Steric hindrance, monomer synthesis

Hydrocarbon Polymers. It is difficult to produce perfluorocarbon polymers by the usual methods. Many monomers, such as hexafluoropropylene, polymerize only slowly because of the steric hindrance of fluorine. Furthermore, some monomers are not very stable and are difficult to synthesize. Direct fluorination can be used for the direct synthesis of fluorocarbon polymers (68—70) and for producing fluorocarbon coatings on the surfaces of hydrocarbon polymers (8,29,44—47,49,68—71). 

The synthetic procedure for the synthesis of the inverse starblock copolymers is given in Scheme 25. Diblock arms (I) having the living end at the PS chain end were prepared by anionic polymerization with sequential addition of monomers. In order to accelerate the crossover reaction from the PILi to the PSLi chain end a small quantity of THF was added prior the addition of styrene. The living diblock (I) solution was added dropwise to a stoichiometric amount of SiCl4 until two arms are linked to the silane. This step was monitored by SEC and is similar to a titration process. The end point of the titration was determined by the appearance of a small quantity ( 1%) of trimer in the SEC trace. The diblock (I) was selected over the diblock (II) due to the increased steric hindrance of the styryl anion over the isoprenyl anion, which makes easier the control of the incorporation of only two arms into the silane. 

Simpler procedures are of course available for the preparation and characterisation of carbenium ions in solution, particularly for the more stable ones. Concentrated sulphuric acid was extensively used as protogenic medium before the superacid mixtures were shown to be superior, but many of the spectroscopic assignements in those earlier studies were later proved erroneous, particularly in the case of such reactive entities as the 1-phenylethylium ion Model monomers which cannot polymerise because of steric hindrance can generate fairly stable carbenium ions by interacting with Lewis or Br nsted acids in normal cationic polymerisation conditions. Thus, 1,1-diphenylethylene and its dimer, and 1,1-diphenylpropene give rise to typical visible absorption bands from which the concentration of the corresponding diphenyl-methylium ions can be accurately calculated. As for carbenium ions capable of forming stable salts, their synthesis and characterisation is obviously easy. 

Arabino oligonucleotides up to a hexamer have been prepared by solid phase synthesis using (118) as the monomer the phosphoramidites (118) could be obtained rather pure without 2 -OH protection due to steric hindrance for phosphity-lation at this position. Deoxyoligonucleotides containing flexible nucleoside analogues have been synthesized using (119) or (120) as the modified monomers in both cases hybridization was greatly impeded. 

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

Linear poly(n-propylenimine) (PnPI) was first synthesized by the alkaline hydrolysis of poly(2-oxazine)s. This synthesis of poly(2-oxazine)s is analogous to the polymerization of 2-oxazolines, albeit the polymerization rate constants are approximately four times slower for the six-membered ring 2-oxazine monomers due to less ring tension and higher steric hindrance during polymerization. In theory, other PHAIs can also be prepared by hydrolysis of poly(cyclic imino ether)s, but this has not been reported so far. For example, poly( -butylenimine) may be synthesized by hydrolysis of poly(oxazepine)s, although such polymers are difficult to prepare, and poly(tert-butylenimine) may be synthesized by hydrolysis of the poly(cyclic imino ether) resulting from commercially available 2,5,5-trimethyl-2-oxazoline. 

In contrast to bis-SiMey-norbornene derivatives, disubstituted tricyclononenes turned out to be active monomers in AP [238, 240]. In tricyclononene molecule both MesSi-groups are moved by an additional one C-C bond away from the double bond and therefore from the reaction catalytic center. Synthesis of bis-MesSi-substituted tricyclononene was carried out from quadricyclane and fran5-l,2-bis(trichlorosilyl)ethylene. This route of synthesis provided formation of norbomene-type monomers with 100% exo-configuration of cyclobutane fragment that reduced steric hindrances in AP. That is why this monomer was active in AP catalyzed with common Ni- and Pd-catalyst systems. As a result, the formation of highly molecular weight polymer (Af up to 500,000 was observed [196]. 

Electrochemical polymerization of bi- and terthiophenes is a facile method for the synthesis of copolythiophenes due to their lower oxidation potentials relative to singlering monomers. Some chemical polymerizations of these monomers have also been carried out. These monomers yield well-defined alternating structures and provide a means to control the effective conjugation length of the resulting polymer. Since substituents are present in every other or every third ring, steric hindrance is decreased, resulting in increased planarity of the main chain. Tables 10.2 and 10.3 show some of alkyl-substituted bi- and terthiophenes that have been polymerized. 

---