Formation of polyurethanes

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). 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Metal salts of neodecanoic acid have also been used as catalysts in the preparation of polymers. For example, bismuth, calcium, barium, and 2kconium neodecanoates have been used as catalysts in the formation of polyurethane elastomers (91,92). Magnesium neodecanoate [57453-97-1] is one component of a catalyst system for the preparation of polyolefins (93) vanadium, cobalt, copper, or kon neodecanoates have been used as curing catalysts for conjugated-diene butyl elastomers (94). 

Isocyanates are highly reactive materials and enter into a number of reactions with groups containing active hydrogen. The reactions of most important in the formation of polyurethanes are ... 

The unique surface characteristics of polysiloxanes mean that they are extensively used as surfactants. Silicone surfactants have been thoroughly studied and described in numerous articles. For an extensive, in-depth discussion of this subject, a recent chapter by Hill,476 and his introductory chapter in the monograph he later edited,477 are excellent references. In the latter monograph, many aspects of silicone surfactants are described in 12 chapters. In the introduction, Hill discusses the chemistry of silicone surfactants, surface activity, aggregation behavior of silicone surfactants in various media, and their key applications in polyurethane foam manufacture, in textile and fiber industry, in personal care, and in paint and coating industries. All this information (with 200 cited references) provides a broad background for the discussion of more specific issues covered in other chapters. Thus, surfactants based on silicone polyether co-polymers are surveyed.478 Novel siloxane surfactant structures,479 surface activity and aggregation phenomena,480 silicone surfactants application in the formation of polyurethane foam,481 foam control and... 

Snow, S. A. Stevens, R. E. The Science of Silicone Surfactant Application in the Formation of Polyurethane Foam. In Silicone Surfactants-, Hill, R. M., Ed. Surfactant Science Series Dekker New York, 1999 Vol. 86, Chapter 5, pp 137-158. 

The International Union of Pure and Applied Chemistry [IUPAC, 1994] suggested the term polycondensation instead of step polymerization, but polycondensation is a narrower term than step polymerization since it implies that the reactions are limited to condensations—reactions in which small molecules such as water are expelled during polymerization. The term step polymerization encompasses not only condensations but also polymerizations in which no small molecules are expelled. An example of the latter is the reaction of diols and diisocyantes to yield polyurethanes (Eq. 1-6). The formation of polyurethanes follows the same reaction characteristics as the formation of polyesters, polyamides, and other polymerizations in which small molecules are expelled. 

Offering an inside look at the logic behind the formation of polyurethane components, Castable Polyurethane Elastomers enables material scientists, engineers, and chemists to understand the total production process and reproduce the procedures to achieve desired results. 

The compound acts as a hydroxy curing agent in the formation of polyurethane binders of -> Composite Propellants-, - also Casting of Propellants. 

TDI acts as a hydroxy curing agent in the formation of polyurethane... 

The viscoelastic properties of carbon nanotube/polymer composites have both practical importance related to composite processing and scientific importance as a probe of the composite dynamics and microstructure. The viscosity for CNT/PU dispersion at mixing is also very important for in-situ formation of polyurethane nanocomposite. Lower viscosity means a better flow ability and more homogenous mixing with isocyanate. Furthermore, low viscosity is very helpful to remove the bubbles before curing, which is a key step for polyurethane preparation. 

As a process characterizing name, the term step polyaddition is preserved for cases where a monomer with multiple bonds is copolymerizing with saturated monomers. The formation of polyurethanes is an example of such a process... 

It should be stressed that the viscosity changes during formation of polyurethanes even from bifiinctional compounds can be correlated with gelation most likely they are connected with the formation of a physical network then crosslinks arise from sufficiently strong specific interactions like hydrogen bonds [50]. An example of such a process is the reaction of macro (diisocyanate) with 3,3 -dichloro-4,4 -diaminodiphenylmethane [43]. 

The peculiarities of the process described above are explained by the fact that branchings may create nuclei of insoluble fractions [43]. This assumption is confirmed by the data given in Fig. 8 where the viscosity increase is compared for network formation of polyurethanes from bi- and multi-functional diamine. The fact that the shape of the rj (t) dependences is similar and the exponents a = 4.6 indicate that equivalent physical effects are operative. 

The uncatalysed reaction of diisocyanates with polyols does not have any significance in the formation of polyurethanes. Reactions are catalysed by acids and organic bases. Among the good catalysts are tertiary amines and organometallic compounds, mainly tin derivatives. 

The polycondensation reactions leading to the formation of polyurethanes are influenced by a number of factors (Wells, 1969), prominent among which are the ... 

Silicone organic hybrids play a critical role as surfactants in the formation of polyurethane foam (5). These surfactants are... 

Use Catalyst for polymerization of ethylene and formation of polyurethane foam. 

Viscosity measurements helped to understand the reaction rate in the formation of polyurethane in the presence of lead powder (Figure 9.3). A smaller addition of lead powder (10%), does not accelerate the reaction rate but larger amounts of lead powder increase the reaction rate rapidly. At 30% lead powder, the reaction rate is increased by a factor of 3. 

Figure 9.3. Viscosity of formation of polyurethane in the presence of lead powder. [Data from Caillaud J L, Deguillaume S, Vincent M, Giannotta J C, Widmaier J VI. Polym. Int., 40, No.l, 1996, 1-7.]...

In this work, we describe an automated differential scanning calorimetric (DSC) technique that can be used to measure polymerization kinetics for formation of urethane block polymers. The same technique is also used to measure phase separation during formation of polyurethane elastomers and the effect that separation has on the polymerization. The elastomer formulations consisted of modified liquid p,p -diphenyImethyl diisocyanate (MDI), 1,4-butanediol and an (oxypropylene-oxyethylene) macroglycol for the soft segment. 

Although the overlap of terms is great, many exceptions exist. For example, the formation of polyurethanes typically occurs through stepwise kinetics. The polymers are classified as condensation polymers, and the backbone is heteratomed, yet no byproduct is released when the isocyanate and diol are condensed. The formation of nylon 6, a condensation polymer, from the corresponding internal lactam occurs through chain-growth kinetics. 

The addition of diols to diketene acetals is similar to the addition of diols to diisocyanates that leads to the formation of polyurethanes. And, like in polyurethanes, mechanical properties can be widely varied by using different diols. Further, because the condensation between a diketene acetal and a diol, just like... 

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