investigations as to polymerization of 1,2-disubstituted ethylenes including DRF have revealed the feature of this polymerization in detail, i.e., polymerization rate, absolute rate constants, and reactivities of the monomer and the polymer radical, as well as the polymer structures and some properties such as tacticity of the polymer, rigidity of the chain, thermal properties, and some applications. Radical polymerization of 1,1-disubstituted and 1,1,2-trisubstituted ethylenes was also investigated. The detailed results are described and discussed in this article. 

Polymers containing rings incorporated into the main chain (e.g., by double-bond polymerization of a cycloalkene) are also capable of exhibiting stereoisomerism. Such polymers possess two stereocenters—the two atoms at which the polymer chain enters and leaves each ring. Thus the polymerization of cyclopentene to polycyclopentene [IUPAC poly(cyclopen-tane-l,2-diyl)] is considered in the same manner as that of a 1,2-disubstituted ethylene. The... 

Steric hindrances prevent the polymerization of most 1,2-disubstituted ethylenes by any mechanism. However, 1,1 -disubstituted monomers and vinylidene monomers usually polymerize more readily than the corresponding vinyl analogs. 

Generally, 1,2-disubstituted ethylene derivatives have only a small tendency for radical homopolymerization. An exception is vinylene carbonate (VCA) which can be easily polymerized by chemical as well as radiation initiation. However, the reaction is strongly affected by traces of impurities formed during the synthesis. Inhibition experiments are discussed with regard to the nature of the inhibiting impurities. The copolymerization behavior of VCA with some halo-substituted olefins was studied with chlorotrifluoroethylene (CTFE), a statistical copolymer with a slight tendency for alternation was obtained. 

From elementary organic chemistry, we know that the positions and hence reactivities of the electrons in unsaturated molecules are influenced by the nature, number, and spatial arrangement of the substituents on the double bond. As a result of these influences, the double bond reacts well with a free radical for compounds of the types CHj = CHY and CHj = CXY. These compounds constitute the so-called vinyl monomers where X and Y may be halogen, ally l, ester, phenyl, or other groups. It must, however, be noted that not all vinyl monomers produce high polymers. In symmetrically disubstituted double bonds (e.g., 1,2 disubstituted ethylenes) and sterically hindered compounds of the type CHj = CXY, polymerization, if it occurs at all, proceeds slowly. 

Abstract Novel vinyl polymers were synthesized from 1,1- or 1,2-disubstituted and 1,1,2-trisubstituted ethylenes by radical polymerization. The polymers obtained consist of a rigid chain structure on account of the bulky side groups compared with flexible poly(monosubstituted ethylene)s. The substituted polymethylenes obtained from 1,2-disubstituted ethylenes such as fumaric and maleic derivatives were revealed to have new properties different from ordinary vinyl polymers. Radical polymerization behaviors of these multi-substituted ethylenes and some properties of the resulting polymers were investigated. 

Formation of such less-flexible polymers is expected from polymerization of not only 1,2-disubstituted ethylenes but also 1,1-disubstituted ethylenes when the substituents are bulky. It has been generally recognized that 1,1-disubstituted ethylenes with bulky substituents polymerize hardly because of their low ceiling temperatures due to the steric effect of the substituents, but itaconic acid (lA), a-(hydroxycarbonylmethyl)-... 

Several possibilities exist for ditacticity in macromolecules formed by polymerizing 1,2-disubstituted ethylenes of the type RHC=CHR, structures (XVIII)-(XX). It can be seen that each unit contains two different asymmetric carbon atoms in the chain. The original definitions of tacticity have been extended to include these modifications and Newman s (1956) definitions of erythro and threo structures. Structures (XVIII)-(XX) illustrate ifereo-diisotactic, erythro-diisotactic, and disyndiotactic poisoners, respectively. 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

Chain growth (step b in Equation 22.5) occurs by a combination of olefin coordination and migratory insertion. A vacant site is required for the olefin to coordinate before alkyl insertion can take place. Because coordination is required, fast insertions occur with less-hindered olefins, such as ethylene, propylene, linear a-olefins, and vinylarenes. The relative rates for the polymerization of aUcenes typically follow the trend ethylene > propylene > a-olefin 1,2-disubstituted olefin = 1,1-disubstituted olefin, and enchainment of tri-and tetra-substituted olefins is rare or unknown. Many polymerization catalysts are sensitive to poisoning by impurities that bind the open coordination site. 

The general reaction equation for alkene metathesis in a simple system, cross-metathesis of two different disubstituted alkenes, is depicted in Scheme 1. In this reaction, a transition metal catalyst establishes equilibrium between the starting alkenes, the ( )- and (Z)-stereoisomers of all possible substituent combinations, and ethylene. Related reaction processes have also been reported for alkynes (aikyne metathesis) and for combinations of alkenes and alkynes (enyne metathesis). Aikyne metathesis is less well developed compared to alkene metathesis and enyne metathesis. This review has been organized according to the basic modes of metathesis depicted in Scheme 2. Alkene metathesis is the more developed process and numerous examples of all the variants have been reported. Aikyne metathesis is less well developed and three variants exist aikyne cross-metathesis, aikyne metathesis polymerization, and ring-closing aikyne metathesis. 

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