Template Controlled Oligomerization Studies

Treatment of template 67 with 10 equivalents of MMA and 1.2 equivalents of Mo(CO)g in benzene at 70 °C furnished a 21% yield of material which appeared by H-NMR spectroscopy to consist of a mixture of diastereomeric template-bound MMA tetramers. Unfortunately, all attempts to confirm this molecular composition by mass spectral analysis were not fruitful, 

Finally, the ability of the silylacetate, fluoromethacrylate-bearing template 69 to enforce oligoselectivity on the group transfer polymerization of MMA was examined. However, no fluoride-initiated polymerization conditions could be identified which led to template-bound oligomer. Typically, off-template initiation (F -i-MMA ) furnished uncontrolled polymer. In these cases, the template 69 suffered simple desilylation to deliver the acetate 70. Perhaps a template-bound silyl ketene acetal initiator would have performed in a more desirable manner, but that species remained elusive. 

This curious observation refocuses concerns about yield loss to a critical choice that a template-bound pMMA radical 99 faces macrocyclization to afford the controlled- 

The yield of macrocycle 94 was monitored as a function of the amount of 106 or 107 added in both the optimum solvent benzene and, independently, in a solvent chosen to resemble the growing pMMA chain, methyl isobutyrate. 

These four experiments were designed to test two related hypotheses (1) in an actual template system, bimolecular termination is suppressed as a consequence of the demanding steric environment about the terminator moiety. Observation of greater macrocycle yield with the bulkier terminator 107 present compared with the smaller analogue 106 in either solvent, given that these species have equivalent intrinsic reactivity with a small radical, would support this premise. (2) More pronounced solvent-solute interactions in benzene as compared to non-aromatic solvents contribute to favoring intramolecular termination (— 94) over bimolecular monomer addition 100) for radical 99. Observation of rela- 

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Exploration of the template controlled free-radical oligomerization of other activated olefins began with standard monomers utilized in bulk polymer synthesis and the template 63. Vinyl acetate and acrylonitrile led only to uncontrolled polymerization, while vinylene carbonate did not react under the standard experimental conditions. More exotic monomers, such as vinyl trifluoroacetate and rert-butyl acrylate, were also unsuccessful. Only methyl acrylate polymerization was arrested by template 64 to provide the macrocyclized product 96 in modest yield as a mixture of five diastereomers (Scheme 8-25). Subsequent studies with the more effective thiophenyl-bearing template 63 at lower temperatures improved this yield to 35%. The diastereomer distribution was reminiscent of the methyl methacrylate-derived product, although no stereochemical assignments were made in this case either. 

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