Basic catalysts polymerization mechanism

Exercise 16-20 Write a reasonable mechanism for the polymerization of methanal in water solution under the influence of a basic catalyst. Would you expect base catalysis to produce any 1,3,5-trioxacyclohexane Why ... 

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes sympy, then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, then show how further condensation leads to cross-linking. (Hint The carbonyl group lends acidity to the N—H protons of urea. A first condensation with formaldehyde leads to an inline, which is weakly electrophilic and reacts with another deprotonated urea.)... 

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

The polymerization of a-reported that a 1 1 mixture of succinodinitrile and dimethylsulfoxide solidified within a few hours at 135 °C, in the presence of basic catalysts such as sodium methoxide, ammonia, amines, etc. Peebles and Brandrup suggested the foolowing mechanism ... 

Equilibrium polymerization, which can be anionic or cationic, is utilized to convert cyclic organosiloxanes into polydiorganosiloxane polymer chains. In the chemical industry octamethylcyclotetrasiloxane is preferred as such, or as a mixture with other siloxanes for chain termination and/or production of copolymers for specific applications. Particularly indu.strially important is anionic polymerization with basic catalysts such as alkali hydroxides, whereby the activity falls off in the order Cs > Rb > K > Na > Li. KOH is most frequently used e.g. as a suspension in octamethylcyclotetrasiloxane at 140°C, the catalyst being active from a concentration of several ppm. According to the assumed mechanism of this catalytic process, potassium siloxanolate is initially formed, which leads to cleavage of the Si-O-Si bonds and chain formation ... 

Compared with the great success in commercial applications, academic progress on the Phillips catalyst is lagging far behind, in spite of numerous research efforts during the past 60 years. Aspects of the Phillips catalyst concerning the formation, structure, oxidation state of active sites, and polymerization mechanisms, especially the initiation mechanism, are still mysterious. The difficulties for basic smdies on this important industrial catalyst system are mainly derived from the low percentage of active Cr species, the complexity of heterogeneous catalyst systems, the multiple valence states of Cr, the instant encapsulation of active sites by produced polymer, and the ultrafast polymerization rate. 

Parallel to the progress in the basic understanding on the nature of active sites and polymerization mechanisms, several modified Phillips catalysts with better performance and improvements in the structures and properties of PE products through surface modification of the silica support and catalyst with Ti, F, Al, or B compounds have been successfully developed and commercially applied during... 

What conditions (free-radical initiator, acid catalyst, basic catalyst, etc.) would you consider using in this polymerization Propose a polymerization mechanism as far as the tetramer. 

In this section, we will briefly examine the salient features of the basic classes of metallocene catalysts. Polymerization activity comparisons and the mechanisms that govern polymerization will be treated in subsequent sections. 

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. 

Polymerization proceeding by activation of the monomer is referred to as the activated monomer mechanism (AMM) -both nudeophilic and electrophilic - whereas polymerization by activation of the rnitiator/potymer chain ends is referred to as the active chain end mechanism (ACEM). Note that, in the context of tme organocatalysis, only basic catalysts are able to... 

Some experiments on the polymerization of BCMO with the chelate catalyst are recorded in the patent literature( ). In this work, high molecular weight polymer (ninh up 4.3) was obtained at very high temperatures (200-250 C.) in a continuous, bulk polymerization method. These data indicate that the chelate catalyst polymerizes BCMO by a coordination mechanism. Presumably the very high temperature works well because of the relatively low basicity of BCMO. ... 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

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