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Chapter 10
Reversible Deactivation Radical Polymerization of Vinyl Chloride Carlos M. R. Abreu,1 Ana C. Fonseca,1 Nuno M. P. Rocha,2 James T. Guthrie,3 Arménio C. Serra,1 and Jorge F. J. Coelho*,1 1CEMMPRE,
Department of Chemical Engineering, University of Coimbra, Rua Sílvio Lima-Pólo II, 3030-790 Coimbra, Portugal 2INEGI – Institute of Mechanical Engineering and Industrial Management, University of Porto, Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal 3Department of Colour Science, School of Chemistry, University of Leeds, LS2 9JT, United Kingdom *E-mail:
[email protected].
Poly(vinyl chloride) (PVC) is one of the higher consumed polymers (more than 40 million tons per year) and can only be prepared on an industrial scale by free-radical polymerization (FRP). Several intrinsic limitations of FRP have triggered interest in synthesizing this polymer by reversible deactivation radical polymerization (RDRP) methods. Despite the many achievements that have been made, the RDRP of nonactivated monomers, such as vinyl chloride (VC), presents several challenges to the scientific community. Several features of VC make its control by RDRP techniques particularly difficult. The most recent developments on RDRP of VC are critically discussed.
Introduction Presently, poly(vinyl chloride), also known as PVC is the second most used polymer. PVC use has gone through steady and continuous growth, despite the various public debates that have been held because of the need to find halogen free polymers, for several health and environmental reasons. PVC has many important characteristics amongst which are low cost, general versatility and good flame retardancy. These have caused it to be widely used in applications © 2018 American Chemical Society Matyjaszewski et al.; Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
from packaging to construction. Also, as a result of its industrial production, PVC plays an important role because of the incorporation of chlorine (approximately 57% of the VC (vinyl chloride) mass), making use of the chlorine that is obtained as a secondary product of chlor-alkali industry. This factor means that VC supply has been less affected by fluctuation in oil prices compared with other fossil-based monomers. The presence of polar chloro groups improve the options provided by PVC in its blend with other compounds (plasticizers) or polymers. Currently, the only available way to synthesize PVC on a large scale is by free radical polymerization (FRP). This process has some side reactions that inevitably lead to structural defects. These unwanted structural defects are the cause of the low thermal stability of the polymer from which some major technological limitations arise (1, 2). In the PVC using industries, other issues faced include the need to make use of stabilizers and the restricted temperature range in which PVC can be processed, even when there are stabilizing compounds in the formulation. The impossibility of processing PVC at higher temperatures entails some problems. These include products’ incompatibility, incomplete fusion of PVC crystallites and the high melting viscosities that result in the need for specialized equipment. In addition, FRP makes it impossible to adopt the use of macromolecular engineering in creating PVC-based controlled structures. With the newly developed reversible deactivation radical polymerization (RDRP) methods, there are now some options when considering the synthesis of polymers that possess a controlled molecular weight, functional chain ends, complex architectures and designed topologies (3). The possibility of using a radical-based method that is capable suppressing most of the side reactions is very important when trying to prevent the occurrence of structural defects, especially if the PVC is to be used in subsequent macromolecular engineering (4). Before RDRP methods were adopted, the majority of the innovations that arose in the PVC industry were largely devoted to polymerization process optimizations, which aimed to improve the production efficiency at more reduced costs. Little has been achieved as far as understanding the polymerization mechanism is concerned. Previous efforts to polymerize VC with the use of ionic initiators, such as alkyl lithium, or metal atom-containing systems (e.g. metallocenes) have either not produced a PVC-based material that has the desired structures, or has made use of technologies that were impossible to replicate on an industrial scale.
Polymerization of Vinyl Chloride VC (CAS number 75-01-4) is an organo-chloro compound whose formula is H2C=CHCl. It has Q- and e- values of 0.44 and 0.2 respectively which indicates the low reactivity of the VC monomer and the very high reactivity of the corresponding radical. VC is categorized as a non-conjugated, weak-electron-withdrawing vinyl monomer (5). The free radical polymerization of VC gives one of the higher values of chain transfer (C) to monomer amongst the more commonly used monomers. For instance at 60 ºC, the values are 1.0 x 10-3
