Isomerism repeat unit

The reactivity sequence shown above corresponds well to Mayr s [18] model reactions of the electrophilic addition of benzhydryl carbenium ions to substituted alkenes. Table 2 lists the second-order rate constants for the addition of a diarylcarbenium ion to various alkenes and dienes [36]. One alkyl group offers little activation of the double bond a-olefins therefore form only oligomers with isomerized repeat units in low conversions under cationic polymerization conditions. One vinyl group activates the double bond slightly more than alkyl groups do. Table 2 also demon-... 

These polymerizations proceed mostly by a Michaelis-Arbuzov (M-A) type of rearrangement (Scheme 1), involving cyclic phosphonium intermediates to produce poly-phosphinates or polyphosphonates (39). The presence of isomerized repeating units (40) was the result of the occurrence of side reactions. Sometimes the proportion of the isomerized units was higher than that of the normal units (39). ... 

Table V. Kinetics of Repeating Unit Isomerization in Poly[(chloromethyl)thiirane] and Poly(3-chlorothietane)...

Repeating unit isomerization is similar in several respects to isomerization polymerization (26,27). Isomerization polymerization may be defined as a process whereby a monomer of structure A is converted to a polymer of repeating unit structure B, wherein the conversion of A to B represents a structural change which is not a simple ring opening or double bond addition ... 

We define repeating unit isomerization as a process subsequent to polymerization, in which an intramolecular rearrangement of the repeating unit leads to a thermodynamically preferred structure ... 

C NMR analysis, 356,360,3611-362/ Reduction, 2,2-bis[4 -(4"-phenylsulfonyl phenoxyl)phenyl] propane, H NMR, 9 Repeating unit isomerization vs. 

Dendritic polymers are most often reported to be amorphous, which can be anticipated from their highly branched architecture. However, some exceptions are presented in the literature. Percec et al. [34,35] reported on liquid crystalline (LC) hyperbranched polymers where the LC-phase was achieved by conformational isomerism. Various repeat units of A2B type have been used where a flex-... 

Polymerization leads to a polymer structure (VII) with a repeating alkene double bond in the polymer chain. The double bond in each repeating unit of the polymer chain is a site of steric isomerism since it can have either a cis or a trans configuration. The polymer chain segments on each carbon atom of the double bond are located on the same side of the double... 

Macromolecules having identical constitutional repeating units can nevertheless differ as a result of isomerism. For example, linear, branched, and crosslinked polymers of the same monomer are considered as structural isomers. Another type of structural isomerism occurs in the chain polymerization of vinyl or vinylidene monomers. Here, there are two possible orientations of the monomers when they add to the growing chain end. Therefore, two possible arrangements of the constitutional repeating units may occur ... 

Linear macromolecules having a constitutional repeating unit such as -CH2-CHX- (X H) show two further stereoisomerisms, i.e., optical isomerism and tacticity. The stereoisomerism named tacticity has its origin in the different spatial arrangements of the substituents X. When we arrange the carbon atoms of the polymer main chain in a planar zigzag conformation in the paper plane. 

The above considerations concerning structural isomerism and stereoisomerism are not restricted to homopolymers but can occur in copolymers as well. Here, moreover, structural isomerism can have its origin additionally in different distributions of two (or more) types of constitutional repeating units within the polymer chain. 

Numerous polymers autooxidize to form peroxides. These compositionally, and thus calorimetrically, ill-defined products may be considered polymeric peroxides. However, one well-defined polymeric peroxide is that of polystyrene with the repeat unit —CHa-CH(CeH5)-0-0-. Through a combination of combustion and reaction calorimetry (chain degradation to benzaldehyde and formaldehyde), a solid phase enthalpy of formation of this species was found to be 27 21 kJ mol . Much the same procedure was used to determine the enthalpy of degradation for the polyperoxide polymers of 2-vinylnaphthalene and the isomeric 1- and 2-propenylnaphthalene to form the related acylnaphthalene and formaldehyde. Numerically, the reaction enthalpy values for these last three polyperoxides were —206+4, —222 + 8 and —222 + 10 kJmol, to be compared with the aforementioned polystyrene with a value of —209 + 8 kJ mol. However, in the absence of enthalpy of formation data for the decomposition products in the naphthalene case, we hesitate to derive enthalpies of formation for these three species. ... 

The observed low Tg s of most polyphosphazenes are consistent with the low barrier to internal rotation predicted for them and indicate the potential these polymers have for elastomeric applications, Theoretical calculations, based on rotational isomeric models assuming localized it bonding, predict the lowest ( 100 cal per mol of repeating unit) known polymer barrier to rotation for the skeletal bonds of polydifluorophosphazene,... 

Previous work, (T-5) has shown that cationic isomerization polymerization of 3-methyl-1-butene produces a copolymer of two repeat units, the 1,2- i-CH2CH(iPrH-, and the 1,3- fCH2CH2C(CH3)2. Similarly, 4-methyl-l-pentene produces a terpolymer of 1,2-, 1,3-, and 1,4- -fCH2CH(iBu)-)-, -fCH2CH2CH(iPr)-f, and CH2CH2CH2C(CH3)2f, respectively. 

In this instance, Ziegler-Natta polymerization yields a soluble, linear polymer 2, containing a six-membered cyclic ring fused at each repeat unit. Unfortunately, this polymer undergoes isomerization to form a non-conjugated polymer, disrupting the electronic properties of the backbone [31]. It was found that this isomerization could be prevented by the introduction of heteroatom functionality into the diyne architecture, as exemplified by the polymerization of propiolic anhydride 3, which yielded a stable polymer 4 as shown in Scheme 11 [32]. 

Olefin isomerization has also been mediated by the photolysis of Fe(C0)s.144 Recently, a detailed study of alkene isomerization by photolysis of Fe(CO)5 has shown that the reaction is truly photocatalytic.14S The very high quantum yields ( 1.0), Table 24, and the fact that the pentenes are ultimately equilibrated to the thermodynamic ratio support the notion that the role of the light is to generate a thermally active catalyst. A mechanism similar to that in reactions (53)-(57) involving Fe(CO)3 as the repeating unit can be used to account for the results. 

By comparing the stability constants relative to different polymers, and models, it may be observed that they increase with the number of basic nitrogens present in the repeating unit. Furthermore, the stability constants of the Cu2+ complexes of the polymers of the first class are higher than those of the isomeric polymers of the second class. This fact, and the lower d-d energy band of the complexes of the second class, has been explained with the lack of partecipation in the latter of the C = O groups to the metal coordination l02). Viscosity measurements show that for the complexes with polymers of the first class, the viscosity monotonously decreases upon increasing the pH until the formation of the complex CuL is complete, and then remain nearly constant. On the contrary, in the case of the polymers of the second class, risp/c is... 

Stereoisomerism, on the other hand, arises primarily because of the two or more distinct ways in which adjacent repeat units containing asymmetric groups can be superimposed. The stereoregular isotactic and syndiotactic polymers are examples of this type of isomeric structures. The repeating structural unit in some polymers may exhibit both stereo and geometric types of isomerism. 

The enumeration and geometry of the possible stereoregular structures of polypropylene oxide illustrate the value of the above concepts of symmetry. Propylene oxide monomer has a truly asymmetric carbon, and the repeat unit in the polypropylene oxide chain can be either of the two optical isomeric... 

The isomerized structure dominates even at - 100° C, but accounts for only 70% of the repeat units at - 130° C. Similar but more complicated structures are formed in 4-methyl-1-butene polymerizations by competing hydride and methide shifts [298]. Other monomers whose propagating carbenium ions isomerize include 5-methyl-l-hexene, 4,4-dimethyl-1-pen-tene and some terpenes [299]. 

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