Carbocationic Polymerization.pdf

CARBOCATIONIC POLYMERIZATION

Introduction

Cationic polymerization is a chain-growth reaction where the active center is pos-

itively charged. In the case of carbocationic polymerization , the active center is

a carbenium ion. The term cationic polymerization also covers polymerizations

where the active center contains a heteroatom (eg, oxonium, sulfonium, ammo-

nium, or phosphonium ion); these are not included in this chapter. The focus will

be on carbocationic polymerization.

In order to get a perspective about the importance of carbocationic polymer-

ization in polymer science and engineering, its positioning in the overall picture

needs to be considered. The estimated total volume of polymers produced commer-

cially by cationic polymerization is around 3–3.5 million tons per annum (1). This

constitutes about 3% of the total synthetic polymer market. Polymers produced

commercially by carbocationic polymerization make up about 2% of the total poly-

mer market. On the basis of these numbers, carbocationic polymerization appears

to be insigniﬁcant.

The main products are poly- and oligoisobutylenes, poly(butenes), hydrocar-

bon resins, and poly(vinyl ethers)—these will be discussed in more detail under

Industrial Carbocationic Processes. Estimated worldwide production capacities

are listed in Table 1.

It can be seen that isobutylene-based products are the most important; they

range from viscous oils to high molecular weight elastomers, such as butyl and

halobutyl rubbers (see B UTYL R UBBER ). The latter exemplify most strikingly the

practical importance of carbocationic polymerization . Butyl and halobutyl rubbers

are used as inner tubes or inner liners in vehicle tires, because of their excep-

tional impermeability toward gases and moisture (3,4). Had butyl and halobutyl

382

Vol. 5

CARBOCATIONIC POLYMERIZATION

383

Table 1. Worldwide Production Capacities of Commercially

Produced Carbocationic Polymers a

Thousand Metric

Polymer

Tons/year

Isobutylene-based elastomers

760

(butyl, halobutyl)

Polybutenes

650

Polyisobutylenes

100

Resins

C 9 + indene–coumarone

350

C 5 + dicyclopentadiene

274

Vinyl aromatics

128

Polyterpenes

25–30

Poly(vinyl ethers)

20–25

a Ref. 2.

rubbers not been discovered, we would have to pump up our car and bicycle tires

daily as the air would escape through the rubbers used to build the body of these

tires. Butyl and halobutyl rubbers have very high damping properties and there-

fore are used in machinery mounts to reduce vibration; unique examples include

dampeners for bridges and foundations for earthquake-resistant buildings. They

also have outstanding oxidative, environmental, and chemical stability, rendering

them suitable for applications as industrial rubber goods, seals, adhesives, con-

denser caps, and pharmaceutical stoppers. An interesting application is chewing

gums (considered essential by our children as any parent can testify). Emerging

applications of polyisobutylene-based rubbers include biomedical implants that

could save lives (5). These examples highlight the importance of carbocationic

polymerization.

This article discusses the most important aspects of carbocationic polymer-

ization, with references to more detailed information for specialists.

Historical Background

Carbocations are extremely reactive. Because of their high reactivity, identiﬁca-

tion or direct observation of carbocations is extremely difﬁcult—although they

are the intermediates of many organic reactions. The existence of carbocations

became generally accepted in the 1920s and 1930s through the work of Whitmore,

Ingold, and Meerwein (6). Analytical proof was ﬁrst given by Olah in 1963 (7,8).

Reacting SbF 5 and alkyl halides in “superacid” media, he was able to prepare

carbocations stable enough for 1 H NMR characterization. He proposed to use the

term carbenium ion for classical trivalent sp 2 carbocations (such as CH 3 + ) and

carbonium ion for the pentavalent carbocation (CH 5 + ) (7,8). This nonclassical en-

tity has a two-electron, three-center-bound pentacoordinated carbon; ammonium

(NH 4 + ), oxonium (R 3 O + ), or sulfonium (R 3 S + ) ions can be considered as similar

“nonclassical” structures.

The ﬁrst example of carbocationic polymerization, the resiniﬁcation of tur-

pentine oil by sulphuric acid (9), dates back a couple of centuries. Other examples

384

CARBOCATIONIC POLYMERIZATION

Vol. 5

from the 1800s include the polymerization of pinene with SnCl 4 and BF 3 (10), tur-

pentine with BF 3 (11), and styrene and isobutylene with sulphuric acid (11,12).

Hunter and Yohe laid out the ﬁrst mechanism of carbocationic polymeriza-

tion in 1933 (13). According to their proposal, the metal halide–monomer interac-

tion leads to the formation of a “zwitterion” (the German word zwitter stands for

“between”):

MtX n +

CH 2 C(CH 3 ) 2 →

MtX n

CH 2 (CH 3 ) 2 C +

Polymerization then proceeds by reaction of monomer with the carbenium

ion. In this system, the metal halide was considered the “catalyst.” For thermo-

dynamic reasons, this mechanism was criticized (14,15) and later abandoned as

Polanyi’s school put forth the idea of coinitiation on the basis of the study of the

isobutylene–TiCl 4 system (16). Polanyi suggested that the true initiator in this

system would be derived from the reaction between a cationogen such as water,

and TiCl 4 , the coinitiator (eq. 1):

TiCl 4 +

H 2 O

CH 2 C(CH 3 ) 2 →

CH 2 (CH 3 ) 2 C + TiCl 4 OH −

(1)

This reaction can be written in a more generalized form (eq. 2):

MtX n +

I B

CH 2 C(CH 3 ) 2 →

I CH 2 (CH 3 ) 2 C + MtX n B −

(2)

where I represents the true initiating species, such as a proton or carbenium ion,

and B represents a basic group, eg, a hydroxide or a halide. The metal halide

coinitiators are still called “catalysts” in the industry, as the processes used to-

day were originally developed many decades ago. It would be helpful to all if the

scientiﬁcally accepted terms were more commonly used.

Classical carbocationic polymerization is very rapid and therefore difﬁcult to

control, and is riddled with side reactions and incomplete monomer conversion. In

contrast, living (or controlled ) carbocationic polymerization opened new avenues

for macromolecular engineering, ie, the design and synthesis of well-deﬁned struc-

tures with efﬁcient processes (17–20).

Monomers for Carbocationic Polymerizations

The most common monomers in carbocationic polymerization are alkenes, often

termed “vinyl” monomers. Carbocationic polymerization of vinyl monomers pro-

ceeds by the electrophilic attack of the initiating and/or propagating carbenium

ion on the double bond of the monomer. In order to sustain the chain reaction of

propagation in competition with termination or transfer reactions, the carbenium

ion has to be sufﬁciently stable and the monomer has to be sufﬁciently nucleophilic

to ensure long enough lifetime of the active center to propagate.

Stability of Carbenium Ions. In terms of thermodynamic stability, car-

benium ions can be ranked on the basis of gas-phase hydride afﬁnity, heats of

ionization, ionization equilibrium in solution, and S N 1-solvolysis rate (21). The

“stability scales” obtained by these different methods correlate reasonably well

Vol. 5

CARBOCATIONIC POLYMERIZATION

385

Table 2. Ranking of Carbenium Ions by Stability

R H 0 , kJ/mol

log k

Carbenium ion

(R H → R + + H − ) a

(R Cl, EtOH, 25 ◦ C) b

CH 3 +

1306

—

CH 3 CH 2 +

1143

—

CH 3 CH + CH 3

1034

−

10.8

Ar CH 2 +

—

−

9.4

(CH 3 ) 3 C +

963

−

7.1

Ar CH + CH 3

963

− 6.8

(CH 3 ) 3 CH 2 C(CH 3 ) 2 +

—

− 5.6

(CH 3 ) 2 C CH CH 2 +

940

− 4.6

Ar C(CH 3 ) 2 +

921

− 3.4

CH 3 O Ar CH + CH 3

—

− 1.7

(Ar) 3 C +

—

0.8

[CH 3 O Ar] 2 CH +

—

1.8

a Hydride afﬁnity of carbocations (21). Original data from Ref. 22.

b Solvolysis rate constant of the corresponding alkyl chlorides in ethanol (21). Original data

from Ref. 23.

(21). Table 2 lists selected carbenium ions in order of increasing stability, along

with their hydride afﬁnity and the solvolysis rate constants of the corresponding

alkyl halides.

It can be seen that as the stability of carbenium ions increases as a result of

inductive or resonance effects, their reactivity (electrophilicity) decreases.

Nucleophilicity of Monomers. Figure 1 shows cationically polymeriz-

able monomers in order of increasing nucleophilicity. The numbers listed are nu-

cleophilicity parameters ( N ) developed by Mayr (24,25). N values were derived

by measuring the rate of reaction between diarylcarbenium ions and various

monomers using UV–vis spectroscopy and are discussed more later in this article.

Optimum combination of carbocation stability and monomer nucleophilic-

ity is necessary for effective polymerization. Indeed, isobutylene positioned in the

middle of both the carbocation stability and monomer nucleophilicity scales can be

polymerized effectively to yield high molecular weight polymers, while the other

monomers listed above mostly yield low molecular weight products by carboca-

tionic polymerization.

Examples of Active Monomers. A vinyl monomer can undergo carboca-

tionic polymerization only if the double bond is the most basic site of the monomer.

For example, acrylates and vinyl acetate cannot be polymerized cationically

OEt

N :

−2.63

−1.15

0.78 1.07

1.12

2.39

3.9

5.02

Fig. 1. Ranking of monomers by nucleophilicity.

386

CARBOCATIONIC POLYMERIZATION

Vol. 5

H 2 C

H 3 C

H 2

Fig. 2. Release of steric hindrance by isomerization in the carbocationic polymerization

-pinene.

donor monomers is N -vinylcarbazole. In these monomers the combination of an

unsaturation with an electron-donating heteroatom leads to very easy polymer-

ization (26); the monomers are highly nucleophilic and form very stable carbenium

ions (see Table 2 and Fig. 1) As a consequence, their polymerization can be initi-

ated by initiators that would be inefﬁcient with less-reactive monomers. However,

the high stability of their carbenium ions restricts their ability to copolymerize

with monomers of lower nucleophilicity. Ethylene, propene, or n -butenes only give

oligomers because of the low stability/high reactivity of the corresponding carbe-

nium ions. Hydride transfer from oligomers prevents the formation of high molec-

ular weight product. For example, the primary cation formed from ethylene will

abstract a hydride ion from the secondary carbon of the oligomer.

In addition to carbocation stability and monomer nucleophilicity, steric

factors are also important. For example 1,1-diphenylethylene, a very reactive

monomer can only be dimerized, because of the formation of a sterically buried

carbenium ion (27). Other monomers that cannot be polymerized to high polymers

because of steric congestion in the transition state are 2,4,4-trimethyl-1-pentene,

1,2-diphenylethylene (stilbene), 3-methylindene, and 2-methylenenorbornane.

In some monomers, such as

-pinene (Fig. 2) or methylene–norbornene, iso-

merization of the initially formed carbenium ion helps overcome the steric restric-

tion (27,28):

Monomers containing more than one double bond (conjugated or nonconju-

gated dienes and trienes) with similar reactivity lead to ill-deﬁned mixtures of

cyclized, branched, or cross-linked polymers (27).

Monomers active in carbocationic polymerization have been periodically re-

viewed over the years (26–30). Isobutylene remains the monomer with optimum

combination of nucleophilicity, carbocation stability, and steric factors, yielding

high molecular weight elastomers.

Initiating Systems

Carbocationic polymerization can be initiated by a wide variety of chemical and

physical methods. Examples are initiation by Bronsted acids, Lewis acids, Lewis

because the carbonyl oxygen of the ester group is more basic than the vinyl

group. If the stability of the chain-carrier carbenium ion forming from a particular

monomer is too high or too low, it cannot undergo carbocationic polymerization;

N -vinyl amines readily react with cationic initiators, but the formation of stable

ammonium salts prevents their polymerization. Alkyl vinyl ethers can be poly-

merized readily, although the oxygen is certainly more basic than the vinyl group.

However, in this case protonation of the vinyl group leads to the formation of a

carbocation stabilized via charge delocalization. Another example of these n ,

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: