Diad effect

It is striking that, despite its small size, the tetrad effect was discovered before the diad effect. This is because the diad effect occurs in d-electron systems and is therefore masked by the orbital stabilization energies produced by the stronger ligand field. 

Such configurational as well as conformational effects have been also reported by MILLAN et al. in the case of nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate (14) and with sodium isooctylthioglycolate or isooctylthiosalicylate (15). The authors have shown that these reactions proceed selectively on the isotactic TT diads which can only exist either in the GTTG isotactic or in the TTTG heterotactic triads, the former ones being much more reactive than the latter ones. 

Since the (n-Bu)3Sn radical (R ) is nucleophilic (17), a partial negative charge must be produced at the methine carbon whose chlorine is being abstracted. The rate of this abstraction should clearly be enhanced by electron-withdrawing groups on R due to their stabilization of this charge by inductive effects. As observed, the removal of Cl from EW (or W diad) is expected to be more facile than from EVE (or VE diad) as a result of a -halogen effect in the former structure. 

Thus, the enhancements in chlorine removal from W diads compared to EV diads and from m-W diads compared to r-W diads observed in the (n-Bu)3SnH reduction of DCP, TCH, and PVC are consistent with the free-radical chain reaction mechanism. Inductive effects produced by neighboring 7-Cl s tend to favor the reduction of W diads relative to EV diads and steric interactions resulting from different preferred conformations in each isomer favor the removal of Cl from m-W diads relative to r-W diads. 

There has been some question as to the effect on the microstructure of annealing the 73/27 HBA/ HNA copolymer at a temperature 70 DC below Tcn-As shown in Fig. 15, distinct changes can be detected in the DSC scan as a result of annealing. However, examination of the two specimens by 13CNMR shows no changes in the diad sequences (see Fig. 16). 

The influence of the chain expansion produced by excluded volume on the mean-square optical anisotropy is studied in six types of polymers (PE, PVC, PVB, PS, polylp-chlorostyrene), polylp-bromostyrenel. RIS models are used for the configuration statistics of the unperturbed chains. The mean-square optical anisotropy of PE is found to be insensitive to excluded volume. The mean-square optical anisotropy of the five other polymers, on the other hand, is sensitive to the imposition of the excluded volume if the stereochemical composition is exclusively racemic. Much smaller effects are seen in meso chains and in chains with Bernoullian statistics and an equal probability for meso and racemic diads. 

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

They correspond to a first-order Markov process for the stereocontrol— i.e., a penultimate effect of the last diad on stereocontrol. 

Cyclopolymerization of bifunctional monomers is an effective method of chirality induction. Optically active vinyl homopolymers and copolymers have been synthesized by using optically active distyrenic monomers (41) based on a readily removable chiral template moiety. Free-radical copolymerization of 41a with styrene and removal of the chiral template moiety from the obtained copolymer led to polystyrene, which showed optical activity ([Oc]365 -0.5-3.5°) (Scheme 11.6) [84], The optical activity was explained in terms of chiral (S,S)-diad units generated in the polymer chain through cyclopolymerization of 41a [85], Several different bifunctional monomers have been synthesized and used for this type of copolymerization [86-90]. 

The stereochemistry of GTP of MMA polymerization was measured for Lewis acid as well as for bifluoride catalysis. Lewis acid catalysts gave a ratio of syndiotactic heterotactic triads of 2 1 while bifluoride catalysis gave ratios near 1 1 [9, 41]. The amount of isotactic triads was about 5%. The effect of temperature on triad and diad composition provided data to calculate the difference in activation enthalpy (AAH ) and activation entropy (AAS ) for... 

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

BD/St-copolymers were also prepared by the use of the catalyst system Nd(OCOCCl3)3/TIBA/DEAC [503]. The copolymer exhibited 79% cis-1,4-structure in the BD units at a content of incorporated styrene of 23 mol%. In this study diades were also determined. According to the authors BD moieties which are adjacent to St moieties predominantly exhibit a transit-configuration whereas BD moieties in BD-BD-diades exhibit a cis-1,4-structure [503]. It therefore can be concluded that the microstructure of an entering BD monomer is controlled by a penultimate effect. This effect can be best described by a model in which backbiting coordination of a penultimate BD unit to Nd is involved [177,367]. 

The dependence the polymer microstructure on the propylene concentration in the reactor is shown in Table XVI. By varying the monomer concentration in operation with the supported catalyst, one can obtain polypropylenes with relatively low concentration of rrrr pentads. At low propylene concentrations, it is possible that m insertions are formed by epimerization. The yield of these isolated m diads is significantly increased when the catalyst is supported. Similar effects were shown by catalysts producing isotactic polymer. 

On the other hand presence of B -N dyad in the polymer could only arise from a transesterification reaction where the B was inserted between the -BN-. As shown in Figure 3, NMR displays a small but distinct peak (ca.14%) at the resonance position corresponding to B -N diad. Since the polymerization was run at the same temperature of 245°C forl70 min and to the same degree of polymerization (M n 2252) as the earlier experiment on the reactivity ratios, one can conclude that the role of interchain transesterification is relatively small and that the monomers have approximately equivalent reactivity ratios. The reaction of B with the HBA-HNA dimer was also examined at 225° and 285°C to determine the possibility of a temperature effect The times of reaction were 20 hrs and one hour, respectively,... 

Excimer formation and its potential applications for polymers continues to be widely studied. Poly(N-vinylcarbazole) continues to be investigated with the monomer exhibiting yet again a new emission below 400 nm assigned to a second excimer state similar to that seen in the homopolymer. Poly(3,6-di-tert-butyl-9-vinylcarbazole) has been found to give unique sandwich excimer formation due to syndiotactic diads while the effect of hydrostatic pressure on intramolecular excimer formation for meso-2,4-di-N-carbazolylpentane dissolved in poly(propylene oxide) has indicated that the compressibility of free volume cannot be considered to be constant. A new scheme has been proposed to... 

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