Terminal Group Effects

Whereas in compounds with the general formula based on structure (23)  

In the case of compounds with the general structure typified by (24)  

It is clear that no consistent order exists for the effectiveness of the groupings and careful consideration has to be given to all the possible influences on the force field in predicting the nature of a particular system  

---

The designation of chain configuration in terms of d and l units we consider to be unsatisfactory for two reasons a) it is not operational, i.e., it draws a distinction between enantiomers which is not reflected in optical properties, terminal group effects being essentially nil in medium to high polymers and b) it is not convenient, because it fails to make evident the experimentally observable classes of polymer structure. 

Hittle, L. R., Altland, D. E., Proctor, A., and Hercules, D. M. Investigation of molecular weight and terminal group effects on the time-of-flight secondary ion mass spectra of polyglycols. Anal. Chem., 66, 2302, 1994. 

Thus, when proposing the chemical mechanism, we will separate the perturbation effects into polymer chain (or backbone) and terminal group effects. As we could see the effects caused by polymer backbone reactions are most important at low polymer concentrations but when the PEG concentration is high terminal group reactions dominate. 

M. Nakayama and T. Okano, Polymer terminal group effects on properties of thermoresponsive polymeric miceUes with controlled outer-shell chain lengths. Biomacromolecules, 6,2320-2327 (2005). 

Specialized orbitals tailored for analysis of phenomena such as bonding in molecular clusters and electron excitations are obtained by maximization of suitably chosen functionals. The so-called pseudo-Wannier orbitals are produced by a procedure that maximizes similarities between one-electron wavefunc-tions localized within analogous units of a given molecular cluster. These orbitals reveal terminal-group effects in linear polymers and provide systematic schemes for partitioning of the total energy and one-electron properties of finite clusters. 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

The first set of dyes, so called visible set , is presented by polymethine dye PD 2630, squaraine dye SD 2243, and tetraone dye TD 2765, all with benzo[e]indolium terminal groups. The second set of dyes, so called NIR set , is presented by polymethine dye PD 2658, squaraine dye SD 2878, and tetraone dye TD 2824, all with 5-butyl-7,8-dihydrobenzo rJ furo[2,3-/]indolium terminal groups. A distinguishing feature seen from this figure is a remarkably large, 300 nm, red shift of the absorption bands for PD 2658 and SD 2878 as compared to PD 2630 and SD 2243. The absorption spectrum of TD 2824 is red-shifted by 200 nm as compared to TD 2765. Thus, the effect of the 5-butyl-7,8-dihydrobenzo[coT]furo[2, 3-/]indo-lium terminal groups is equivalent to the extension of the chain to three vinylene groups. 

On well characterised non-stabilized PP samples [48] having molar mass within 45-180 kg/mol with differing tacticity and crystallinity, we can see that the increasing molar mass leads to an increase of induction time and reduction of the maximum chemiluminescence intensity (Figure 14). The polymer with higher average molar mass appears to be more stable than that with lower molar mass. This may be ascribed to the effect of increased concentration of more reactive terminal groups, which promote initiation of thermal oxidation. 

Systematic variation in chirality at both the chelate backbone and the terminal groups revealed a remarkable effect on the enantioselectivity of the catalysts. Ligand (109) generates chiral cooperativity between the backbone and the terminal moieties in Pt-catalyzed hydroformylation. The highest ee (65%) for 2-phenylpropanal was found for the ligand R-bis(S)-(110) in combination with Pt. The chemoselectivities with all ligands described in association with Pt were rather low. The comparative... 

Anderson, P.W., Pichichero, M.E., Stein, E.C., Porcelli, S., Betts, R.F., Connuck, D.M., Korones, D., Insel, R.A., Zahradnik, J.M., and Eby, R. (1989) Effect of oligosaccharide chain length, exposed terminal group, and hapten loading on the antibody response of human adults and infants to vaccines consisting of Haemophilus influenzae type b capsular antigen uniterminally coupled to the diphtheria protein CRMI97./. Immunol. 142, 2464-2468. 

---