Radical Polymerization.pdf

RADICAL POLYMERIZATION

Introduction

Free radical polymerization is one of the most studied chemical processes. This

is not surprising, because free radical polymerization is carried out on a large in-

dustrial scale; the world production of polymers by this method is in the range of

100 million tons per year. This enormous production corresponds to nearly 50% of

all synthetic polymers. The importance of free radical polymerization is likely to

increase in the coming years as new controlled/living radical polymerization tech-

niques ﬁnd industrial applications. Free radical polymerization has been known

for more than 60 years. As far back as the 1950s, the basic theory and compre-

hension of radical polymerization was established (1–5). It included the thorough

understanding of the mechanism of the process, encompassing the chemistry and

kinetics of the elementary reactions involved, with the determination of the corre-

sponding absolute rate constants, the structure, and concentrations of the growing

species, as well as a correlation of the structure of the involved reagents and their

reactivities. Similar to other chain reactions, the radical polymerization process

may be subdivided into initiation, propagation, transfer, and termination steps as

depicted in Scheme 1. The radicals necessary to initiate the chain process have to

be generated in situ in most cases.

A multitude of monomers can be used in radical polymerization and it is

impossible to list them all. The fundamental feature of the vast majority of

monomers in question is the vinylic double bond. Thus the simplest monomer

is ethylene, which, however, can only be polymerized under high pressure and

high temperature to the commercially very important low density polyethy-

lene. Common monomers are monosubstituted or unsymmetrically (1,1-) disubsti-

tuted ethylenes, CH 2 CHR or CH 2 CRR . The substituents R and R determine

the properties of resulting polymers and also the kinetic and thermodynamic

359

360 RADICAL POLYMERIZATION

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Scheme 1.

polymerizability of monomers. Polymerizable monomers under radical condi-

tions include styrene and its substitution products, dienes, mono- and disubsti-

tuted ethylene derivatives, such as vinyl acetate, acrylonitrile, (meth)acrylates,

(meth)acrylamides, and vinyl chloride, and a variety of halogenated alkenes.

Carbon–heteroatom (N,O) double bonds only rarely polymerize via radical poly-

merization (eg, CF 3 CHO (6)).

The essential polymerization step is a repetitive free radical addition to

the monomer double bonds, forming chains of carbon atoms constructed of units

( CH 2 CHR ) or ( CH 2 CRR ) linked together predominantly head-to-

tail (the substituted carbon atom is denoted as the head). Since the free radical

is essentially sp 2 hybridized, very limited control of tacticity is observed, with

many monosubstituted monomers providing atactic polymers and disubstituted

like methacrylates showing (thermodynamic) preference for syndiotacticity.

Monomers with exocyclic double bond can polymerize via ring-opening poly-

merization and incorporate heteroatoms to the backbone. The systematic study of

ring-opening polymerization started in the 1950s (7). Ring-opening polymerization

has several characteristic features, and it can produce a wide variety of polymers,

many of which have found important industrial applications (8,9). A characteristic

feature of ring-opening polymerization is the smaller volume shrinkage of cyclic

monomers during polymerization in comparison with vinyl monomers (10). Con-

sequently, cyclic monomers are better as adhesives, curing resins, and molding

and ﬁlling materials. Vinyl monomers show volume shrinkage about two times

larger than the shrinkage of cyclic monomers of identical molecular weight. This

is due to the compact structures of cyclic monomers and the compensation of the

closeness of monomer molecules by ring opening during polymerization.

Most cyclic monomers polymerize ionically, but there are several monomers

undergoing radical ring-opening polymerization (RROP). The copolymerization

of radically polymerizable cyclic monomers with common vinyl monomers can

add functions to common vinyl polymers. Scheme 2 illustrates the typical pat-

terns of RROP, depicting vinyl-substituted cyclic and exo -methylene-substituted

monomers. The radical species formed by the radical addition to the double bond

undergoes ring opening to form a propagating radical species. RROP is commonly

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RADICAL POLYMERIZATION 361

Scheme 2.

accompanied by so-called vinyl polymerization without ring opening of the cyclic

structure.

Although fewer monomers undergo RROP than cationic, anionic, and coor-

dination ring-opening polymerizations, novel monomers undergoing RROP are

steadily being developed, some of which were recently examined as monomers for

living radical polymerization (11). RROP can provide a wide variety of functional

polymers with industrially promising radical polymerization.

1,6-Dienes having vinylic double bonds that are not polymerizable can form

polymers consisting of repeat units made up of ﬁve- or six-membered rings.

Such processes are called cyclopolymerization (qv), and typically the intramolec-

ular cyclization followed by intermolecular addition of the cyclized radicals (see

Scheme 3) is orders of magnitudes faster than individual propagation via the

carbon–carbon double bonds (12–15).

Typical cyclopolymerizable monomers include N-substituted dimethylacry-

lamides where the nonpolymerizability of the N,N-disubstituted methylacry-

lamido group results in the generation of ﬁve-membered cyclic structures

involving head–head linkages. In cases where one of the carbon–carbon

double bonds is severely sterically hindered, cyclopolymerization (qv) may

not occur and a linear polymer with pendant double bonds is formed

(16). Other examples of monomers that undergo cyclopolymerization are 4-

( N,N -diallylamino)pyridine, 5,6-di- O -isopropylidene- D -mannitol, 1,5-hexadiene,

N -methyl- N -methallyl-2-(methoxycarbonyl)-allyl amine, and dipropargyl com-

pounds (13). One of the most efﬁcient ways to achieve structural control in radical

polymerization is via cyclopolymerization. For example, the free radical cyclopoly-

merization of a diacryloyl monomer involving a chiral template results in the

generation of stereogenic centers and four different stereoisomers (17).

Another essential component of free radical polymerization systems are ini-

tiators which should provide initiating free radicals via homolytic cleavage of co-

valent bonds, redox reaction, photochemical or other stimuli. In addition, various

Scheme 3.

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compounds which can regulate molecular weights (transfer agents) or rates

(retarders/inhibitors) can be used. Free radical polymerizations tolerate many

protic impurities such as water or alcohols but require absence of oxygen, which

act as a powerful inhibitor. Various solvents can be used which should not (unless

desired) participate in transfer.

Initiation

The initiation process constitutes the ﬁrst reaction step in free radical polymer-

ization, leading to the generation of (primary) radicals. The kinetics of the initia-

tion process, ie its rate and effectiveness, are of fundamental importance in both

theoretical studies and commercial applications. Commercial procedures mainly

rely on the formation of primary radicals via thermal decomposition processes us-

ing azo- and peroxy-type compounds. Investigative kinetic studies are—to a large

extent—carried out using photoinitiators, which decompose upon irradiation with

UV or visible light. The main reason for this choice is the possibility to deﬁne exact

start and end times of the initiation and subsequently the polymerization process.

The decomposition scheme (eq. 1) describing the generation of radicals is

common to both thermal and photoinitiators.

(1)

The measurable decrease of the initiator concentration [I] in a polymerizing

systems with time is given by

−

d[I]

d t =

k d [I]

(2)

Integration of equation 2 leads to equation 3, an expression which describes

the decreasing initiator concentration as a function of time.

[I]

[I] 0 e − k d t

(3)

However, the rate of the formation of initiating primary radicals is of greater

interest in kinetic studies. The rate of generation of radicals that are capable of

initiating the polymerization process, R d , is described via the following general

ﬁrst-order rate law:

R d =

d[I • ]

d t =−

2 f d[I]

d t =

2 fk d [I]

(4)

where k d corresponds to the rate coefﬁcient of initiator decomposition and f de-

notes the initiator efﬁciency (see below). It should be noted that in the case of

photoinitiation, k d is a composite of various variables.

In order to initiate the polymerization process via reaction with a monomer

unit, the generated primary radicals, I 1 • and I 2 • , have to leave the solvent cage

that surrounds them. The ability of the primary radicals to leave the solvent cage

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RADICAL POLYMERIZATION 363

indicates that every generated primary radical escapes the solvent cage and sub-

sequently initiates polymerization. Typical values of f are between 0.5 and 0.8,

depending on the viscosity of the reaction medium, indicating that the escaping

process is diffusion controlled. It should be noted that in the case of an unsymmet-

rical initiator molecule, I 1 • and I 2 • do not necessarily display the same reactivity

toward the monomer unit (18–20). Hence, the initiation process may be described

by equations 5 and 6:

(5)

(6)

where I 1 • and I 2 • represent initiator fragment 1 and 2, respectively, M indicates a

monomer unit, R 1 • corresponds to a macroradical of chain length 1, and k i (1) and

k i (2) refer to the individual initiation rate coefﬁcient of the respective fragments.

The rate of initiation, R i , is given by equation 7:

R i =

d[R 1 • ]

d t =−

d[I 1 • ]

d t −

d[I 2 • ]

d t =

k (1)

[M][I 1 • ]

k (2)

[M][I 2 • ]

(7)

[I • ]/2, the rate coefﬁcient of initiation, k i , is a composite

of the individual rate coefﬁcients of initiation for the initiator fragments I 1 • and

I 2 • .

[I 2 • ]

R i =

k i [M][I • ] with k i =

k (1)

k (2)

(8)

Thermal Initiation. Thermally decomposing initiators (mainly) fall into

two classes: azo- and peroxy-type molecules. The general structures of azo- and

peroxy-initiators are represented by 1 and 2 respectively.

An important quantity of a thermal initiator is its half-life t 1 / 2 (at a certain

temperature), given by equation 9. The half-life is the time period during which

half of the initiator molecules initially present is decomposed.

t 1 / 2 =

ln2

k d

(9)

unreacted and to start the polymerization process is quantiﬁed by the initiator

efﬁciency f , with theoretical values between zero and unity. Not all generated pri-

mary free radicals initiate polymer growth. Shortly after decomposition, the free

radicals are very close to each other and recombination can occur. In addition,

they can also react in alternative ways before they can react with a monomer

unit. An efﬁciency of zero corresponds to no initiation taking place, whereas f

Because [I 1 • ]

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