Hydrogenation stereoregular

Molecular alignment in the monomer crystals is controlled by several intermolecular interactions, such as strong and weak hydrogen bonds, leading to the formation of various types of stereoregular polymers via a topochemical polymerization process. This approach to the stereocontrol of polymers differs from other conventional ways in the control of the propagating chain end using catalysts or additives in solution polymerization. 

The 100 MHz H-NMR spectrum of polypropylene oxide is too complex to be analyzed accurately even when decoupled from methyl protons. It is reasonable to ascribe the complexity of the spectrum to the spin coupling between hydrogen atoms linked to contiguous carbon atoms in the main chain. If this interpretation is correct, the spectrum should be simplified by substituting the methine proton by a deuterium atom. In fact, the spectrum of polypropylene oxide-a-d was simpler than that of the undeuterated one. Methyl protons lie at higher field and methylene protons at lower field. The stereoregularity was analyzed on the basis of the spectrum of the methylene proton absorption other than that of the methyl proton absorption. 

I. R. spectra of polymers of optically active and racemic monomers (12) having similar stereoregularity are identical in the case of poly-5-methyl-l-heptene, but slightly different in the case of poly-3-methyl-l-pentene and poly-4-methyl- 1-hexene. A very characteristic crystallinity band has been found in the I. R. spectrum of poly-5-methyl-l-heptene at 12.06 fi bands which seem connected with stereoregularity have been found in the I. R, spectrum of poly-4-methyl-l-hexene at 10.06 fi the nature of these bands has been proved when preparing a practically atactic sample by hydrogenation of poly-4-methyl-l-hexyne (24). 

The temperature-dependent stereoregular MAO-promoted polymerization of a-olefins502 has been explained by /i-hydrogen interactions in the olefin insertion and formation of a six-membered — 0 —Cp—D — Al—O—A1 ring TS. The stereoselective isotactic product formation occurs as a result of the substituent orientation at the -carbon (R1 vs CH2CHR2R4 in the conformationally restricted 426 equation 255). 

Stereoregularity has relatively little effect on the mechanical properties of amorphous vinyl polymers in which the chiral carbons are trisubstituted. Some differences are noted, however, with polymers in which X and Y in 4-14 differ and neither is hydrogen. Poly(methyl methacrylate) (Fig. 1-4) is an example of the... 

In addition to having a specific orientation of the monomer adsorbed on the catalyst surface or complexed with a catalytic site, a stereoregular polymerization requires orientation of the monomer molecule with respect to the growing polymer end and restricted rotation around the C1-C2 axis in the metal-Ci-C2 sequence. Furukawa and Tsuruta (2) have proposed that rotation is restricted by the formation of a bond between a hydrogen atom attached to C2 and a metal atom on the surface. 

For propylene polymerization, it has been demonstrated that the chain transfer is dependent not only on regio -but also on stereo -selectivity.412 This is in keeping with the tendency that, with catalyst systems of the type MgC /TiCU/phthalate ester-AlR3-alkoxysilane, the silanes which give the most stereoregular isotactic polymer also give relatively low hydrogen response. 

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