Step polymerization ring formation

To explain the formation of non-crosslinked polymers from the diallyl quaternary ammonium system, Butler and Angelo proposed a chain growth mechanism which involved a series of intra- and inter-molecular propagation steps (15). This type of polymerization was subsequently shown to occur in a wide variety of symmetrical diene systems which cyclize to form five or six-membered ring structures. This mode of propagation of a non-conjugated diene with subsequent ring formation was later called cyclopolymerization. 

Neglect of the formation of polymeric rings, however, is sometimes too crude an approximation. It may happen that the cyclisation constant C2 of the linear dimer is larger than the cyclisation constant Cl of the monomer. It may also happen that the concentration of the monomer is comparable to Ct but smaller than C2. When this is the case, the open chain dimer, once formed, will show a higher tendency to cyclise than to react with the monomer to give the linear trimer. Under the above conditions the stepwise polymerisation is truncated after the first step, and the system is described to a useful approximation by scheme (6). 

Many of the common condensation polymers are listed in Table 1-1. In all instances the polymerization reactions shown are those proceeding by the step polymerization mechanism. This chapter will consider the characteristics of step polymerization in detail. The synthesis of condensation polymers by ring-opening polymerization will be subsequently treated in Chap. 7. A number of different chemical reactions may be used to synthesize polymeric materials by step polymerization. These include esterification, amidation, the formation of urethanes, aromatic substitution, and others. Polymerization usually proceeds by the reactions between two different functional groups, for example, hydroxyl and carboxyl groups, or isocyanate and hydroxyl groups. 

The production of linear polymers by the step polymerization of polyfunctional monomers is sometimes complicated by the competitive occurrence of cyclization reactions. Ring formation is a possibility in the polymerizations of both the A—B and A—A plus B—B types. 

The overall rate of conversion of e-caprolactam to polymer is higher than the polymerization rate of e-aminocaproic acid by more than an order of magnitude [Hermans et al., 1958, I960]. Step polymerization of e-aminocaproic acid with itself (Eq. 7-57) accounts for only a few percent of the total polymerization of e-caprolactam. Ring-opening polymerization (Eq. 7-58) is the overwhelming route for polymer formation. Polymerization is acid-catalyzed as indicated by the observations that amines and sodium e-aminocaproate are poor initiators in the absence of water and the polymerization rate in the presence of water is first-order in lactam and second-order in COOH end groups [Majury, 1958]. 

The first step is the formation of an alkoxide anion by the initiating alcohol (allyl alcohol is the initiator most commonly used, although other initiators have been suggested). The appropriate oxide(s) is (are) then added to the alcohol initiator. This causes the opening of the oxirane ring in the oxide and propagates the chain growth of the alkylene oxide on the initiator. The last step is the neutralization of the alkoxide anion to terminate the polymerization. 

An example of a kinetic template effect is provided by the reaction between this complex and l,2-bis(bromomethyl)benzene. Complexation orients the sulfur atoms of the quadridentate N2S2 ligand to favour ring formation in a two-step reaction with l,2-(BrCH2)2C H, as shown in Scheme 5.4. In the absence of the metal ion, reaction of the free ligand with l,2-(BrCH,)2C H4 gives polymeric materials. 

Assuming that the condensation reaction (142) is much faster than the ring opening (139) and (140), 73 > fevi and 72, then the rate determining step is the formation of the aminoacyl chloride (XXXII). For large rings, the depolymerization can be neglected (k-j i > fe 7i and 7 2 7 2) and the rate of polymerization should obey the expression... 

In this case the driving force for the ring-opening step is the formation of the euromatic ring. Finally the ring-opening polymerization of Sg has been postulated to involve free radicals ( ). 

If we apply the generally assumed mechanism of the CALB-catalyzed ringopening polymerization of lactones [7h] to the polymerization of P-lactam the first step would be formation of a first acyl-enzyme intermediate as a consequence of the ring-opening of P-lactam by Ser10s assisted by His 224 and Aspi87 as depicted in Figure 14.8. 

Step-growth polymerization can be complicated by cyclization reactions that accompany formations of linear polymers. Such ring formations can occur in reactions of monomers with either the same type of functional groups or with different ones. Some illustrations of cyclization reactions follow ... 

Discuss ring formations that can accompany step-growth polymerizations. 

More recent experiments proving the presence of very large rings in step reactions and a discussion of the role of ring formation in step polymerization are summarized by Kricheldorf HR (2003) The Role of Ring-ring Equilibria in Thermodynamically Controlled Polycondensation, Macromol Symp 199 15-22 see also other papers in the same issue and the introduction What Does Polycondensation Mean Ibid pp 1-13. 

There are two major kinetic mechanisms for the formation of a polymer, namely, chain polymerization reactions and step polymerization reactions. Typically, vinyl and diene monomers will undergo polymerization by a chain mechanism. Monomers containing, for example, -OH and -COOH functionality will polymerize by a step kinetic mechanism. Ring monomers can polymerize by either a chain or step polymerization mechanism depending on the precise nature of the monomer and method used to start the polymerization reaction. 

In two-step polymerization, initially the polyamic acid is formed from an equimolar mixture of dianhydride and diamine in a polar aprotic solvent such as A,A-dimethylacetamide (DMAc) or A,A-dimeth-ylformamide (DMF). The reaction pathway for the formation of poly(amic acid) involving intermediates is presented in Scheme 3.2. The reaction mechanism involves nucleophilic attack of the amino group to the electrophilic carbonyl carbon of the anhydride group. This opens the anhydride ring to form an amic acid group. Formation of the poly(amic acid) is an equilibration reaction in which the forward reaction... 

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