Polyaddition systems

Isocyanate polyaddition systems are also of considerable interest in view of the way the properties of the polymer can be varied. A description of the characteristic two-step network... 

The pol3rmerization systems, the second group of pol3nners mentioned before, are not as readily tailored to requirements as the polyaddition systems described above, where presynthesized elements can be used to determine network structures. Nevertheless, pol3mierization systems could also be suitable materials for low-temperature applications, since it would appear possible to impart... 

The polyaddition reaction is influenced by the stmcture and functionaHty of the monomers, including the location of substituents in proximity to the reactive isocyanate group (steric hindrance) and the nature of the hydroxyl group (primary or secondary). Impurities also influence the reactivity of the system for example, acid impurities in PMDI require partial neutralization or larger amounts of the basic catalysts. The acidity in PMDI can be reduced by heat or epoxy treatment, which is best conducted in the plant. Addition of small amounts of carboxyHc acid chlorides lowers the reactivity of PMDI or stabilizes isocyanate terrninated prepolymers. 

The kinetics of polyhydrosailylation reactions has been studied. Quantum-chemical calculations of the model system and data of NMR 1H spectra of the real products of the polyaddition reaction have confirmed probability of passing polyhydrosilylation reaction according to the aforementioned two concurrent directions obtaining both a and (5 adducts. For the evaluation of relative activity for selected monomers the algebraic-chemical approach has been used. 

First let us consider the initiation process in the presence of proton-donor compounds specially introduced into the system. Two contradictory viewpoints about the reaction mechanism may be distinguished in this case. One of them 138,139f l45,154) presupposes a molecular mechanism of the reaction, i.e. a stepwise polyaddition of the epoxy compound to the alcohol group, e.g. according to Scheme (32)... 

Aldersley, J. W., and M. Gordon Polyaddition and polycondensation-substitution effects in polycondensation systems. IUPAC Symposium on Macromolecular Chemistry, Prague 1965. Preprint p. 584. 

The polyaddition reaction is influenced by the structure and functionality of the monomers, including the location of substituents in proximity to the reactive isocyanate group (steric hindrance) and the nature of the hydroxy] group (primary or secondary). Impurities also influence the reactivity of tlie system. 

Equations of an analogous type can be used for other polyurethane-forming systems, although the numerical values of the constants may be different, because Eq. (2.29) must be treated primarily as empirical. It is reasonable to expect that an adequate approximation for other systems would be n equal to 2 (which is typical for polyaddition reactions) and m also equals to 2. Such values of the constants m and n are valid, for example, for RIM-2200 (produced by Union Carbide ), which consists basically of polyester and 4.4-diphenylmethane diisocyanate.4... 

If the initial polymerization system contains a single monomer as in Eq. (2.1), the constitutional repeating unit (CRU) of the polymer will contain only one monomer-based unit and the structure of the CRU will be derived from the monomer (polyaddition case), possibly through the elimination of a small molecule (polycondensation case). 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Nowadays the reactions of hydride polyaddition are in a great use in siliconorganic chemistry. Recently a great importance is paid to this reaction in the field of obtaining complicated monomers [1] and also in the field of investigation the mechanisms of additions on the various catalyst systems Pt, Pd, Co, carbonyl of metals and investigation of specificity of actions of various catalytic systems [2-5],... 

Hydride polyaddition of divinyl-containing compounds was carried out for various lengths of a,co-dihydridedimethylsiloxanes. The reaction run was searched by a decrease of active =Si-H groups concentration. It was found that for rhodium acetylacetonatedicarbonyl as a catalyst, copolymers soluble in organic solvents were obtained, which were structured after some time. This may be explained by the fact that in spite of polymers re-precipitated from toluene solution by methyl alcohol, rhodium catalyst remains in polymeric systems, which decompose and induce structuring (cross-linking) of copolymers. 

Synthesis of carboorganosiloxane copolymers and polymers is based on hydride polyaddition of organohydrosiloxanes to organoalkenyl silanes [69], Recently, this reaction is of great interest in the field of obtaining complex monomers [70], as well as in the field of study of addition mechanisms on various Pt, Pd, Co and metal carbonyl catalysts [71-74] and specificity of actions of one or other catalytic systems [75], Besides carboorganosiloxane oligomers and linear polymers, other compo-unds were also synthesized by this method [76 -78],... 

Data on MMD (Table 14) indicate that cyclolinear polymers with the most homogeneous structure and composition are formed on the reduced catalyst. The more so, every particular case displays its own optimal type of the reduced form, i.e. catalytic system for synthesizing cyclolinear polymers should be selected with regard to activity of dihydrorganocyclosiloxane in polyaddition reaction. It should be noted that as yellow colloid is applied as the catalyst, the reaction temperature has no ef-fect on the shape of MMD curves for ethyl-substituted polymers with tetra- and hexasiloxane cyclic fragments. 

As a third possibility, nanocapsules in a miniemulsion system could be achieved using different interfacial reactions in inverse miniemulsions. The formation of polyurea, polythiourea, and polyurethane nanocapsules synthesized through the polyaddition reaction has been described in detail [110-112], The size of the nanocapsules could be controlled by the amount of surfactant used and the addition time of the diisocyanate. The wall thickness was adjusted by the amount of employed monomers. dsDNA molecules were successfully encapsulated into poly-butylcyanoacrylate (PBCA) nanocapsules by anionic polymerization, which took place at the interface between the miniemulsion droplets and the continuous phase [113]. 

Photocrosslinking 61-77 Photoinitiators 63 f., 69 Physical aging 132 Plasticity at the crack tip 135 Plastic zone 135 Poisson distribution 21 Polyaddition, irreversible step 18 Polyamines, addition to polyepoxides 25 f. Polycarboxylic acids 47 Polyepoxides and polyamines 25 f. Polyepoxy-polyamine systems, multicomponent 36... 

Various kinds of polymerization, polyaddition, substitution and polycondensation reactions of monomers bearing stabilizing moieties have been involved. Almost every kind of synthetical approach has been tested. However, only some of them are of practical interest Examples of monomers and/or of the respective ma-cromolecular systems proposed as stabilizers were selected from many literature data to show the diversity of structures. Only representative references are reported. 

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