In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed.12 This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar (i.e. they have a very low polydispersity index). Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.
Living polymerization in the literature is often called "living" polymerization or controlled polymerization. Living polymerization was demonstrated by Michael Szwarc in 1956 in the anionic polymerization of styrene with an alkali metal / naphthalene system in tetrahydrofuran (THF). He found that after addition of monomer to the initiator system that the increase in viscosity would eventually cease but that after addition of a new amount of monomer after some time the viscosity would start to increase again.3
The main living polymerization techniques are:
- Living anionic polymerization
- Living cationic polymerization
- Ring opening metathesis polymerization
- Living free radical polymerization
- Group transfer polymerization
- living Ziegler-Natta polymerization
As early as 1936, Karl Ziegler proposed that anionic polymerization of styrene and butadiene by consecutive addition of monomer to an alkyl lithium initiator occurred without chain transfer or termination. Twenty years later, living polymerization was demonstrated by Szwarc through the anionic polymerization of styrene in THF using sodium naphthalenide as celerator.567
Monomers for living cationic polymerization are electron-rich alkenes such as vinyl ethers, isobutylene, styrene, and N-vinylcarbazole. The initiators are binary systems consisting of an electrophile and a Lewis acid. The method was developed around 1980 with contributions from Higashimura, Sawamoto and Kennedy.
Given the right reaction conditions ring-opening metathesis polymerization (ROMP) can be rendered living. The first such systems were described by Robert H. Grubbs in 1986 based on norbornene and Tebbe's reagent and in 1978 Grubbs together with Richard R. Schrock describing living polymerization with a tungsten carbene complex.8
Starting in the 1970s several new methods were discovered which allowed the development of living polymerization using free radical chemistry. These techniques involved catalytic chain transfer polymerization, iniferter mediated polymerization, stable free radical mediated polymerization (SFRP), atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and iodine-transfer polymerization.
Group-transfer polymerization also has characteristics of living polymerization.9 It is applied to alkylated methacrylate monomers and the initiator is a silyl ketene acetal. New monomer adds to the initiator and to the active growing chain in a Michael reaction. With each addition of a monomer group the trimethylsilyl group is transferred to the end of the chain. The active chain-end is not ionic as in anionic or cationic polymeriation but is covalent. The reaction can be catalysed by bifluorides and bioxyanions such as tris(dialkylamino)sulfonium bifluoride or tetrabutyl ammonium bibenzoate. The method was discovered in 1983 by O.W. Webster10 and the name first suggested by Barry Trost.
Several reported methods exist that introduce livingness in Ziegler-Natta polymerization.11 The monomer in this type of polymerization (a subset of coordination polymerization) is an alpha-olefin and the active site contains an alkyl to metal bond. Chain growth is based on the Cossee-Arlman mechanism. An early method (Doi, 1979) describes propene polymerization in toluene at −50°C using diethylaluminium chloride and a vanadium catalyst for example V(acac)3 to syndiotactic polypropylene with a polydispersity index of 1.05 to 1.4.1213 Another living system as described by McConville in 1996 is based on titanium using 1-hexene, [RN(CH2)3NR]TiMe2 and tris(pentafluorophenyl)boron14
- IUPAC Gold Book Definition
- Living Ziegler-Natta Polymerization Article
- Living polymers 50 years of evolution Article
- Halasa, A. F. Rubber Chem. Technol., 1981, 54, 627.
- (2006) The Chemistry of Radical Polymerization - Second fully revised edition (Graeme Moad & David H. Solomon). Elsevier. ISBN 0-08-044286-2
- Webster, O. W. Science, 1991, 251, 8877.
- "Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". Pure and Applied Chemistry 68 (12): 2287–2311. 1996. doi:10.1351/pac199668122287.
- M. Szwarc, Nature 1956, 178, 1168.
- Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78, 2656.
- US 4 158 678 (priority date 30 June 1976).
- "Ring-opening polymerization of norbornene by a living tungsten alkylidene complex" R. R. Schrock, J. Feldman, L. F. Cannizzo, R. H. Grubbs Macromolecules; 1987; 20(5); 1169–1172. doi:10.1021/ma00171a053
- Polymer chemistry: a practical approach 2004 Fred J. Davis
- "Group-transfer polymerization. 1. A new concept for addition polymerization with organosilicon initiators" O. W. Webster, W. R. Hertler, D. Y. Sogah, W. B. Farnham, T. V. RajanBabu J. Am. Chem. Soc., 1983, 105 (17), pp. 5706–5708 doi:10.1021/ja00355a039
- organicdivision.org Essay: Living Ziegler-Natta Polymerization 2002 Richard J. Keaton PDF
- "'Living' Coordination Polymerization of Propene Initiated by the Soluble V(acac)3-Al(C2H5)2Cl System" Yoshiharu Doi, Satoshi Ueki, Tominaga Keii Macromolecules, 1979, 12 (5), pp. 814–819 doi:10.1021/ma60071a004
- "Living coordination polymerization of propene with a highly active vanadium-based catalyst" Yoshiharu Doi, Shigeo Suzuki, Kazuo Soga Macromolecules, 1986, 19 (12), pp. 2896–2900 doi:10.1021/ma00166a002
- "Living Polymerization of α-Olefins by Chelating Diamide Complexes of Titanium" John D. Scollard and David H. McConville J. Am. Chem. Soc., 1996, 118 (41), pp. 10008–10009 doi:10.1021/ja9618964