Polymers steric effect

Fig. 31 Overall interaction energy between two DNA-coated colloids, (a) Sketch of the interacting surfaces of two spheres of radius R0 separated by d. The maximum length of hybridized strands is 2L. (b) Total interaction energy as a function of d. It is the sum of the attractive I/DNA from the binding of accessible DNA strands, the repulsive I/rep from electrostatics and/or polymer steric effect, and the van der Waals attraction t/vdw. (c) For weak, short-range I/rep, particles which are unbound at high temperatures are irreversibly trapped in the van der Waals well after DNA hybridization at low temperatures, (d) For strong, medium-range I/rep, DNA binding produces a secondary minimum of reversible aggregation. Reproduced with permission from [138]...

Optically active monomers 108 and 109 with a stereocenter located relatively far from the isocyanide moiety give single-handed helical polymers. " Steric effect between the relatively stiff spacer moieties connecting the isocyanide moiety and the stereocenter has been considered to be responsible for the long-range chirality transfer from the side chain to the polymer backbone. 

In mechanistic studies, the use of optically active reagents seems to be a rather elegant and useful tool. It allows to follow the stereocontrolled reactions simply by looking on optical activity of the monomer or the polymer. Steric effects are thus substantially amplified, but still many chiral recognitions or interactions are not yet completely understood. 

The addition of functional groups to the pyrrole monomer influences the electronic properties of the resultant polymer. Steric effects introduced by bulky functional groups, for example, decrease the conductivity, particularly when present as N-substituents. The presence of substituents on the 3-position of the pyrrole ring has less effect, but still results in a decrease in conductivity (see Table 3.3). 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The steric effects in isocyanates are best demonstrated by the formation of flexible foams from TDI. In the 2,4-isomer (4), the initial reaction occurs at the nonhindered isocyanate group in the 4-position. The unsymmetrically substituted ureas formed in the subsequent reaction with water are more soluble in the developing polymer matrix. Low density flexible foams are not readily produced from MDI or PMDI enrichment of PMDI with the 2,4 -isomer of MDI (5) affords a steric environment similar to the one in TDI, which allows the production of low density flexible foams that have good physical properties. The use of high performance polyols based on a copolymer polyol allows production of high resiHency (HR) slabstock foam from either TDI or MDI (2). 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

In several cases, the experimental plots were indeed linear. According to a molecular theory,11 39 40 the slope Hq and the intercept Sq are related to the chiral dispersion forces and to the entropic gain obtained by a twisted relationship between the polymer molecules (Figure 7.4), respectively. The value of Sq reflects the fact that the helices take up less space when packed in a helical arrangement and should be related to the hard-core steric contribution to the twist. Positive or negative values of Sq should be related to a right- or left-handed twist between the polymer molecules, respectively, and should be determined solely by steric effects. 

Assuming that all B groups have the same reactivity, the chemical reaction giving rise to a branched molecule is identical to the reaction resulting in a linear polymer. Statistically this will eventually result in a hyperbranched polymer. However, dependent on the chemical structure of the monomer, steric effects might favor the growth of linear polymers. Computer simulations of of ABX-monomer condensation and AB -monomers co-condensed with B-functional... 

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