Contents 12 Steric factors

Numerous studies have probed how novolac microstmcture influences resist hthographic properties. In one example, a series of resists were formulated from novolacs prepared with varying feed ratios ofpara-jmeta-cmso. These researchers found that the dissolution rate decreased, and the resist contrast increased, as thepara-jmeta-cmso feed ratio increased (33). Condensation can only occur at the ortho position ofpara-cmso but can occur at both the ortho- and i ra-positions of meta-cmso. It is beheved that increased steric factors and chain rigidity that accompany increasedpara-cmso content modify the polymer solubihty. 

The effect of the nature of the substituent at the acetylene bond is not so noticeable. Substitution reduces the C-3 activity due to polarization effects and steric factors. As aresult, in the cyclization with hydrazines and hydroxylamines an increase in the content of 5-substituted pyrazoles and isoxazoles is observed (81UK1252). As mentioned above, nonsymmetiic nitrogen-containing binucleophiles H2N—YH (Y = O, NMe, NPh) react with l-heteroalk-l-en-3-ynes in two alternative pathways by functions H2N and YH. 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

The overall bias in polymers of low or intermediate cis content may reflect to some extent the polarity of the Mt=C bond, though steric factors may also be involved. If the carbene carbon has cationic character the polymerization of 14 should propagate mainly through Pjj rather than Px in accordance with the C2+2il interaction illustrated in Equation 1. 

Generally, monomer molecules can adsorb onto AC-1 with their other more bulky C = C-groups, however, both calculations and numerical experiment show that chain growth on AC-1 is not caused by the steric factor. For example, the dependence of AC-1 stereospecificity and r(3ws-l,4-polydiene yield on the chemical structure of monomers can be observed experimentally. The more bulky substituent is in the 1,3-diene molecule, the lower the stereospecific polymerisation rate, the higher the content of r(3ns-l,4-isomers of the respective polydienes. 

Comparison of dimensions of the PVB chains with different hydroxyl contents requires measurement of the chain dimensions under 6 conditions where A2 vanishes and the chain assumes its unperturbed dimensions. The Matsuda and Inagaki calculation of freely rotating chain dimensions assumes that excluded volume effects are absent, and that there is no hindrance to rotation about C-C bonds. The ratio of the experimental unperturbed dimensions to the calculated freely rotating dimensions is the steric factor o, a characteristic parameter for a given polymer which is dependent on the hindrance to rotation. Although we have not directly measured the chain dimensions even in good solvents, estimation of the unperturbed dimensions can be made with several theories developed for use with intrinsic viscosities in good solvents, These include the theory of Berry 13... 

Finally, the new environmental regulations limiting the sulfur content of gas oil used for vehicle transportation involves a new and increasing effort to understand the mechanisms of desulfurization of the most refractive compounds. These molecules are mainly DBTs with alkyl groups near the sulfur atom (fourth and sixth positions) (160). The reasons for their low HDS reactivity is still intensively discussed and two hypotheses have been proposed. The first proposal suggests that the transformation of 4-alkyl- and 4,6-dialkyl-DBT is limited by the adsorption step by the sulfur atom, which would be sterically hindered by the presence of the alkyl groups (161). The second hypothesis supposes a fiat adsorption of the DBT compounds on the catalyst (162). In this case, sterical factors do not hinder the first step of adsorption on the catalyst but more an intermediate step of elimination. 

When the adsorption from ethanol, n-propanol, and n-butanol + benzene mixtures is compared the adsorption of alcohol decreases with increase in molecular weight both on untreated and HDP-content clay this is attributed to steric factors. 

The uranium content of the complexed PANa is fairly constant (about 46 uranium). Based on a product composed of units of form 1, this would require about 85 of the carboxylate groups to be chelated. The constancy of the amount of iiranium found in the products synthesized over a wide variety of reaction conditions may be due to control by steric factors on the cross-... 

Structure and is defined by not only electron bnt also steric factors. With branching of the radical at the oxime function, the content of 4-methyl-snbstitnted pyrrole in the mixtnre increases. It is clearly seen if to compare the resnlts obtained for acetoxime, methyl isopropyl ketoxime and pinacoline oxime. Bulkier oximate anion, likely dne to the steric reasons, preferably attacks the C atom of methylacetylene. At the same time, cyclohexanone and 2-acetylfuran oximes react with the propyne in a regiose-lective manner to afford 5-methylpyrrole only. With oximes of ethyl methyl ketone, acetophenone, and 2-acetylthiophene, the yields of 5-methyl-snbstitnted pyrroles range 90%-92%. 

In most cases we can assume that the equilibrium distribution and partitioning of organic chemicals in both mammalian and nonmammalian systems is a function of lipid content in the animal and that the lipid-water partition coefficient (AW) is equal to K0w Instances where this is not the case include specific binding sites (e.g., kepone in the liver) and nonequilibrium conditions caused by slow elimination rates of higher level organisms or structured lipid phases that sterically hinder accumulation of very hydrophobic chemicals. For aquatic organisms in constant contact with water, the bioconcentration factor or fish-water partition coefficient (AW) s simply ... 

---