Monomer isomerization, substituted

Two isomeric acetylenic benzothiazole monomers, 2-(3-ethynyl-phenyl)-5-ethynylbenzothiazole (3) and 2-(3-ethynylphenyl)-6-ethynyl-benzothiazole (4) were prepared according to the general reaction scheme shown below. The synthesis of the benzothiazole heterocyclic structure was carried out by the condensation of m-bromobenzoic acid with isomeric bromo-substituted o-aminomercaptobenzenes in poly-phosphoric acid (PPA). The bis-bromobenzothiazoles were converted to the acetylene systems by the reaction with 2-methyl-3-but3m-2-ol and subsequent displacement of acetone with base. The bromo displacement reaction utilized a catalyst composed of triphenylphosphine, (bis-triphenylphosphine)palladium dichloride and cuprous iodide. 

The second type of isomerism we discuss in this section is stereo isomerism. Again we consider the number of ways a singly substituted vinyl monomer can add to a growing polymer chain ... 

A peptoid pentamer of five poro-substituted (S)-N-(l-phenylethyl)glycine monomers, which exhibits the characteristic a-helix-like CD spectrum described above, was further analyzed by 2D-NMR [42]. Although this pentamer has a dynamic structure and adopts a family of conformations in methanol solution, 50-60% of the population exists as a right-handed helical conformer, containing all cis-amide bonds (in agreement with modeling studies [3]), with about three residues per turn and a pitch of 6 A. Minor families of conformational isomers arise from cis/trans-amide bond isomerization. Since many peptoid sequences with chiral aromatic side chains share similar CD characteristics with this helical pentamer, the type of CD spectrum described above can be considered to be indicative of the formation of this class of peptoid helix in general. 

The isomeric bibenzoic acids (BBs), would appear to share similar structural features with naphthalene dicarboxylic acid. Like the PET-naphthalate copolymers, PET-bibenzoates have been demonstrated to possess moduli and glass transitions temperatures which increase with increasing levels of rigid comonomer [37-39], Unlike the PET/PEN copolymers, when the symmetrical 4,4 - I f I f monomer is substituted into a PET backbone, virtually every composition of PET-BB is semicrystalline the 2,4- and 3,4- isomers of BB, when... 

The present study reports the synthesis, characterization and thermal reactions of phenyl and carbomethoxy substituted norbornenyl imides. These substrates were designed to model the reactive end-caps of the PMR-15 resin and allow an assessment of the effect that conjugating substituents would have on the high temperature cure of such systems. The effect of these substituents on both monomer isomerization and polymerization is reported and a possible use of the phenyl substituent as a probe of polymer structure is suggested. 

While exo/endo isomerization of the substituted monomers had been found to be comparable to that of the parent monomer, polymerization of the substituted monomers was much faster. Under the conditions indicated above for <20% polymer formation in neat samples of PN or PX, both the phenyl and carbomethoxy monomers show >80% polymer formation. This greatly complicates any attempt to dissect isomerization from polymerization in these neat samples. Nevertheless, we were able to establish that, in the lower temperature (195 C) polymerization, the isomeric mixture undergoing polymerization was the same (for the carbomethoxy system) whether we started from CBN or from CVN (195 , 1 hr 71% VN, 23% VX, 6% BN + BX). A similar result was obtained by comparing the polymerization of < >VN and <)>BN. 

Having established the effect of substitution on the rates of both monomer isomerization and polymerization, we addressed the question of polymer structure. Specifically, are norbornenyl imide units incorporated into the fully cured polymer with their norbornyl rings intact If so, does the polymer also reflect the equilibrium ratio of exo and endo ring fused monomers For our parent monomers, PN and PX, this question has been unanswerable. We have not found any direct probe that allows an unambiguous assessment of specific substructures within the cured polymer. We do, however, have some evidence bearing on this question for the phenyl substituted monomer. This evidence is attributable in part to our discovery of an unexpected side-reaction in the cure of the phenyl substituted monomer, and in part to the presence of a unique NMR diagnostic for phenyl substituted, endo norbornyl N-phenyl imides. Both of these results are detailed below. 

Optical isomerism is possible whenever the substituents X contain centers of asymmetry polymers obtained from pure enantiomeric monomers are optically active. However, the specific rotation of the polymers is in general clearly different from that of the monomers. Optical isomerism is also possible when asymmetrically substituted carbon atoms are placed in the main chain (see Example 3-25). 

Willow-like cascade construction was facilitated by the preparation of the AB2-type building blocks, e.g., 6-bromo-l-(4-hydroxy-4 -biphenylyl)-2-(4-hydroxyphenyl)hexane (40 Scheme 6.12), 13-bromo-l-(4-hydroxyphenyl)-2-[4-(6-hydroxy-2-naphthalenylyl)-phenyljtridecane (41), and 13-bromo-l-(4-hydroxyphenyl)-2-(4-hydroxy-4"-p-terpheny-lyl)tridecane (42). Scheme 6.12 illustrates the monomer preparative strategy for the incorporation of the monomeric mesogenic moieties predicated on conformational (gauche versus anti) isomerism. Thus, one equivalent of 4-hydroxybiphenyl was allowed to react with 1,4-dibromobutane to yield the monobromide 43, which was converted under Finkelstein conditions to the corresponding iodide 44. Treatment of the latter with ketone 45 under phase transfer conditions afforded the a-substituted ketone 46. Reduc-... 

Unsubstituted polymer chains cannot form different stereo isomers, while substituted polymers can have a large number of different possible isomeric forms. As a result it is possible to have various configurations for substituted polymers. For example polystyrene produced by radical polymerization is atactic which means the phenyl groups bound to every second C-atom are randomly distributed on both sides of the polymer chain. Polymers produced using Ziegler catalysts, made from monomers like styrene, propene and others are isotactic (Figure 2-2) ... 

If the diene fragment of the 2-pyrone and the acetylene dienophile are unsymmetrically substituted, the formation of two isomeric benzene products is possible. The model reaction between 4,5,6-triphenyl-2-pyrone (20) (XXIII) and phenylacetylene yields approximately equal amounts of 1, 2, 3, 4-(XXV) and 1,2,3,5-tetraphenylbenzene (XXVI). Therefore, this position isomerism would be expected to materialize during polymer formation when similar bispyrone monomers are polymerized with diethylnylbenzene. 

Propenyl Ethers and Unsaturated Cyclic Ethers Propenyl ethers (CH3—CH=CH—OR R = ethyl, isobutyl, etc. cis- and trans-isomers) and 3,4-dihydrofuran are linear and cyclic a,/3-unsaturated ethers, that can be regarded as / -substituted vinyl ether derivatives. For these monomers a few controlled/living cationic polymerizations have been reported. The HI/I2 system is generally effective for both linear and cyclic monomers [181,182,183], whereas a recent study by Nuyken indicates that the IBVE-HI adduct coupled with nBu4NC104 is suited for 3,4-dihydrofuran (see Section V.A.4) [184]. A variety of mono- and bifunctional propenyl ethers can readily be prepared by the ruthenium complex-catalyzed isomerization of corresponding allyl ethers [185]. 

Cyclohexyl-substituted monomer 53 containing the cyclohexyl group exists in three isomeric forms in the presence of catalytic amounts of acid [Eq. (11)]. The polymer obtained from the cyclohexyl-containing monomer contains 52 exclusively as the structural unit [16]. In this case there is a high degree of steric hindrance in the transition state that would lead to the formation of 49. 

The properties of four-coordinate Si centers bonded to an alkylenedioxy substituent derived from a furanoidic diol also were investigated with the two isomeric oxolane-3,4-diols AnEryt and L-anhydrothreitol [92]. Thus, substitution of silicium with two phenyl residues leads to Ph2Si( AnErytH 2) 85 in the case of AnEiyt. The molecular structure is that of a monomer. The five-membered chelate ring is almost planar, with a diol torsion angle close to 0°. Such geometrical parameters cannot be met by L-AnThre. However, this diol provides another example... 

Ring-openTng polymerizations involve a variety of the ionic growing species. Moreover, some of the heterocyclic monomers may react ambidently and, therefore, produce chemically isomeric structures of active centers. Polymerization of lactones or polymerization of substituted a-oxides, both with two possible ways of ring-opening, are the typical examples. 

Anionic polymerization. For some heterocyclic monomers the unique chemical structure of the growing species follows unequivocally from the monomer structure. However, in many cases isomeric structures have to be taken into account. For instance, for symmetrical monomers, like thietane, the carbanion but not the thiolate anion was proposed (4). Unsymmetrically substituted monomers can provide active species by a- or B- ring scission. Unusual structure of activated monomer was proposed for NCA and lactams. These structures can not be distinguished by spectrophotometric methods, and application of H- or 13C-NMR looks more promising. 

This work focused on frans-cinnamic acid and some of its ring substituted derivatives. In homogeneous solution, the distribution of the photoproducts is dependent on the steric and electronic effects of the reagents. Irradiation of a melt or solution of frans-cinnamic acid does not cause dimerization, only isomerization (9,10). Dimers of frans-cinnamic acid can form in the solid state with retention of crystal symmetry because the monomers are held rigidly within the lattice in a uniform and repeating manner forcing an association between monomers. 

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