Preparation of Cyclic Polymers

Laurent BA, Grayson SM (2009) Synthetic approaches for the preparation of cyclic polymers. Chem Soc Rev 38 2202-2213... 

A similar solid state method for the preparation of cyclic polymer has been reported. This method involves the synthesis of cyclic polymer obtained by a combination of inter-alkylation and intra-alkylation, i.e., cyclization of 11-bromo-decanoic acid. The cyclic polymer can be liberated from an anionic exchange resin. Thus, this method has the advantage that the cyclic product from the mixture after reaction is easily produced, as indicated in Fig. 50 [161]. 

This work also described the preparation of cyclic polymers with two attached branches [282] using the following procedure (Scheme 110). 

The preparation of cyclic polymers from a,(B-heterodifimdional polymers has been explored first by Rique-Lurbet et In this approach, a heterodifimdional... 

The preparation of cyclic polymers has been particularly challenging for polymer chemists as the vast majority of polymerization methods involve the linear propagation from an initiator to a terminal end group. A post... 

Very recently, Monteiro and Jia [85] reported a thiol-ene reaction for the preparation of cyclic polymer with inherent alkyne functionality from RAFT polymerization. An alkyne-hydroxyl-RAFT agent, through post-polymerization functionalization, was used to introduce an acrylate to the polymer via the reaction of the hydroxyl group with acryloyl chloride. Through the successive aminolysis of the RAFT and thiol-ene reaction between the acrylate group and thiol group, which was generated from aminolysis under hexylamine, alkyne functional monocyclic PSTY, PDMA, PNIPAM, and P BA were obtained (Scheme 37). 

In a KCP, cyclics cannot be formed by back-biting , and thus the formation of cyclic oligomers and polymers is indicative for the intermediate formation of linear chains having two reactive end groups. In other words, the formation of cyclics is indicative for simultaneous ROP and (poly)condensation steps. If side reactions are absent, and if the linear chains are mobile enough for cyclization, cyclics will be the only reaction products, and combined ROP and polycondensation is an eflhcient strategy for the preparation of cyclic polymers in high yields. 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

Scheme 3.1 Preparation of cyclic terephthalate esters via acid chlorides or depolymerization, and polymerization to high-molecular-weight polymer...

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. 

MALDI-TOF mass spectrometry analysis of poly(methyl acrylate) prepared by the free-radical polymerization of methyl acrylate (MA) in the presence of a cyclic dixanthate under y-ray irradiation revealed that there are at least three distributions, i.e., molecular mass for [ 1-(MA) -H]+ of cyclic polymers, [1-(MA) -THF-H]+, and [1-(MA) -(THF)2-H]+ of linear polymers were observed. The relative content of the cyclic polymers markedly increases at a lower temperature, which may be related to the reduced diffusion rate and the suppressed chain-transfer reaction at the low reaction temperature [39]. 

Oxidative polymerization of aryl disulfides has been carried out under high dilution conditions for the preparation of cyclic arylene sulfides for synthesizing cyclic arylene sulfide polymers. The readily available diphenyl disulfide and dichloro disulfide with diphenyl sulfide, diphenyl ether, or p-xylene are useful monomers for the synthesis of the corresponding cyclic oligo(thio arylene)s. The reaction scheme is shown in Fig. 30 [101]. 

Alternatively, ring closure metathesis reaction is applied to the formation of cyclic polymers, although the main chain does not consist of hydrocarbon. Such a method was reported in the preparation of cyclic poly(THF) as shown in Fig. 33 [112]. Such ring-close metathesis was also applied for a preparation of catenane [113]. 

For a synthetic polymer chemist the important question is whether the cyclization processes in cationic ring-opening polymerization can be controlled. If the preparation of linear polymer is attempted, then cyclic oligomers are undesirable side products. This is especially important in synthesis of telechelic polymers containing reactive end groups, because macrocycles would be unreactive admixtures. On the other hand, cyclic polymers, if prepared selectively, could be a valuable materials. 

If, however, the polymerization process is aimed at the preparation of functional polymers (telechelics, macromonomers), then the problem of cyclization becomes much more important, because cyclic oligomers do not contain functional end groups. 

Thus, cationic polymerization of oxiranes is of little synthetic value, if the preparation of linear polymers is attempted. The high tendency for cyclization may be employed, however, for preparation of macrocyclic polyethers (crown ethers). Polymerization of ethylene oxide in the presence of suitable cations (e.g., Na+, K+, Rb +, Cs + ) leads to crown ethers of a given ring size in relatively high yields, due to the template effect [105], Thus, with Rb+ or Cs+ cations, cyclic fraction contained exclusively 18-crown-6. 

If R = R (bifunctional polymers), reaction (119) does not affect the functionality but leads to the broadening of the molecular weight distribution, which is occurring anyway, due to the reversibility of propagation. Thus, several bifunctional polymers of 1,3-dioxolane were prepared and used, for example, to form the networks containing degradable and hydrolyzable polyacetal blocks (cf., Section IV.B). Reaction (119), however, may effectively prohibit the preparation of monofunctional polymers, e.g., macromonomers. Indeed, two recent attempts to prepare macromonomers by cationic polymerization of cyclic acetals led to nearly statistical... 

This equation shows that (a) the more dilute the polymer solution, the more probable the cyclization over the polycondensation is, and (b) at the same molecular weight, double the concentration (higher yield) can be used for heterodifunctional than for homodifunctional polymers. The practical strategies used for the preparation of cyclic copolymers will be presented below. 

The monomers that have been explored most extensively are propylene carbonate methacrylate (PCMA) and propylene carbonate acrylate (PCA). These monomers are readily copolymerized with other commonly used unsaturated monomers to yield polymers with cyclic carbonate functionality. There are a few patents discussing the formation of coatings by the amine cross linking of these cyclocarbonate functional polymers. However, they do not appear commercially available. Thus, their use in the preparation of cyclic carbonate functional polymers has been limited. 

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