Step polymerization Completeness

Figure 3.13 shows the variation of the gel conversion of the limiting reactant as a function of the stoichiometric ratio. For r > 3, no gel is formed and the polymer remains in the liquid state after complete reaction of epoxy groups. If the amount of epoxy monomer necessary to obtain a stoichiometric system is added in a second step, polymerization restarts, leading to gelation and the formation of a network. The two-step polymerization is the basis of several commercial thermosetting polymers. 

Other variables can also be used to influence the thermodynamics of a polymerization. For example, most step polymerizations involve equilibrium reactions, which may be driven to completion by removing the small molecule by-product in an open system. Addition polymerizations are influenced by the solvent used. That is, [ML depends on both the nature of the solvent and on [M]o- For example, the equilibrium monomer concentration of THF increases as the acidity of the solvent increases due to complex formation. In other cases, solvation of a polymer segment may be more exothermic than monomer solvation, resulting in a more exothermic AHP compared to the bulk polymerization. 

Floiy Statistics of the Molecular Weight Distribudon. The solution to the complete set (j= 1toj = 100,000) of coupled-nonlinear ordinary differential equations needed to calculate the distribution is an enormous undertaking evai with the fastest computers. However, we can use probability theory to estimate the distribution. This theory was developed by Nobel laureate Paul Flory. We have shown that for step polymerization and for free radical polymerization in which termination is by isproportionation the mole fraction of polymer with chain lengthy is... 

Phosphine-derivatised poly(4-/i r/-butylstyrene) has been prepared for use as a soluble support in homogeneous catalysis.It was used in a monophasic medium and separation of the catalysts after reaction was effected either by cooling- or water-induced phase separation. The support was prepared by co-polymerisation of /i r/-butylstyrene with a phosphine oxide-containing styrene monomer (Scheme 19) A small quantity of a methyl red-labelled comonomer was also added to act as a colorometric tag to facilitate studies of the extent of separation and recycling of the polymeric material. The phosphine oxide was reduced to the free phosphine after the polymerisation step was complete. 

When the polymerization step is complete, it is necessary to recover of the solvent, and to wash and dry the resin. The work-up consists basically of the following steps ... 

Once the fragmentation step is complete, the particle continues to grow as long as monomer arrives at the active sites and the catalyst does not deactivate. The final morphology, size, shape and porosity of the particles will be the result of a complex balance between the conditions during the fi-agmentation step, the phase in which the reaction takes place, the polymer properties in the particle and, obviously, the polymerization rate. 

The distribution of chain lengths that comes from a step polymerization is termed the most probable distribution. One aspect of this distribution is that the PDI goes to 2 as the reaction approaches completion (i.e., p 1). Since high molecular weights are achieved only at high conversions, most polymers produced by step polymerization have PDI values that are quite close to 2. 

Allen, G. and J. C. Bevington (eds.), Comprehensive Polymer Science , Pergamon Press, Oxford, 1989. A series of volumes that show the relationship between methods of preparation, treatment, structure and properties. It is organized into seven volumes Volume 1, Polymer Characterization Volume 2, Polymer Properties Volumes 3 and 4, Chain Polymerization Volume 5, Step Polymerization Volume 6, Polymer Reactions Volume 7, Specialty Polymers and Polymer Processing. A cumulative subject index completes the final volume. 

In recent years we have been able to delineate the key to success in ADMET chemistry, that being the selection of a catalyst free of Lewis acids. Acidic catalysts permit the intervention of vinyl-addition chemistry which precludes formation of high polymers through step polymerization. To prove the point, styrene was used as a model compound to explore possible mechanistic paths for reacting olefin systems. It was shown that if Lewis-acid containing catalyst systems are used, then vinyl-addition chemistry predominates, whereas the choice of a Lewis-acid free catalyst system (Shrock s tungsten catalyst is used in this example) results in complete domination of metathesis chemistry instead. These observations... 

Polymerization, the reaction through which small hydrocarbon molecules are combined to form a single large molecule of high molecular weight. The result of this reaction is the formation of coke. This process is initiated in the liquid phase and continued in different steps. Polymerization reactions require long reaction time and the coke drums provide the necessary residence time for these reactions to proceed to completion. 

The reaction of NaOH with bisphenol A generates water. This water must be thoroughly removed from the system to allow the reaction to be driven to completion, and more importandy, to preclude any residual water in the system from hydrolyzing part of the DCDPS monomer (2). Before the introduction of DCDPS for the polymerization step, all but traces of water must be removed. Failure to do so results in regeneration of NaOH, which rapidly reacts with DCDPS to form the monosodium salt of 4-chloro-4 -hydroxydiphenylsulfone [18995-09-0] (3) (6). 

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