Chapter 1
Principles of Anionic Polymerization: An Introduction
Downloaded by UNIV OF GUELPH LIBRARY on July 18, 2012 | http://pubs.acs.org Publication Date: June 30, 1998 | doi: 10.1021/bk-1998-0696.ch001
Roderic P. Quirk, Qizhuo Zhuo, Sung H. Jang, Youngjoon Lee, and Gilda Lizarraga Maurice Morton Institute of Polymer Science, University of Akron, Akron, OH 44325-3909
The general mechanistic aspects of living alkyllithium-initiated polymerization of styrenes and dienes are presented. The structural variables which can be controlled are discussed including molecular weight and molecular weight distribution. Methods for the synthesis of well-defined block copolymers, star polymers and chain-end functionalized polymers are described. New developments in polymer synthesis using functionalized alkyllithium initiators are presented. The controlled alkyllithium-initiated polymerization of alkyl methacrylates is considered in terms of proper choice of initiator, solvent and temperatures. New additives which permit anionic alkyl methacrylate and acrylate polymerization at higher temperatures are discussed. The anionic polymerization of alkylene oxides (epoxides) is discussed with emphasis on ethylene oxide and propylene oxide. The mechanism and characteristics of anionic polymerization of lactams are also described.
The almost coincident reports of the delineation of the characteristics of living anionic polymerization by Szwarc, Levy and Milkovich (7,2) and of the uniqueness of lithium metal-initiated polymerization of isoprene to form high tis-1,4polyisoprene by Firestone researchers (3) heralded the advent of controlled polymerization to form a wide variety of polymers with low degrees of compositional heterogeneity. Shortly thereafter, Tobolsky and coworkers (4,5) reported that the same high tis-l,4-polyisoprene was obtained in hydrocarbon solutions using n-butyllithium as initiator. In addition, they reported that the high stereospecificity was lost in polar solvents such as tetrahydrofuran or diethyl ether and also using either sodium or potassium counterions in hydrocarbon solution (6). A living polymerization is a chain polymerization that proceeds in the absence of the kinetic steps of termination and chain transfer (7,8). Such polymerizations provide versatile methodologies for the preparation of polymers with control of the major structural variables that affect polymer properties. The following sections will 2
©1998 American Chemical Society
In Applications of Anionic Polymerization Research; Quirk, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
3 describe general mechanistic aspects of living alkyllithium-initiated polymerizations of styrenes and dienes and the applications of controlled anionic synthesis for a variety specific polymer structures. A brief discussion of anionic polymerization of alkyl methacrylates, epoxides and lactams is also included.
Downloaded by UNIV OF GUELPH LIBRARY on July 18, 2012 | http://pubs.acs.org Publication Date: June 30, 1998 | doi: 10.1021/bk-1998-0696.ch001
Mechanism of Alkyllithium-initiated Polymerization. The mechanism of alkyllithium-initiated polymerization is analogous to other chain reaction polymerizations, except that the kinetic steps of random termination and transfer are absent, as shown in Scheme 1 for sec-butyllithium-initiated polymerization of styrene (8). Although alkyllithium compounds are generally soluble in hydrocarbon media, they are associated into dimers, tetramers and hexamers (9). Polymeric organolithium compounds are also associated into dimers, tetramers and higher aggregates (8,10). As a consequence of this association behavior of the initiator and propagating polymeric organolithium chain ends, the actual kinetics of initiation and propagation are complicated in hydrocarbon media. A further complication results from cross-association of the unreacted initiator with the growing Scheme 1. Mechanism of alkyllithium-initiated polymerization of styrene. Initiation ^eq l/4(seç-C H Li) 4
9
^
4
^eq ^ C4H9L1
or
l / 6 ( n - C H L i ) ^ — ^ - C4H9L1 4
9
6
RH C4H9L1 + C H
2
RH
= C H
•
C, =*Η C H CH ÇHLi 4
9
K
2
d
C4H9CH2ÇHLJ
C H
5
6
^
1 / 2
[ C 4
H CH CHLi]2 9
2
w
C H 6
5
J C H
5
6
5
Propagation k
C4H9CH2ÇHU + η C H = C H C H 6
Î: H 6
P •
2
C4H9—[CH CH] CH2^HLi 2
ir
C H
5
6
5
C H 6
5
5
polymeric organolithium chain ends (eq. 1); because of this complication, reliable kinetic data for the initiation process can only be obtained at low conversions of the initiator.
(RLi)
n
+ (PLi)
m
=^=S=
(RLi) (PLi) x
y
(1)
In Applications of Anionic Polymerization Research; Quirk, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
Downloaded by UNIV OF GUELPH LIBRARY on July 18, 2012 | http://pubs.acs.org Publication Date: June 30, 1998 | doi: 10.1021/bk-1998-0696.ch001
4 In general and especially in aromatic solvents, the kinetics of initiation exhibit a fractional order dependence on the concentration of alkyllithium initiator (8). For example, a one-sixth-order dependence on η-butyl lithium concentration is observed for initiation of styrene polymerization in benzene solution (eq. 2). Since η-butyl lithium is aggregated predominantly into hexamers in hydrocarbon solution, the fractional kinetic order dependence of the initiation process on total concentration of initiator has been rationalized on the basis that the species that reacts with styrene monomer must be the unassociated form of the initiator which is in equilibrium with the hexameric aggregate (see Scheme 1 ). Similarly, the analogous one-fourth order dependence observed for initiation using sec-butyllithium (eq. 3) is also consistent with reaction of the unassociated alkyllithium form, since sec-butyllithium is associated predominantly into tetramers in benzene solution (see Scheme 1). R =k K [n-BuLi]l/6 ] (2) i
i
eq
[S
1/4
Ri=kiKe [sec-BuLi] [S] (3) However, the observed inverse correlation between reaction order dependence for alkyllithium and degree of alkyllithium aggregation is not observed in aliphatic solvents. The use of aliphatic solvents leads to decreased rates of initiation and pronounced induction periods. In fact, a different reaction mechanism involving the direct addition of monomer with aggregated organolithium species has been proposed for aliphatic solvents (8). The anionic propagation kinetics for styrene (S) polymerization with lithium as counterion are relatively unambiguous. The reaction order in monomer concentration is first order as it is for polymerization of all styrene and diene monomers in heptane, cyclohexane, benzene and toluene (1-3). The reaction order dependence on total chain end concentration,[PSLi] , is one-half as shown in eq. 4 (4). q
0
Rp = -d[S]/dt = k [ P S L i ] l / 2 [ S ] (4) Investigations of the kinetics of propagation for dienes have shown that although the rates exhibit first order dependence on monomer concentration, fractional order dependencies are generally observed for the concentration of active centers (8). Isoprene exhibited one-fourth order kinetics and butadiene exhibited one-fourth or onesixth order dependencies. Recent kinetic studies for isoprene reported that the kinetic order dependence on active center concentration is 1/4 at [PLi]>10"^M and 1/2 at [PLi]
