R7.1 Polymerization

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Polystyrene Coffee Cups 361 R7.1.2.1 Steps in Free-Radical Polymerization Chap. The basic steps in free-radical polymerization are initiation, propagation, chain transfer, and termination. Initiation. Chain polymerization reactions are different because an initiation step is needed to start the polymer chain growth. Initiation can be achieved by adding a small amount of a chemical that decomposes easily to form free radicals. Initiators can be monofunctional and form the same free radical: k0 Initiation I2 ⎯⎯→ 2 I Propagation Assumption of equal For example, 2,2-azobisisobutyronitrile: or they can be multifunctional and form different radicals. Multifunctional initiators contain more than one labile group3 [e.g., 2,5 dimethyl-2,5-bis(benzoylperoxy)hexane]. For monofunctional initiators the reaction sequence between monomer M and initiator I is For example, Propagation. The propagation sequence between a free radical monomer unit is In general, 3 ( CH3) 2CN—NC( CH3) 2 ⎯⎯→ 2 ( CH ) R 1 with a J. J. Kiu and K. Y. Choi, Chem. Eng. Sci., 43, 65 (1988); K. Y. Choi and G. D. Lei, AIChE J., 33, 2067 (1987). 3 2 C � CN CN CN k i I� M ⎯⎯→ R 1 ( CH3) 2C � � CH2—CHCl ⎯⎯→ ( CH3) 2 � N2 C CH C � CN CN Cl k p R1 � M ⎯⎯→ R 2 kp R2 � M ⎯⎯→ R 3 reactivity R j� M ⎯⎯→ R j k p �1 2 H
Chain Sec. R7.1 Polymerization For example, ( CH3) 2CCH ( 2CHCl) j CH2C � � CH2C � CHCl ⎯⎯→ ( CH3) CN Cl The specific reaction rates kp are assumed to be identical for the addition of each monomer to the growing chain. This is usually an excellent assumption once two or more monomers have been added to R1 and for low conversions of monomer. The specific reaction rate is often taken to be equal to . k i Chain Transfer. The transfer of a radical from a growing polymer chain can occur in the following ways: 1. Transfer to a monomer: R j k p 362 Here a live polymer chain of j monomer units tranfers its free radical to the monomer to form the radical R1 and a dead polymer chain of j monomer units. 2. Transfer to another species: 3. Transfer of the radical to the solvent: H �M ⎯⎯→ P j� R1 transfer R j C ⎯⎯→ P j R j k m kc � � k s The species involved in the various chain transfer reactions such as CCl3� and C6H5CH2� are all assumed to have the same reactivity as R1. In other words, all the R1’s produced in chain transfer reactions are taken to be the same. However, in some cases the chain transfer agent may be too large or unreactive to propagate the chain. The choice of solvent in which to carry out the polymerization is important. For example, the solvent transfer specific reaction rate ks is 10,000 times greater in CCl4 than in benzene. The specific reaction rates in chain transfer are all assumed to be independent of the chain length. We also note that while the radicals R1 produced in each of the chain transfer steps are different, they function in essentially the same manner as the radical R1 in the propagation step to form radical . R 2 R 1 �S ⎯⎯→ P j � R1 CCH ( CHCl) 2 2 H j�1CH2C � CN Cl
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R7.1 Polymerization

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