Chain polymerization depolymerization

Polymerization occurs very quickly and the process is controlled via kinetic effects rather than thermodynamic ones. The net result is that the molecular weight distribution of the product does not match the thermodynamically stable one. If the chains were not capped with monofunctional phenols, the polymer chains would depolymerize, allowing the monomers to rearrange themselves at elevated temperature to approach the thermodynamically stable... 

Polymerization depolymerization equilibria are more often encountered in ROP than in the chain polymerizations, both radical and ionic, of carbon-carbon double-bond monomers. 

Since the equilibrium constants for the addition of monomer onto the sequence of length 1 or to longer sequences are so different, the following situation must be considered. The living sequences with two monomer units cannot split off a monomer unit, but higher sequences are subject to polymerization-depolymerization equilibria with an equilibrium constant which no longer depends on chain length. We consider the special case in which the equilibrium constant K2 and the constants... 

Cleavage of formaldehyde from the active centers and polymerization of formaldehyde at the same cationic chain ends (polymerization-depolymerization equilibrium of formaldehyde) (9). 

Macrocycles are sometimes only formed in the presence of monomers. When all monomer has been consumed by polymerization, depolymerization also stops [333]. The tendency of common heterocycles to the production of giant molecules, and the possibility of generating cyclic fractions from their chains is illustrated in Table 6. 

Depolymerization is the reverse of addition polymerization [reaction (23-50)]. It only occurs without side reactions when the bonds to the side groups are much more stable than the main-chain bonds. Depolymerization begins spontaneously only in living problems. In all other macromolecules, bonds in the main chain must first be broken homolytically in a start reaction. In these cases, therefore, depolymerization proceeds according to a free radical mechanism. 

The glass-transition temperature depends on the mobility of the chain segments and can therefore be raised by stiffening the chain (see Section 10.5.3). Thus, a-methyl styrene forms a polymer that, in contrast to poly-(styrene), does not deform at lOC C, because of a glass-transition temperature of 170°C. However, since the thermodynamic ceiling temperature for for the polymerization/depolymerization equilibrium is also simultaneously lowered (see Section 16.3), poly(a-methyl styrene) degrades more easily than poly(styrene), so that it is not so easy to work by injection molding. 

The experiments, however, where cyclic material was added at the start of the reaction, suggest that under these conditions the tetracyclic material is either incorporated into the polymer or isomerized to the pentacyclic, and only reforms at the end of the reaction. The most reasonable route for isomerization is the polymerization/depolymerization route, with the proportions governed more by kinetic effects connected with the ultimate loss of chain end activity. 

The polymerization-depolymerization equilibrium is not between the growing chain and the originally propagating monomer, trioxane, but to formaldehyde [Eq. (60)] [241, 242]. Thus formaldehyde should be considered as a comonomer [236, 243], which will be accumulated until its equilibrium concentration and pressure is reached. 

Random scission can be thought of as the converse of stepwise polymerization. Chain rupture occurs at random points along the chain leaving fragments which are large compared to the monomer units. Chain depolymerization can be considered the reverse of chain polymerization and involves successive release of monomer units from a chain end in a depropagation reaction. 

The primary polymerization product ia these processes has a relatively wide molecular weight distribution, and a separate step is often used to narrow the polydispersity. Such a narrowkig step may consist of high vacuum stripping to remove volatile polymer chains, often followed by a solvent fractionation step (35,36), sometimes a solvent fractionation step alone (37,38), or a fractional precipitation from organic solvent (32). The molecular weight distribution can also be narrowed by depolymerization at elevated temperatures ia the presence of a depolymerization catalyst (217—220). 

Kawakami, Suzuki and Yamashita showed that compound 7, among many others, could be polymerized to derivatives of the corresponding open-chained species by treatment with boron trifluoride ether complex. Yamashita and Kawakami formed these same sorts of materials by heating the glycols and paraformaldehyde in the presence of toluenesulfonic acid. This led to prepolymers which were then thermally depolymerized to afford the cyclic oligomers which were separated by fractional distillation. 

Upon thermal destruction of polyethylene the chain transfer reactions are predominant, but depolymerization proceeds to a much lesser extent. As a result, the products of destruction represent the polymeric chain fragments of different length, and monomeric ethylene is formed to the extent of 1-3% by mass of polyethylene. C—C bonds in polypropylene are less strong than in polyethylene because of the fact that each second carbon atom in the main chain is the tertiary one. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

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