Step-growth thermal polymerization

Gandon S, Mison P, Sillion B (1996) Mechanism of step-growth thermal polymerization of arylacetylene. In Hedrick JL, Labadie JW (eds) Step-growth polymers for high performance materials, new synthetic methods. Am Chem Soc Symp Series 624 306... 

Mechanism of Step-Growth Thermal Polymerization of Arylacetylene... 

Spindler and Frechet1391 prepared hyperbranched polyurethanes by step-growth polymerization (Scheme 6.8) of protected, or blocked , isocyanate AB2 monomers. The method is dependent on the thermal dissociation of a carbamate unit into the corresponding isocyanate and alcohol moieties.140,411 Decomposition temperatures range from ca. 250°C for alkyl carbamates to ca. 120°C for aryl carbamates.1401... 

This is one of the simplest methods of polymerization. It is often used in the polymerization of step-growth polymers.28 In these types of systems the viscosity remains low for a large portion of the reaction and heat transfer is easily controlled. Chain-growth polymers are more difficult to polymerize by this method due to the rapid and highly exothermic reactions. As the viscosity increases, thermal control becomes more difficult and may result in thermal runaway or localized hot spots. Commercial use of bulk polymerization for vinyl polymers is rather limited for... 

ADMET polymerization is performed on a,co-dienes to produce strictly linear polymers with unsaturated polyethylene backbones, as shown in Scheme 2. This step-growth polymerization is a thermally neutral process driven by the release of a small molecule condensate, ethylene [16-20]. Ring-opening metathesis polymerization (ROMP) is widely used to polymerize cyclic olefins and is performed with the same catalysts as in ADMET polymerizations. 

Feast. Dibenzoylbenzene derivative (114) was irradiated with tetramethylallene for 3.5 minutes in benzene to produce a polymeric structure in which there were 90% oxetane structures incorporated along the backbone. This process constitutes an example of step-growth photopolymerization via the triplet state of an aromatic carbonyl moiety. The products are thermally unstable and readily degraded by acids, but are quite soluble in organic solvents and may possess useful properties as new materials. 

Several epoxy formulations are cured by both step-growth and chain-growth polymerizations occurring sequentially or in parallel. For example, BF3 complexes or tertiary amines may be added as catalysts of an amine-epoxy reaction, leading to different reaction mechanisms taking place whose relative significance depends on the cure temperature (or thermal cycle) and the initial stoichiometry. The structure and properties of the resulting polymer networks depend on the relative contribution of both mechanisms. 

Many step-growth polymerizations are carried out by mass or bulk-type polymerization. This is commonly done not only for convenience, but also because it results in minimum contamination. Few step-growth reactions are highly exothermic, so thermal control is not hard to maintain. Because equilibrium considerations are very important, the reactions are usually carried out in a way that allows continuous removal of the byproduct. Occasionally, the polymerizations are carried out in dispersion in some convenient carriers. Solution polymerizations are sometimes used as a way of moderating the reactions. 

The step-growth polymerization of dibasic lithium phosphate is discussed next as an example of a polymerization reaction followed by immediate crystalhzation to the polymer, studied largely by thermal analysis [14,15]. It illustrates that not only organic molecules can be flexible macromolecules, but also inorganic ones. The two major techniques used for the analysis are differential scanning calorimetry (DSC, see Sect. 4.3) and thermogravimetry (TGA, see Sect. 4.6). The reaction equation is ... 

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