Polyurethane step-growth polymerization reaction

Polymers in this category are synthesized by routes similar to the routes used to synthesize regular step-growth polymers. The difference, however, is that the monomer units contain a metal-metal bond. A sample step-growth polymerization reaction is shown in Eq. 7.1, which illustrates the reaction of a metal-metal bonded dialcohol with hexamethylene diisocyanate (HMDI) to form a polyurethane.4... 

Hyperbranched polyurethanes have also been synthesized by step-growth polymerization reactions such as the reaction between 3,5-diaminobenzoic acid and... 

Step-growth polymerization is characterized by the fact that chains always maintain their terminal reactivity and continue to react together to form longer chains as the reaction proceeds, ie, a -mer + -mer — (a + )-mer. Because there are reactions that foUow this mechanism but do not produce a molecule of condensation, eg, the formation of polyurethanes from diols and diisocyanates (eq. 6), the terms step-growth and polycondensation are not exactly synonymous (6,18,19). 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

Another example of the step-growth polymerization is the synthesis of polyurethanes. Here, linear polyurethanes are produced by the reaction of bifunctional alcohols, HO — R — OH, with bifunctional isocyanates, OCN — R — NCO, to produce a polyurethane (see Figure 3.21). 

This experiment is another example of a step-growth polymerization, one that produces a crosslinked polymer. Two liquids are mixed, beginning the chemical reactions that cause polymerization and foam generation. The result is a hard polyurethane foam, similar to the material commonly used for insulation, for flotation in boats and canoes, and in furniture. This activity works well either as a laboratory experiment or as a demonstration. ... 

Condensation polymers result from formation of ester or amide linkages between difunctional molecules. Condensation polymerization usually proceeds by step-growth polymerization, in which any two monomer molecules may react to form a dimer, and dimers may condense to give tetramers, and so on. Each condensation is an individual step in the growth of the polymer, and there is no chain reaction. Many kinds of condensation polymers are known. We discuss the four most common types polyamides, polyesters, polycarbonates, and polyurethanes. 

Some step-growth polymerizations involve reactions where no small molecules (like HCI or HjO) are split out at all, as in the synthesis of polyurethanes, shown schematically in Figure 3-14. Isocyanates are... 

Condensation polymers by the above delinition are usually produced by step-growth polymerizations but not all step-growth syntheses are condensation reactions. Thus there is no elimination product in polyurethane synthesis from a diol and a diisocyanate (cf. reaction (1-12)) ... 

Step-growth Polymerization The simplest scheme of this polymerization involves the reaction of a difunctional monomer AB, which contains both functional groups A and B in the molecule. For example, A can be an amine and B a carboxylic acid group. Another scheme involves the reaction between two difunctional monomers of the type AA and BB. In any case, each polymer linkage will have involved the reaction of the functional groups A and B coming from two molecules (monomers or chains). Some examples of polymers synthesized by this mechanism are polyurethane, polyamide, and polyester. 

Step-growth polymerization is a very important method for the preparation of some of the most important engineering and specialty polymers. Polymers such as polyamides [7], poly(ethylene terephthalate) [8], polycarbonates [9], polyurethanes [10], polysiloxanes [11], polyimides [12], phenol polymers and resins, urea, and melamine-formaldehyde polymers can be obtained by step-growth polymerization through different types of reactions such as esterification, polyamidation, formylation, substitution, and hydrolysis. A detailed list of reaction types is shown in Table 3.2. 

A typical polyurethane adhesive may contain, in addition to the urethane linkages, aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea, and allophanate groups. Polyurethanes are formed by the addition reaction of diisocyanates or polyisocyanates with polyols (Figure 3.12) through a step-growth polymerization... 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

The second general pattern of polymerization is step-growth or condensation polymerization. While terminal alkenes are the most common monomers in chain-growth polymerization, bifunctional molecules are the characteristic monomers for step-growth polymerization. The polyester-, polyamide-, and polyurethane-forming reactions shown below are examples of step-growth polymerizations ... 

Reaction injection molding (RIM) is suitable for some step-growth polymerization processes in which no condensation byproducts are generated, and reactions are very rapid (e.g., polyurethane synthesis from diisocyanates and diols or multifunctional alcohols) ... 

Furan, Furfural, Hydroxymethylfurfural, Furan polymers. Chain polymerizations. Step-growth polymerizations, Furan polyesters, Furan polyamides, Furan polyurethanes, Furan-containing conjugated oligomers, Diels-Alder reaction, Dendrimers, Reversible crosslinking... 

A related family of materials is shown in Figure 13.15 B, along with a highly efficient synthetic route. Diisocyanates are common, especially attractive monomers. Reaction with diols produces polyurethanes [the -NH-C(0)-0- functional group is also known as a carbamate], while reaction with diamines produces polyureas. Technically, no small molecule such as water or ammonia is given off, but such reactions are generally viewed as condensation polymerizations. More accurately, they are step-growth polymerizations. 

Polyurethanes are condensation polymers formed by step-growth polymerization in which the chain length of the polymer increases steadily as the reaction progresses. Two major routes of polymerization of polyurethanes are one-step and two-step synthesis methods. In one-step synthesis, the diisocyanate, polyol, and a chain extender are mixed together and allowed to react. In the two-step route, an oligomer or prepolymer synthesized from diisocyanate and polyol and the chain extender are allowed to react. The two-step route offers some advantages over the one-step route, such as a reduced polydispersity index and a higher extent of phase separation. 

Important polymers that are produced by polyaddition are polyamide 6 (nylon) and all kinds of polyurethanes. In polycondensation one mol of a small molecule (typically H2O) is liberated per step of chain growths, important polymers that are produced by polycondensation are polyamide 6.6, poly(ethylene terephthalate) (PET), polycarbonate, polyarylate, and polysulfide. Step growth polymerization is usually slow, equilibrium limited and isothermal to slightly exothermic. Polyaddition and polycondensation reactions of monomers with three or more reactive end groups lead to three-dimensionally crosslinked resins. 

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