Poly isomeric forms

Variations in the proportions of the different components of product mixtures are observed in reactions that involve anhydrous HF31-80-82-84-85 and in pyridinium poly(hydrogen fluoride).86 These variations can also be explained in terms of kinetic and thermodynamic control. Thus, less stable, but more rapidly formed, dianhydrides isomerize under thermodynamic conditions to give more-stable products. It has also been noted that the starting isomeric forms of the ketose influence the kinetic outcome of the reaction.119... 

Poly(lactide) can be synthesized from two optically isomeric forms of lactic acid L-lactic acid... 

The chromatographic conditions specified produced a chromatogram in which, for Uni-dyme-18-dimer acid, there are four broad peaks corresponding to neutrals, monobasic, dibasic, and poly basic components. Each peak represents the elution of many isomers for example, the dibasic peak is composed of linear, cyclic, and aromatic dimer, each in various geometric, structural, and conformational isomeric forms (the peaks are very broad as a result of the presence of these isomers). 

The ester models, as stated above, were all LC, but two of the ether model compounds, the methoxy- and ethoxy-substituted derivatives, were not, despite the fact that each of these mesogens yielded some liquid-crystallinity in the poly(ether) form. Therefore it seems that the polymers in this system tend to be "more liquid crystalline" than the related small molecules. This hypothesis is supported by the fact that Memeger(H) found liquid crystallinity in allhydrocarbon polymers incorporating the distyrylbenzene mesogen, even in cases where the cis/trans ratio of the unsaturations was as large as 0.3, while Campbell and McDonald (10) noted that iodine isomerization to the all-trans form was essential for the observation of an LC phase in the small-molecule derivatives which they prepared. 

Such differences in structure can have a profound effect on the physical properties of a polymer. Thus natural rubber, which comprises cis-1,4-poly(isoprene), is a soft rubbery material at room temperature, whereas guttapercha, which comprises the corresponding ftms-isomer, is semi-crystalline and hard. The method of polymerisation determines the isomeric form of the polymer. 

Each of the different groups of simple sugars, from tri-oses to non-oses, exists in two structurally isomeric forms (a) that of a polyhydroxy aldehyde, and (6) that of a poly-hydroxy ketone. To dis-... 

We can easily calculate that, due to different arrangements of the four units, there are eight possible isomeric tetra-peptides containing these same four amino acids of the same stereo-isomeric forms. One of these eight isomers may prove to be actually identical with the hydrolytic tetra-peptide but it may be necessary to wait until these eight isomers have all been synthesized before we can state positively that a synthetic poly-peptide is identical with a poly-peptide obtained by the hydrolysis of a protein. The probability is that one of these possible synthetic compounds will be found to be identical with the hydrolytic. When this is done we shall have proven that proteins are poly-peptide combinations... 

Isomerism.— As all of the second group, in which substitution occurs in the side chain, considered as benzene derivatives, are mono-substituted benzenes, they do not exist in isomeric forms, and only one compound of each formula is known. The first group, however, in which halogen substitution occurs in the ring, are all poly-substitution products of benzene, since toluene itself is a mono-substituted benzene. Mono-chlor toluene is, therefore, a di-substituted benzene, and occurs in the three forms, as follows ... 

The substituted phenols result from the substitution of an additional element or group in the original benzene ring. Derivatives of only the mono-phenols will be mentioned. Considered as benzene derivatives these compounds will be poly-substitution products and, therefore, possible of existence in isomeric forms. We thus shall have ... 

Reaction of dineopentylosmium dioxide with 2 equiv. of Ta(CHR)(CH2R)3 was reported to result in the formation of osmium dineopentylbisneopentylidene [269, 270]. Interconversion of the two isomeric forms [syn-anti and anti-anti) of this compound was postulated to proceed via an osmium tris(neopentyl)neo-pentylidene. So far, no reports on the ROMP activity of these complexes have been given. Similar to OsCls [271] and (p-cyrnene)OsCl2P(c-H,.) j [272], (/z-H)20s3(CO)io was reported to be active in the ROMP of NBE to yield all cis-poly-NBE [273]. 

MM calculations with full geometry optimization have been carried out to calculate the molecular structure and conformational energies od, ..modeJ -.cpmpounds for poly (phenylmethylsilylene) [-SiPhMe-] and poly(silastyrene) [-SiPhH-SiH2-]. In each case the calculations considered both the isotactic and syndiotactic stereochemical isomeric forms of the model compounds. The structures considered are illustrated in Figures 6 and 7. 

Lactide is the cycUc dimer of lactic acid, which exists as two optical active isomers d and l. L-Lactide is the naturally occurring isomer, while OL-lactide is the synthetic blend of D-lactide and L-lactide. The polymerization of these monomers leads to either a semicrystalline polymer or an amorphous polymer. Poly(L-lactide) (PLLA), for example, is a semicrystaUine polymer with a degree of crystallinity around 37%. It has a glass transition temperature of 60-65 C and a melting temperature of approximately 175 °C. Conversely, poly(DL-lactide) (DLPLA) is an amorphous polymer with random distribution of both isomeric forms of... 

In the 1970s, DuPont commercialized a silk-like Qiana fiber based on poly(bis[4-aminocy-clohexyljmethane dodecaneamide) or PACM,12 [87]. The base polymer is synthesized from bis(4-aminocyclohexyl)methane and dodecanedioic acid. The diamine exists in three isomeric forms trans-trans, cis-trans, and cis-cis. Its content of trans-trans isomer can be controlled by specific catalysts during its preparation. A normal diamine intermediate contained 51% trans-trans, 40% cis-trans, and 9% cis-cis isomers [88,89]. For Qiana fiber, the trans-trans content was about 70%. The polymer had a melt temperature of 290°C. 

Poly(lactic acid) has three typical optical isomeric forms (i) optically active and crystalliz-able poly(L-lactide) (i.e. poly(L-lactic acid) (PLLA)), (ii) optically active and crystallizable poly(D-lactide) (i.e. poly(D-lactic acid) (PDLA)), and (iii) optically inactive and noncrystal-lizable poly(DL-lactide) [i.e. poly(DL-lactic acid) (PDLLA)]. Of these isomeric polymers, PLLA is most frequently used because its production cost is lower due to its joint mass production of 1.4 X 10 metric tons per year by NatureWorks LLC, which is owned by Cargil... 

The great influence exerted by the structure of macromolecular compounds on properties, in contrast to low-molecular-weight structures, becomes obvious in a comparison of the two 1,2-dimethylethylenes with the l,4-poly(butadienes). The melting point of the trans-butene-2 is about 34°C higher than that of the cis-butene-2, while the boiling points Tbp only differ by about 3°C. While butene-2 shows two isomers, poly-(butadiene) can occur in five isomeric forms. 

Poly(bis-[p-aminocyclohexyl]methane-co-dodecanedioic acid), PACM-12 [24936-74-1], introduced by DuPont as Qiana in 1968 was later withdrawn from the market. This diamine exists in several cis-trans and trans-trans isomeric forms that influence fiber properties, such as shrinkage and tenacity. The product offered silk-like hand and luster, dimensional stability, and wrinkle resistance similar to polyester. The yam melted at 280°C, had a high wet glass-transition temperature of 85°C, and a density of 1.03 g/cm the last was lower than that of nylon-6 and nylon-6,6. Qiana required a carrier for elFective dyeing. 

Of the three possible isomeric forms, poly(L-lactic acid) and pofy(D-lactic acid) are both semicrystalline in nature, and pofy(mesolactic acid) or poly(D,L-lactic acid) is amorphous. Racemic PLA - synthesized from petrochemicals - is atactic, i.e. it exhibits no stereochemical regularity of strueture, is highfy amorphous and has a low glass transition temperature. Amorphous grades of PLA ate transparenL... 

Two types of cteviation from the first-orda kinetics are noted for photo-aialthamal isomerization reactions in polymer films. The first is the normal type, in which the reaction rate is the same as or smaller than that in solutions at the initial stage and then progressively becomes smaller. Typical examples are thermal decoloration of the photocolored merocyanine form of spirobenzopyran molecularly dispersed in or chemically bound to a polymer matrix > and photoisomerization of the transazobenzene residue incorporated in polymer main chains The first interpretation for the decoloration of the merocyanine form assumed the existence of different isomers, each of which fades independently following first-order kinetics On the other hand, Kryszewski et al. proposed the kinetic matrix effect, which means that the distribution of free volume may lead to the deviation from first-order kinetics. His idea was based on the finding that deviations from first-order kinetics can be observol even in simple molecules such as azobenzene which has only one trans or cis isomeric form. The effect of free volume distribution on reactivity was further demonstrated by studies of annealed polymer films The distribution function of free volume as well as the critical free volume v were estimated for the merocyanine form of spiropyran in poly(methyl methacrylate) derivatives of azobenzene in polystyrene and azobenzene in polycarbonate The deviation from first-order kinetics was also observed in cyclizing imidization of model poly(amic acid) in a polyamide matrix... 

Poly(acetylene) (PAC) exists in various isomeric forms ... 

Polylactic acid (PLA) is the world s most popular synthetic biodegradable polymer and has a widespread use in the biomedical field. It maybe obtained directly from lactic acid by condensation polymerization or, more commonly, by ring-opening polymerization from the cyclic dimer of lactic acid lactide. Lactide is a chiral molecule that exists in three isomeric forms D(-), L(+) and racemic (D,L) lactide. Consequently, the polymerization of this monomer can lead to the formation of three different forms of polylactide poly-L-lactic acid (PLLA), poly-D-lactic acid (PDLA), and poly-D,L-lactic acid (PDLLA). The general representation of the chemical structure of PLA is presented in Figure 16.10. 

Poly(lactic add) is present in three isomeric forms D(—), L(- -) and racemic (D,L). and the polymers are usually abbreviated to indicate the chirahty. Poly(L)LA and poly(D)LA are semi-crystalline solids, with similar rates of hydrolytic degradation as... 

An alternative polymer synthesis has been carried out in our laboratory (isomeric forms 33 and 34) (Scheme 11) using tetramethyldisiloxane (TMDS) and DVB. The special feature of the new polymer is that there are no weak Si-O-C linkages, which generally imdergo degradation resulting in the formation of Si-OH bonds. Scheme 12 is a CAChe representation of a poly(tetramethyldisiloxane-divinylbenzene) model segment. 

Polylactic acid (PLA) (Fig. 1.12) is produced by ring opening of the lactide, a diester of lactic acid that can be obtained from oil even though the natural origin is now largely known. PLA exists in three isomeric forms d(-), 1(+) and racimic (d,l). Poly(l)LA and poly(d)LA are semi-crystalline solids having hydrolytic degradation rates similar to PGA. PLA is more hydrophobic and more resistant to hydrolysis than PGA. The (1) isomer of lactic acid (LA) is preferentially chosen because it is better metabolised in the body. 

Dediazoniation of three o-substituted benzenediazonium salts in pyridinium poly(hydrogen fluoride) yields different products depending on the substituent, as Olah and Welch (1975) have found. The 2-methyl derivative gives 2-fluoro-toluene. With the 2-nitrobenzenediazonium ion the main product is 3-nitrofluoro-benzene, the 2-isomer being formed only in small quantities. Finally, the 2-tri-fluoromethyl derivative yields all three isomeric trifluoromethylfluorobenzenes. 

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