Alternating copolymers boron

Lewis acids have long been used in both polymerizations and copolymerizations to enhance the reactivities of monomers. The addition of ZnC, alkyl aluminum compounds, or boron halides has been shown to increase both the rate and degree of polymerization of monomers such as acrylonitrile and methyl methacrylate [8]. The use of Lewis acids to enhance electrophilicity of acrylate monomers has also been exploited to enhance alternation in copolymerizations with electron-rich alkenes such as isobutylene [9], Systems that would never produce alternating copolymers can be induced to do so with as little as 0.1 equivalents of an appropriate Lewis acid. This section focusses on efforts to utilize Lewis acids to both alter reactivity and control stereochemistry in copolymerizations. 

FIGURE 47.15. The SCP route to fluorene-based alternating copolymers tetrakis(triphenylphosphine)palladium mediated condensation 9,9-dialkylfluorene-2,7-bis(trimethylene boronate) and various dibromoarenes. 

Bamford C. Alternating copolymerization in the presence of Lewis acids. In Cowie JMG, ed. Alternating Copolymers. New York Springer US 1985 75—152. Hirai H, Takeuchi K, Komiyama M. Polymerization of coordinated monomers. 19. Kinetic study of the alternating copolymerization of methyl-methacrylate with styrene by boron trichloride. J Polym Sci Part A Polym Chem. 1982 20 159-172. 

The above mentioned polymer-supported oxazaborolidines are prepared from polymeric amino alcohols and borane. Another preparation of polymer-supported oxazaborolidines is based on the reaction of polymeric boronic acid with chiral amino alcohol. This type of polymer can be prepared only by chemical modification. Lithiation of the polymeric bromide then successive treatment with trimethyl borate and hydrochloric acid furnished polymer beads containing arylboronic acid residues 31. Treatment of this polymer with (li ,2S)-(-)-norephedrine and removal of the water produced gave the polymer-supported oxazaborolidine 32 (Eq. 14) [41 3]. If a,a-diphenyl-2-pyrrolidinemetha-nol was used instead of norephedrine the oxazaborolidine polymer 33 was obtained. The 2-vinylthiophene-styrene-divinylbenzene copolymer, 34, has been used as an alternative to the polystyrene support, because the thiophene moiety is easily lithiated with n-butyl-lithium and can be further functionalized. The oxazaborolidinone polymer 37 was then obtained as shown in Sch. 2. Enantioselectivities obtained by use of these polymeric oxazaborolidines were similar to those obtained by use of the low-molecular-weight counterpart in solution. For instance, acetophenone was reduced enantioselectively to 1-phe-nylethanol with 98 % ee in the presence of 0.6 equiv. polymer 33. Partial elimination of... 

One way to modify PPP is to prepare copolymers with only partial substitution. Jing et al. (1994) prepared a random copolymer (Chaturvedi et al. 1992) with 33% of unsubstituted phenylene units by copolymerization of the substituted and unsubstituted bromobenzene boronic acids. Copolymers (Tanigaki et al. 1996) with alternating substituted and unsubstituted phenylenes were obtained by Stille (Bao and Yu 1995 Savenkova et al. 2000) or Suzuki coupling, both resulting in reduction of brittleness of PPP. 

Preparation of a similar copolymer 2.28 containing sexithiophene units which are alternately linked by a hydrophobic alkyl chain and a hydrophilic PEG chain was achieved by Suzuki-type cross-coupling of dibromo derivative 2.23 with bis-boronic ester 2.27 (Scheme 1.6) [94]. The latter compound was synthesized by Stille-type cross-coupling of bis-stannylated derivative 2.26 and a bromobithiophene endowed with boronic ester functionality at other terminal a-position. However, the synthesized copolymer 2.28 was found to be insoluble in common organic solvents. 

Cationic polymerization of MVK is certainly not the method of choice. However, if boron trifluoride etherate was added to a monomer-carbon dioxide mixture in petroleum ether polymerization was observed [298]. Acid-catalyzed polarography of MVK in methanol is also considered to be a cationic polymerization. For the polymer an alternating ketone-ether copolymer structure was suggested [299,300]. The following reaction mechanism is... 

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