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Show by equations the polymerization of melamine and formaldehyde to form a crosslinked structure. Describe by means of equations how random and block copolymers having the following compositions could be synthesized:
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How would you synthesize a block copolymer having segments of the following structures
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Distinguish between spiro, ladder, and semiladder polymers. Give examples of each. Describe the difference between the convergent and divergent approaches to synthesizing dendrimers. For each of the following reactions system indicate whether the product is a linear, branched, crosslinked, or hyperbranched polymer. a. A2 B2 b. AB2 c. AB3 d. A2 B3 e. AB2 B3 f. AB B3
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RADICAL CHAIN POLYMERIZATION
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In the previous chapter, the synthesis of polymers by step polymerization was considered. Polymerization of unsaturated monomers by chain polymerization will be discussed in this and several of the subsequent chapters. Chain polymerization is initiated by a reactive species R* produced from some compound I termed an initiator:
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I R*
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The reactive species, which may be a free radical, cation, or anion, adds to a monomer molecule by opening the p-bond to form a new radical, cation, or anion center, as the case may be. The process is repeated as many more monomer molecules are successively added to continuously propagate the reactive center:
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H R CH2 C* Y
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3-2
Polymer growth is terminated at some point by destruction of the reactive center by an appropriate reaction depending on the type of reactive center and the particular reaction conditions.
Principles of Polymerization, Fourth Edition. By George Odian ISBN 0-471-27400-3 Copyright # 2004 John Wiley & Sons, Inc.
NATURE OF RADICAL CHAIN POLYMERIZATION
3-1 NATURE OF RADICAL CHAIN POLYMERIZATION 3-1a Comparison of Chain and Step Polymerizations
Chain polymerization proceeds by a distinctly different mechanism from step polymerization. The most signi cant difference is that high-molecular-weight polymer is formed immediately in a chain polymerization. A radical, anionic, or cationic reactive center, once produced, adds many monomer units in a chain reaction and grows rapidly to a large size. The monomer concentration decreases throughout the course of the reaction as the number of high-polymer molecules increases. At any instant the reaction mixture contains only monomer, high polymer, and the growing chains. The molecular weight of the polymer is relatively unchanged during the polymerization, although the overall percent conversion of monomer to polymer increases with reaction time. The situation is quite different for a step polymerization. Whereas only monomer and the propagating species can react with each other in chain polymerization, any two molecular species present can react in step polymerization. Monomer disappears much faster in step polymerization as one proceeds to dimer, trimer, tetramer, and so on. The molecular weight increases throughout the course of the reaction, and high-molecular-weight polymer is not obtained until the end of the polymerization. Long reaction times are necessary for both high percent conversion and high molecular weights.
3-1b 3-1b-1
Radical versus Ionic Chain Polymerizations General Considerations of Polymerizability
Whether a particular monomer can be converted to polymer depends on both thermodynamic and kinetic considerations. The polymerization will be impossible under any and all reaction conditions if it does not pass the test of thermodynamic feasibility. Polymerization is possible only if the free-energy difference G between monomer and polymer is negative (Sec. 3-9b). A negative G does not, however, mean that polymerization will be observed under a particular set of reaction conditions (type of initiation, temperature, etc.). The ability to carry out a thermodynamically feasible polymerization depends on its kinetic feasibility on whether the process proceeds at a reasonable rate under a proposed set of reaction conditions. Thus, whereas the polymerization of a wide variety of unsaturated monomers is thermodynamically feasible, very speci c reaction conditions are often required to achieve kinetic feasibility in order to accomplish a particular polymerization. Although radical, cationic, and anionic initiators are used in chain polymerizations, they cannot be used indiscriminately, since all three types of initiation do not work for all monomers. Monomers show varying degrees of selectivity with regard to the type of reactive center that will cause their polymerization. Most monomers will undergo polymerization with a radical initiator, although at varying rates. However, monomers show high selectivity toward ionic initiators. Some monomers may not polymerize with cationic initiators, while others may not polymerize with anionic initiators. The variety of behaviors can be seen in Table 3-1. The types of initiation that bring about the polymerization of various monomers to high-molecular-weight polymer are indicated. Thus, although the polymerization of all the monomers in Table 3-1 is thermodynamically feasible, kinetic feasibility is achieved in many cases only with a speci c type of initiation. The carbon carbon double bond in vinyl monomers and the carbon oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain