Anionic polymerization cyclic carbonates

The reaction of C—Li reagents with carbon-carbon double bonds has great technological relevance because it is the basis of the anionic polymerization processes. This reaction also affords convenient synthetic routes to cyclic compounds, when internal addition to an olefinic double bond present in the metallated molecule takes place (e.g. equations 68, 83 and 84). 

The range of monomers that can be incorporated into block copolymers by the living anionic route includes not only the carbon-carbon double-bond monomers susceptible to anionic polymerization but also certain cyclic monomers, such as ethylene oxide, propylene sulfide, lactams, lactones, and cyclic siloxanes (Chap. 7). Thus one can synthesize block copolymers involving each of the two types of monomers. Some of these combinations require an appropriate adjustment of the propagating center prior to the addition of the cyclic monomer. For example, carbanions from monomers such as styrene or methyl methacrylate are not sufficiently nucleophilic to polymerize lactones. The block copolymer with a lactone can be synthesized if one adds a small amount of ethylene oxide to the living polystyryl system to convert propagating centers to alkoxide ions prior to adding the lactone monomer. 

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

Potassium carboxylate groups introduced onto the surface of carbon fibers initiated anionic polymerization of epoxides (e.g., styrene oxide, epichlorohydrin, and glycidyl phenyl ethers) and cyclic acid anhydrides (e.g., maleic anhydride, succinic anhydride, and phthalic anhydride) in the presence of 18-crown-6 [41]. 

Among the more common thermoplastics from ring opening polymerization of interest in composite processing are polylactams, polyethers, polyacetals, and polycycloolefins. It has also been shown that polycarbonates can be produced from cyclic carbonates [22], Anionic ring opening polymerization of caprolactam to nylon 6 is uniquely suited to form a thermoplastic matrix for fiber-reinforced composites, specifically by the reaction injection pultrusion process [23-25]. The fast reaction kinetics with no by-products and the crystalline... 

Cyclic carbonates are prepared in satisfactory quality for anionic polymerization by catalyzed transesterification of neopentyl glycol with diaryl carbonates, followed by tempering and depolymerization. Neopentyl carbonate (5,5-dimethyl-l,3-dioxan-2-one) (6) prepared in this manner has high purity (99.5%) and can be anionically polymerized to polycarbonates with mol wt of 35,000 (39). 

Some heterocycles have both nucleophilic and electrophilic atoms in their molecule. Thus they can be opened and polymerized by the anionic, cationic or coordination mechanisms. Examples are lactams, lactones, and cyclic siloxanes. Investigations of the mechanism of lactam propagation are complicated by the occurence of side reactions. In principle, the mechanism described in Chap. 3 by the schemes (55)—(57) and (71) is accepted. Anionic polymerization of cyclic esters consists, in most cases (see Chap. 4, Sect. 2.2) of repeated reversible attacks on the carbonyl carbon by the anion 0]-. From e-caprolactone, polyester chains grow according to [315]... 

Anionic polymerizations can be terminated by addition of another molecule, which will introduce an co-functional group in the chain. Excess carbon dioxide or cyclic anhydrides lead to terminal carboxylic groups, whereas addition of excess phosgene produces an acid chloride function. Similarly, isocyanates generate coamide functions, and lactones yield co-hydroxyl groups. 

Aliphatic polysulfides with two or more carbon atoms per monomeric unit are accessible through ring opening polymerization of cyclic sulfides or by the addition of thiol groups onto vinyl groups. In these cases, the anionic polymerization of cyclic sulHdes differs substantially from that of cyclic ethers. The ethyl anion attacks the carbon atom in cyclic ethers. But in the ethyl lithium initiated polymerization of propylene sulfide, a lithium ethane thiolate is first formed, and its anion then starts the polymerization of propylene sulfide ... 

Haba O., Tomizuka H., Endo T, Anionic ring-opening polymerization of methyl 4,6-0-benzylidene-2,3-0-carbonyl-a-D-glucopyranoside A first example of anionic ring-opening polymerisation of five-membered cyclic carbonate without elimination of CO2, Macromolecules, 38, 2005, 3562-3563. 

Matsuo, J., Aoki, K., Sanda, F., Endo, T., 1998b. Substituent effect on the anionic equilibrium polymerization of six-membered cyclic carbonates. Macromolecules 31, 4432—4438. 

Polycarbonates have attracted attention in recent years because of their potential use in biomedical applications based on their biodegradability, biocompatibility, low toxicity and good mechanical properties [67]. These polymers can be prepared by the ROP of cyclic carbonate monomers by anionic, cationic, and coordination catalysts. However, lipase-catalyzed polymerization seems to be a feasible alternative to prepare polycarbonates as chemical methods often suffer from partial elimination of carbon dioxide (resulting in ether linkages), require extremely pure monomers and anhydrous conditions. 

There are a number of heterocyclic monomers, for example, epoxides, cyclic sulfides, lactones and lactides, lactams, cyclic carbonates, and cydosUoxanes, which can be polymerized by ring-opening reactions many of them can he polymerized by an anionic as well as by a cationic mechanism. They cannot all be covered here, but there are a number of monographs and reviews on this subject [181-185]. 

The lack of polyethylene samples with tailored architectures, controlled molecular weight, low PDI, and absence of branching defects has hmited a systematic study of the relationships between molecular structure and physical properties of polyethylene. Several notable exceptions include cyclic polyethylene that has been synthesized by ring-opening metathesis polymerization (ROMP) (PDI of 2) (Bielawski et al., 2002 Bielawski et al., 2003) and star, comb, H-shape and pom-pom polyethylenes, containing 17—25 ethyl branching defects per 1000 carbons, which were produced by anionic polymerization (Hadjichristidis et al., 2000). 

On the other hand, in the purely anionic polymerization of five- or higher-membered cyclic esters, the carbonyl carbon of the monomer is attacked with subsequent acyl-oxygen bond scission and reformation of the alkoxide anion. In the coordination polymerization, this is also the carbonyl carbon that is now first coordinated with alkoxide species and then the acyl-oxygen bond is broken with reforming of the covalent alkoxide chain end. In the already formed macromo-lecular chains, the same ester bonds are present as those being the site of the nucleophilic attack in the monomer molecules. These processes are illustrated in Scheme 12, where the active centers are shown as. ..-OMt, for both anionic and covalent centers. 

The aromatic five-membered cyclic carbonate (benzo-1,3-dioxolan-2-one) appeared inert in anionic polymerization initiated with scc-BuLi and potassium dihydronaphthalide. ... 

GC alone can be a valuable monomer for the synthesis of hyperbranched poly(hydroxyether)s (Scheme 25). In case of polymerization, GC, containing a l,3-dioxolan-2-one ring and hydroxyl group in a single molecule, is considered a latent cyclic AB2-type monomer. The anionic ROP of the GC, which proceeds with CO2 liberation, leads to a branched polyether. l,l,l-Tris(hydroxymethyl)propane or other multihydroxyl molecules are usually used as a initiator-starter and central core of the polyether. The hyperbranched polyglycerol structure is obtained by slow addition of the cyclic carbonate monomer at above 150 °C. Such polymers are characterized by a flexible polyether core and a multihydroxyl outer sphere. They are suitable for preparation of acrylic resins for dental applications or additives for polyurethane foams. Hyperbranched poly (hydroxyether)s from biscyclic carbonate with phenol group (2, Scheme 24) were also reported. 

The polymerization of cyclic carbonate with alkyl halide proceeds according to pathway 1 (Scheme 30) and owing to the higher nucleophilicity of the halide anion, the covalent macrohalide is more favored than the trioalkxycarbenium ion. However, when a solvent of higher polarity is used as the reaction medium, partial decarboxylation takes place due to... 

The anionic polymerization of six-membered cyclic carbonates was first reported in the 1930s by Carothers and Van... 

Generally, six-membered cyclic carbonates easily polymerize with anionic initiators affording high-molecular-weight polymers. In contrast to cationic ROP of cyclic carbonates, these polymers do not contain ether linkages. This feature is especially important when the PCs or their derivatives are utilized in biomedical practice. The presence of ether linkages make such polymers susceptible for oxidation and loss of good mechanical properties. 

Scheme 32 Anionic polymerization of six-membered cyclic carbonate with nucleophilic initiators.

Anionic metal-free initiation was successfully applied to both aliphatic and aromatic cyclic carbonates. This method is based on the reaction of a silyl ether with fluoride anions, for example, tetrabutyl ammonium fluoride (BU4NF) or tris (dimethylamino)sulfonium trimethylsilyl difluoride (TASF, [(CH3)2N]3 SSi(CH3)3p2), to produce an anion with a tetrabutyl ammonium or tris(dimethylamino)sulfonium counterion. The metal-free system is an efficient initiator for neopentyl carbonate polymerization. ... 

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