Linked-ring polymers, synthesis

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

Abstract Thousands of polymeric materials have been made into synthetic polymers, based on a linear structure, and used in commercial applications. The study of synthetic polymeric materials has focused on those derived from long chain linear molecules. Alternatively, cyclic polymers (also referred to as polymer rings or macrocycles) can be prepared, which not only can be branched or cross-linked, but can also form nonco-valently linked structures based on their loop topology. Through a number of different approaches and advances in cyclization techniques, a wide range of novel cyclic polymers have been synthesized in good yields. This review will focus on a variety of synthetic methods and some properties of cyclic polymers using many polymerization mechanisms in various fields of polymer synthesis. 

Finally, we would like to give a short outlook on the stmcture of a single homopolymer chain confined to a very small miniemulsion droplet. Advanced Monte Carlo methods [224] were applied to a simple coarse-grained model of polystyrene in spherical confinement [225]. The polymer chain becomes highly knotted once the confining droplet shrinks (e.g., by evaporation of the solvent) beyond the typical size of the polymer (in good solvent conditions) [225-227]. These simulations may hence lead the way to the synthesis of knotted and unknotted ring polymers in extremely small miniemulsion droplets when the termini of the polymer are chemically linked. 

Scheme 1.3 Bimolecular (A) (Laurent and Grayson, 2009), unimolecular (B) (Hoskins and Grayson, 2009 Kubo etal, 2003 Laurent and Grayson, 2006 Qiu etal, 2007 Ye etal, 2008) and (C) end-to-end linking processes for the synthesis of ring polymers (Adachi et al, 2008 Bielawski and Grubbs, 2007 Guidry et al, 2005 Hayashi et al, 2001 Tezuka and Komiya, 2002).

Scheme 1.4 Ring-expansion polymerization and the subsequent covalent linking for the synthesis of ring polymers (Li et al, 2006).

Zirconocene complexes containing two indenyl or tetrahydroindenyl groups bridged with short links such as —CH2—CH2— or —Si(CH3)— can produce isotactic polymers of higher a-olefins (32). To synthesize syndiotactic PO, bridged zirconocene complexes with rings of two different types are required, one example of which is isopropyl(cyclopentadienyl)(l-fluorenyl)zirconocene. These complexes are used for the synthesis of syndiotactic PB... 

FIGURE 1.9 (a) Amino acids build proteins by connecting the n-carboxyl C atom of one amino acid to the n-amino N atom of the next amino acid in line, (b) Polysaccharides are built by combining the C-1 of one sugar to the C-4 O of the next sugar in the polymer, (c) Nucleic acids are polymers of nucleotides linked by bonds between the 3 -OH of the ribose ring of one nucleotide to the 5 -P04 of its neighboring nucleotide. All three of these polymerization processes involve bond formations accompanied by the elimination of water (dehydration synthesis reactions). 

As early as 1895, the synthesis of polydichlorophosphazene was attempted by H.N. Stokes by thermal ring-opening polymerization of hexachloro-triphosphazene [(NPCl2)3]. The product obtained by H.N. Stokes was a high-molecular weight cross-linked rubbery material called inorganic rubber which is insoluble in all solvents and hydrolytically decomposes into phosphates, ammonia, and hydrochloric acid in the presence of moisture. Because of its insolubility and hydrolytic instability, the polymer found no technological application and remained as a laboratory curiosity. 

The synthesis of the first polymer-supported chiral Mn-salen derivatives was reported independently by Sivaram171 and Minutolo.171-173 Different monomeric Jacobsen-type units, containing two polymerizable vinyl groups, were copolymerized with styrene and divinylbenzene to yield the corresponding cross-linked polymers as a monolithic compact block.174-176 The less mobile system (Figure 19) with no spacer between the aromatic ring and the polymer backbone is less enantioselective. 

Scheme 8.1 Synthesis of cross-linked polymer beads via ring-opening metathesis precipitation polymerization. [Mo] = Mo(N-2,6-i-Pr2-...

Polymers such as PBI have a weak link in them since a single covalent bond connects the phenyl rings in biphenyl. This weakness is overcome by the synthesis of ladder polymers, such as polyquinoxaline structures 4.64 and 4.65, and polydithione (structure 4.66), which have two covalent bonds throughout the chain. Thus, the integrity of the polymer is maintained even if one bond is broken ... 

Uryu and coworkers also reported the first synthesis of a nonhydro-lyzable polysaccharide by cationic, ring-opening polymerization of an oxetane ring. The monomer was 3,5-anhydro-l,2-0-isopropylidene-a-D-xylofuranose, and it formed a polymer of molecular weight of 10,000-11,000. Spectral and optical data indicated regular polymerization to a (3— 5)-linked polymer of 1,2-O-isopropylidene-a-D-xylofuranose.14... 

Because PBI is expensive, other thermostable polymers were explored and tested as catalysts (246). A cross-linked version of a polyimide (PI) support with incorporated triazole rings (12b) gave better results than PBI for the epoxidation of cyclohexene. Moreover, it can be reused in the cyclohexene epoxidation at least 10 times without any loss of activity (247). Even less expensive, but thermooxidatively stable materials include polysiloxane-based resins, which have also been used for incorporation of Ti (see Section II,A). In this case, the synthesis comprises the polymerization of TEOS and an oligomeric dimethyl silanol with the addition of functional trialkoxysilanes such as trimethoxysilyl-2-ethylpyridine instead of Ti(OiPr)4 (248). Preliminary results show that the activity per Mo atom is higher than that of PBI-Mo. Furthermore, the degree of leaching of Mo is very low. Thus, it is expected that the polysiloxane-based systems may soon find wide application in oxidation chemistry. 

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