Polymerization processes, types

Table II. Number of Literature Reference Subdivided by Reactor and Polymerization Process Types...

Figure 31.4 The three distinct methods through which carbenes have been utilized for polymer synthesis. Type 1 Carbenes are essential to the polymerization process Type 2 Carbenes as side-group functionalities or reagents for post-polymerization modification Type 3 Carbenes are used as catalysts or ligands for polymerization catalysts.

Interfacial Polymerization. Many types of polymerization reactions can be made to occur at interfaces or produce polymers that concentrate at interfaces thereby producing microcapsules. Accordingly, this approach to encapsulation has steadily developed into a versatile family of encapsulation processes. Figure 4 schematically illustrates five types of encapsulation processes that utilize these types of reactions. 

Solution Polymerization. Two types of solution polymerization technologies are used for LLDPE synthesis. One process utilizes heavy solvents the other is carried out in mixtures of supercritical ethylene and molten PE as a polymerization medium. Original solution processes were introduced for low pressure manufacture of PE resins in the late 1950s subsequent improvements of these processes gradually made them economically competitive with later, more advanced technologies. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

Besides, polymerization processes were used to obtain PCSs with alternating C=N-bonds in the main chain. Thus, radiation polymerization of methacrylonitrile54 gives rise to a ladder polymer of the following type ... 

Cu-mediated Ullman reaction has been used for the polymerization of dihaloaryls. For example, see ref. 3. This type of polymerization as well as other transition-metal-mediated reactions that involve radicals in the polymerization process is not included in this chapter. 

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

Since Carothers introduced the distinction between condensation and addition polymers based on the composition relationship between monomer and structural unit, a number of examples of polymerization processes have been found which formally resemble the condensation type but which proceed without evolution of a by-product. For example, a glycol and a diisocyanate react intermolecularly as follows ... 

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