Anionic Polymerization.pdf

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ANIONIC POLYMERIZATION

Introduction

This article describes the general aspects of anionic polymerization of vinyl, car-

bonyl, and heterocyclic monomers. Polymerizations involving multicomponent

catalyst systems (coordinated anionic polymerization) are not discussed. Anionic

polymerization is a chain reaction polymerization in which the active species is

formally an anion, ie, an atom or group with a negative charge and an unshared

pair of electrons. Anions can be considered to be the conjugate bases of the corre-

sponding acids, as shown in the following equation. The stability and reactivity

of anionic species can be deduced from p K a values for the equilibria depicted

in this equation for the corresponding conjugate acid. The more acidic conjugate

acids (lower p K a values) are associated with a correspondingly more stable anionic

species.

(1)

In general, these anions are associated with a counterion, typically an alkali

metal cation. The exact nature of the anion can be quite varied depending on the

structure of the anion, counterion, solvent, and temperature. The range of possi-

ble species is depicted in terms of a Winstein spectrum of structures as shown in

equation 2 for a carbanionic chain end (R − ). In addition to the aggregated ( 1 ) and

unassociated ( 2 ) species, it is necessary to consider the intervention of free ions ( 5 )

and the contact ( 3 ) and solvent-separated ( 4 ) ion pairs; Mt + represents a metal-

lic counterion such as an alkali metal cation (1). In hydrocarbon media, species

( 1–3 ) would be expected to predominate. Polar solvents tend to shift the Winstein

112 ANIONIC POLYMERIZATION

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spectrum to the right, ie, toward more reactive, less associated, more ionic species.

With respect to the nature of the bonding in organoalkali metal compounds, it is

generally agreed that the carbon–alkali metal bond is ionic for sodium, potassium,

rubidium, and cesium. For carbon–lithium bonds, however, there is disagreement

regarding the relative amount of covalent versus ionic bonding (1–3). One unique

aspect of anionic polymerization is that the reactive propagating species are not

transient intermediates. Carbanions and organometallic species can be prepared

and investigated independently of the polymerization process. These species can

also be characterized during the polymerization.

(2)

Living Anionic Polymerization

A living polymerization is a chain polymerization that proceeds in the absence of

the kinetic steps of termination and chain transfer (4,5). For living anionic poly-

merization of vinyl monomers, the propagating species is a carbanion associated

with the corresponding counterion, as shown in the scheme below. Living polymer-

izations provide versatile methodologies for the preparation of macromolecules

with well-deﬁned structures and low degrees of compositional heterogeneity. Us-

ing these methodologies it is possible to synthesize macromolecular compounds

with control of a wide range of compositional and structural parameters includ-

ing molecular weight, molecular weight distribution, copolymer composition and

microstructure, stereochemistry, branching, and chain-end functionality. Anionic

polymerization is the archetype of a living polymerization and it embodies the

following deﬁning characteristics of living polymerizations (6).

(1) Initiation

(2) Propagation

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ANIONIC POLYMERIZATION 113

(3) Deliberate termination (no spontaneous termination)

Molecular Weight. The number-average molecular weight ( M n ) in living

anionic polymerization is a simple function of the stoichiometry and the degree of

conversion of the reaction since one polymer is formed for each initiator molecule.

The expected number-average molecular weight can be calculated, as shown in

equation 3, as a function of conversion.

M n =

grams of monomer consumed

moles of initiator

(3)

10 6 g/mol using living anionic polymerizations.

The ability to predict and control molecular weight depends critically on the ab-

sence of signiﬁcant amounts of terminating species that react with the initiator,

decrease the effective number of molecules of initiator, and thus increase the ob-

served molecular weight relative to the calculated molecular weight.

Molecular Weight Distribution. In principle, it is possible to prepare a

polymer with a narrow molecular weight distribution (Poisson distribution) by us-

ing living polymerization when the rate of initiation is competitive with or faster

than the rate of propagation (5,7). This condition ensures that all of the chains

grow for essentially the same period of time. The relationship between the poly-

dispersity and the degree of polymerization for a living polymerization is shown

in equation 4:

10 3 to

X w /

X n =

+ X n ( X n +

1) 2 ≈

X n )

(4)

The second approximation is valid for high molecular weights. The Poisson dis-

tribution represents the ideal limit for termination-free polymerizations. Thus,

it is predicted that the molecular weight distribution will become narrower with

increasing molecular weight for a living polymerization system. Broader molecu-

lar weight distributions are obtained using less active initiators, with mixtures of

initiators or with continuous addition of initiator as involved in a continuous ﬂow,

stirred tank reactor. Thus, living polymerizations can form polymers with broader

molecular weight distributions. It has been proposed that a narrow molecular

weight distribution (monodisperse) polymer should exhibit M w / M n ≤

From a practical point of view, polymers can be prepared with predictable molecu-

lar weights ranging from

≈

1.1 (8).

Block Copolymers. One of the important aspects of living polymeriza-

tions is that since all chains retain their active centers when the monomer has

been consumed, addition of a second monomer will form a diblock copolymer (9–

11). Sequential addition of monomer charges can generate diblocks such as A B,

triblocks such as A B A, A B C, and even more complex multiblock structures.

In principle, each block can be prepared with controlled molecular weight and

narrow molecular weight distribution.

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Chain-End Functionalized Polymers. In principle, the propagating car-

banionic center that remains at the end of the polymerization can react with a

variety of electrophilic species to incorporate functional groups ( X) at the chain

end, as shown in equation 5 (12–14):

(5)

Methods have been reported and tabulated for the synthesis of a diverse array of

functional end groups.

An alternative methodology for the synthesis of functionalized polymers us-

ing living anionic polymerization is the use of functionalized initiators (15,16). If a

functional group (or a suitably protected functional group) is incorporated into the

initiator, then that functional group will be at the initiating end of every polymer

molecule, as shown below:

(1) Initiation

(2) Propagation

(3) Termination

where X is the functional group in the initiating species X I −

and X P is the

-functionalized polymer. In principle, this method can quantitatively produce

functionalized polymers with controlled molecular weights and narrow molecular

weight distributions.

Star-Branched Polymers. An extension of the concept of controlled

termination reactions is the ability to prepare star-branched polymers by

post-polymerization reactions with multifunctional linking reagents as shown in

equation 6, where L is a linking agent of functionality n (17–20):

(6)

For example, termination of a living anionic polymerization with a tetrafunctional

electrophile such as silicon tetrachloride will produce a four-armed star polymer

as shown in equation 7. Given that PLi is a well-deﬁned living polymer, a branched

polymer with a predictable, well-deﬁned structure will be formed from the linking

reaction.

(7)

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ANIONIC POLYMERIZATION 115

A variety of linking agents with variable functionalities have been described and

tabulated in the literature (11,18,19).

General Considerations

Monomers. Two broad types of monomers can be polymerized anionically:

vinyl, diene, and carbonyl-type monomers with difunctionality provided by one

or more double bonds; and cyclic (eg, heterocyclic) monomers with difunction-

ality provided by a ring that can open by reaction with nucleophiles. For vinyl

monomers, there must be substituents on the double bond that can stabilize the

negative charge that develops in the transition state for monomer addition, as

shown in equation 8:

(8)

These substituents must also be stable to the anionic chain ends; thus, relatively

acidic, proton-donating groups (eg, amino, hydroxyl, carboxyl, acetylene functional

groups) or strongly electrophilic functional groups that can react with bases and

nucleophiles must not be present or must be protected by conversion to a suitable

derivative. In general, substituents that stabilize the negative charge by anionic

charge delocalization render vinyl monomers polymerizable by an anionic mecha-

nism. Such substituents include aromatic rings, double bonds, as well as carbonyl,

ester, cyano, sulfoxide, sulfone, and nitro groups. The general types of monomers

that can be polymerized anionically without the incursion of termination and

chain-transfer reactions include styrenes, dienes, methacrylates, vinylpyridines,

aldehydes, epoxides, episulﬁdes, cyclic siloxanes, lactones, and lactams. Monomers

with polar substituents such as carbonyl, cyano, and nitro groups often undergo

side reactions with initiators and propagating anions; therefore, controlled an-

ionic polymerization to provide high molecular weight polymers is generally not

possible. Many types of polar monomers can be polymerized anionically, but do

not produce living, stable, carbanionic chain ends. These types of polar monomers

include acrylonitriles, cyanoacrylates, propylene oxide, vinyl ketones, acrolein,

vinyl sulfones, vinyl sulfoxides, vinyl silanes, halogenated monomers, ketenes,

nitroalkenes, and isocyanates.

The simplest vinyl monomer, ethylene, although it has no stabilizing moiety,

can be polymerized by an anionic mechanism using butyllithium complexed with

N , N , N , N -tetramethylethylenediamine (TMEDA) as a complexing ligand. The

conversion of a double bond to two single bonds provides the energetic driving

force for this reaction. Because of the insolubility of the crystalline, high density

polyethylene formed by anionic polymerization, the polymer precipitates from

solution during the polymerization.

Solvents. The choice of suitable solvents for anionic polymerization is de-

termined in part by the reactivity (basicity and nucleophilicity) of initiators and

propagating anionic chain ends. For styrene and diene monomers, the solvents of

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