Tham khảo tài liệu 'fundamentals of polymer engineering part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 3 Step-Growth Polymerization INTRODUCTION As described in the previous chapters the properties of polymeric materials depend considerably on their molecular-weight distribution MWD . This in turn is completely determined by the mechanism of polymerization. There are various mechanisms by which polymer chains grow this chapter focuses on one of them step-growth polymerization. Monomer molecules consisting of at least two functional groups can undergo step-growth polymerization. In order to keep mathematics tractable this chapter will focus on polymerization of bifunctional monomers. The two reacting functional groups can either be on the same monomer molecule as in amino caproic acid NH2 CH2 5 COOH or on two separate molecules as in the reaction between ethylene glycol OH CH2 2 OH and adipic acid COOH CH2 4 COOH. If they are located on the same monomer molecule represented schematically as ARB the concentrations of the functional groups remain equimolar throughout the course of the reaction which can be schematically represented as follows 1-7 n A-R-B A-R-B A-R-B n_2-A-R-B Here R represents an alkyl or aryl group to which the two functional groups A and B are attached. In case the functional groups are located on two different monomers A R A and B R0 B an analysis similar to the one for ARB 103 Copyright 2003 Marcel Dekker Inc. 104 Chapter 3 polymerization can be conducted. As pointed out in Chapter 1 the overall reaction represented by Eq. consists of several elementary reactions which can be represented as follows k m n Pm IC. - Pm n W k m n m n 1 2 . . . where Pn represents A R B A R B n_2 A R B Pm represents the A R B A R B m_2 A R B molecule and W represents the condensation product. The forward and reverse rate constants km n and k m n are in general chain-length dependent as discussed in the following paragraphs. If two small molecular species A and B react as A B Products it is evident that the reaction will proceed only after a .
