Lecture 6

Question 1

What is a free radical?

Answer 1

Free radicals are independently existing species that possess an unpaired electron and normally are highly reactive with short lifetimes. A carbon-based free radical typically has sp2 hybridization and
has a general structure that can be represented in several ways as shown in the notes

Question 2

What is radical polymerization?

Answer 2

It is a CHAIN polymerization that has been performed since the early days of polymer science and remains the most widely used and versatile type of polymerization for unsaturated monomers (containing
C=C bonds) because almost all are susceptible to this form of chain polymerization. Each polymer molecule grows by sequential additions of molecules of unsaturated monomer to a terminal free-radical reactive site known as an active centre. The growing chain radical attacks
the π-bond of a molecule of monomer causing it to break homolytically. one electron from the π-bond joins with
the unpaired electron from the terminal carbon atom of the chain radical, creating a new bond. the remaining π-bond electron moves to the other C=C carbon atom, which becomes the new active radical site. Hence, upon every addition of a monomer, the active centre is transferred to the newly-created chain end.

Question 3

What are the two types of radical polymerization?

Answer 3

i) Conventional free-radical polymerizations: Discussed below

ii) reversible deactivation radical polymerizations: out of the scope of our course

Question 4

What are conventional free-radical polymerizations?

Answer 4

Free-radical polymerization is the most widely practised method of radical polymerization, and is used almost exclusively for the preparation of polymers from monomers of the general structure
CH2=CR1R2. In common with other types of chain polymerization, the reaction can be divided into three distinct basic stages: initiation, propagation and termination. A further process, known as chain transfer, can occur in all chain polymerizations and
often makes a very significant contribution

Note that higher reactivities occur for monosubstituted alkanes (compared with disubstituted) due to steric hindrance in propagation

Question 5

What is the intention step

Answer 5

Initiation involves the creation of the free-radical active centre and usually takes place in two steps. The first is the formation of free radicals from an initiator and the second is the addition of one of these
free radicals to a molecule of monomer.

What are the two principles in which free radicals are formed?

Question 6

What are the two principles in which free radicals are formed?

Answer 6

i) homolytic scission (i.e. homolysis) of a single bond in which the two bonding electrons go one each onto the two atoms
associated with the original bond (thereby always producing two free radical species)

(ii) transfer of a single electron to or from an ion or molecule (e.g. in redox reactions) which often are termed single-electron transfer processes, some of which produce only a single free-radical species.

Question 7

What is thermolysis?

Answer 7

Homolysis can be affected by the application of heat (Δ) and there are many compounds that contain weak bonds which undergo thermolysis at useful rates above about 50 °C. Compounds containing peroxide (–O–O–) or azo (–N=N–) linkages are particularly useful as initiators and
undergo hemolysis as shown in notes. where R1 and R2 can be aromatic, alkyl or H (often with R1=R2) and R is aromatic or alkyl

Some specific thermal initiators are shown in the notes.

Such homolysis reactions are widely used to initiate free-radical polymerizations in the convenient temperature range of 60–90 °C. Many of the primary radicals produced undergo further breakdown before reaction with monomer, for example, β-scissions as shown in notes.

Question 8

What is photolysis?

Answer 8

Homolysis can also be brought about by the action of radiation (usually ultraviolet) rather than heat, i.e. by photolysis.

photochemical initiators that decompose
efficiently when exposed to ultraviolet radiation, such as benzophenone and benzoin derivatives are shown in notes.

An advantage of photolysis is that the formation of free radicals begins at the instant of exposure and ceases as soon as the light source is removed.

Question 9

How are redox reactions important in making free radicals?

Answer 9

Redox reactions often are used when it is necessary to perform free-radical polymerizations at low temperatures.

Two redox initiation systems will be considered to exemplify these types of reactions:

i) a Fe2+ ion donates an electron to the OH oxygen atom of cumyl hydroperoxide causing the adjacent O–O peroxide bond to break homolytically; the OH oxygen atom thereby receives a second electron and becomes negatively charged (giving the hydroxyl ion) and the remaining oxygen atom from the peroxide bond receives
the other peroxide bonding electron (giving the cumyloxy radical) as shown in notes. Similar chemistry occurs with all peroxides and hydroperoxide and can be activated using many other transition
metals.

ii) In this case, one electron from the lone pair of electrons (negative charge) on the bisulphite ion is donated to one of the peroxide oxygen atoms, causing the peroxide bond to break homolytically,
each of the peroxide oxygen atoms receiving one of the two bonding electrons in the process (hence, one finishes with two electrons, to yield the sulphate ion, and the other with an unpaired electron, to yield the sulphate radical-anion); both the sulphate radical-anion and the bisulphite radical are capable of initiating free-radical polymerization (This is shown in the notes). This is an example of a completely inorganic redox initiation system that is useful for polymerizations carried out in water.

Question 10

What is an active centre?

Answer 10

An active centre is created in the second step of initiation, when a free radical R*
generated from the initiation system attacks the π-bond of a molecule of monomer.

Two modes of addition are possible (shown in notes):

Mode (I) predominates because attack at the methylene (CH2) carbon is less sterically hindered and yields a product radical that is more stable because of the effects of the attached substituent group X (which provides steric stabilization of the radical and often also contributes mesomeric stabilization).

Not all of the radicals formed from the initiator are destined to react with the monomer. Some are lost in side reactions such as those for benzoyl peroxide shown in notes

Question 11

What are the modes of propagation?

Answer 11

There are two possible modes of propagation (as shown in notes):

Mode (I) dominates as it has less steric hindrance.

If a head-to-head addition occurs, it will be followed immediately by a tail-to-tail addition to generate the more stable active centre which will then continue to propagate principally via headto-tail addition. The extent of head-to-head and tail-to-tail additions is immeasurably small for most monomers; mode (II) only contributes significantly for the few monomers for which X is small (e.g.
similar in size to an H atom) and provides no mesomeric stabilization* (e.g. in free-radical polymerization of CH2=CHF). Thus, for the purposes of a more general description of radical polymerizations,
it is entirely reasonable to neglect mode (II) and to assume that propagation proceeds exclusively by mode (I) head-to-tail addition and can be represented in its most general form shown in the notes

*Mesomneric is basically resoance stability.

Question 11

What is propagation?

Answer 11

Propagation involves the growth of a polymer chain by rapid sequential addition of monomer to the active centre. The time required for each monomer addition typically is of the order of a millisecond and so several thousand additions can take place within a few seconds.

Question 12

What is termination and its types?

Answer 12

In the termination stage, the active centre is destroyed irreversibly and propagation ceases.

There are two common termination mechanisms:

i) Combination involves the coupling together of two growing chains to form
a single polymer molecule as shown in ntoes. where x and y are the degrees of polymerization of the respective chain radicals prior to the combination reaction. Note that a single dead polymer molecule (with degree of polymerization=x+y) is produced with an initiator fragment (the R group) at both chain ends and that the radical coupling reaction also gives rise to a ‘head-to-head’ linkage.

ii) disproportionation involves the abstraction of a hydrogen atom from the second to last C atom of one growing chain radical by another, the remaining electron from the C–H bond joining with the unpaired electron on the terminal C atom of that chain to create a terminal π-bond (as shown in notes). Thus, two dead polymer molecules are formed (with degrees of polymerization x and y, respectively), one with a saturated end group and the other with an unsaturated end group, and both with an initiator fragment at the other chain end.

Question 13

Each termination mechanism is dominant under what conditions?

Answer 13

the combination tends to dominate termination in polymerizations of vinyl monomers (CH2=CHX), whereas disproportionation dominates in polymerizations of α-methylvinyl monomers (CH2=C(CH3)X). This is because the α-CH3 group provides an additional
three C–H bonds from which a H atom can be abstracted as shown in the notes.

Examples: it is found that polystyrene chain radicals terminate principally by combination whereas poly(methyl methacrylate) chain radicals
terminate predominantly by disproportionation, especially at temperatures above 60 °C.

Question 14

What are chain transfer reactions?

Answer 14

Chain transfer reactions occur in most chain polymerizations and are reactions in which the active centre is transferred from the active chain end to another species in the polymerization system. In their most generic form, chain transfer reactions in radical polymerizations are shown in the notes. where T and A are fragments linked by a single bond in a hypothetical molecule TA.

The chain radical abstracts T (often a hydrogen or halogen atom) from TA causing homolytic scission of the T–A bond to yield a dead polymer molecule and the radical A*, which if sufficiently reactive may
then react with a molecule of monomer to initiate the growth of a new chain.

Question 15

What are examples of chain transfers with small molecules?

Answer 15

Chain transfer can occur with molecules of initiator, monomer, solvent and deliberately added transfer agents (All of which are small molecules. Examples of such reaction are shown in the notes. Clearly, each of these chain transfer reactions leads to death of a propagating chain:

i) The first chain transfer reaction is another example of induced decomposition of a peroxide.

ii) Chain transfer to monomer is exemplified by the commercially important
monomer, vinyl acetate; the product radical reinitiates polymerization but also possesses a terminal C=C double bond which itself can undergo polymerization, leading to formation of long-chain branches in the resulting polymer.

iii). In the case of toluene (a good solvent for many monomers and polymers), the chain radical abstracts a benzylic H atom (rather than a H atom from the benzene
ring) because this yields the benzyl radical which is stabilized mesomerically (resonance) by delocalization of the unpaired electron around the benzene ring (which cannot occur for the aryl radicals produced by H-abstraction from the benzene ring).

iv) The final example is of a molecule, CBr4, which is so active in chain transfer that it can only be tolerated in very small quantities, otherwise the polymer chains. formed would be too short; such compounds are called transfer agents (or modifiers or regulators) because they can be used effectively in very small quantities to control the degree of polymerization.

Question 16

What is chain transfer to polymers?

Answer 16

Chain transfer to polymer occurs in the polymerization of some monomers and leads to the formation of branched polymer molecules, so it can have a major effect on the skeletal structure and properties of the polymers produced.

Intramolecular chain transfer to polymer (or backbiting) reactions give rise to short-chain branches, whereas long-chain branches result from intermolecular chain
transfer to polymer

There are examples discussed (ALL ARE IMPORTANT) in the next three slides

Question 17

What is the first example of chain transfer to polymers?

Answer 17

The polymerization of ethylene at high temperatures and high pressure (used to
manufacture low-density polyethylene, LDPE) is a classic example of the importance of chain transfer to polymer; the chemistry is shown in Figure 4.2 (shown in notes). Note that the so-called short-chain branches resulting from backbiting are in fact n-butyl, ethyl and 2-ethylhexyl side groups. The short- and long-chain branches in LDPE greatly restrict the ability of the polymer to crystallize and compared to linear polyethylene (see Sections 6.4.7 and 6.5.6), LDPE has a much lower melting point (105–115 °C cf. 135–140 °C) and degree of crystallinity (45%–50% cf. about 90%). The long-chain branches also have a major effect on the rheology of LDPE in the molten state.

Question 18

What is the second example of chain transfer to polymers?

Answer 18

For other monomers, chain transfer to polymer depends on the nature of the monomer structure and may proceed via the abstraction of an atom (usually H) from either the backbone or a substituent
group. For example, in radical polymerizations of acrylic monomers (CH2=CHCO2R where R=H or alkyl), chain transfer to polymer proceeds via H-abstraction from the tertiary backbone C–H
bonds. This occurs because the tertiary radical produced is significantly more stable than the ‘normal’ secondary propagating radical (i.e. the additional group attached to the tertiary carbon atom
provides significant steric stabilization of the radical). The intermolecular reaction is illustrated for methyl acrylate in Figure 4.3 (shown in notes). The H-abstraction can also occur intramolecularly via six-membered cyclic transition states (in backbiting reactions analogous to those shown in Figure 4.2). In fact, as for ethylene, there is evidence that the intramolecular reaction in the polymerization of acrylic monomers is far more frequent than the intermolecular reaction.

Question 19

What is the thrid example of chain transfer to polymers?

Answer 19

In radical polymerization of vinyl acetate, chain transfer to polymer proceeds via H-abstraction from the side group, as shown in Figure 4.4. The reason for this difference compared to methyl acrylate (which is an isomer of vinyl acetate) is the position of the carbonyl group, which for vinyl
acetate is attached to the methyl group; thus the product radical from H-abstraction at the methyl group is mesomerically (resonance) stabilized by electron delocalization with the carbonyl π-bond (this stabilization is not possible for the product from H-abstraction at a backbone tertiary C–H bond). Again chain transfer to polymer can also proceed intramolecularly, though the chemistry will be slightly
different from that shown for ethylene because the H-abstraction is from the side group.

Polymers from disubstituted monomers of structure CH2=CR1R2 have no tertiary backbone C–H bonds, and so chain transfer to polymer proceeds only via the side group and then only if it contains labile H atoms. For some monomers, therefore, chain transfer to polymer is negligible (e.g. in polymerization of methacrylic monomers which have R1=CH3 and R2=CO2R where R=H, alkyl or aryl)

Question 20

For polymerization, what are the two quantities that are of paramount importance?

Answer 20

For any polymerization, two quantities are of paramount importance: the rate at which the monomer is polymerized and the degree of polymerization of the polymer produced. The kinetics of free-radical
polymerization can be analysed to obtain equations that predict these quantities

Question 21

How are kinetics used to find the rate of polymerization?

Answer 21

Look at written notes

Long text:

At the start of the polymerization, the rate of formation of radicals greatly exceeds the rate at which they are lost by termination. However, [M] increases rapidly and so the rate of loss of radicals by termination increases. A value of [M] is soon attained at which the latter rate exactly equals the rate of radical formation. The net rate of change in [M] is then zero and the reaction is said to be under steady-state conditions. In practice, most free-radical polymerizations operate under steady-state
conditions for all but the first few seconds. If this were not so and [M] continuously increased, then the reaction would go out of control and could lead to an explosion. Assuming that chain transfer reactions have no effect on [M*] (i.e. that the product radicals from any chain transfer reactions
immediately reinitiate a new chain), steady-state conditions are defined by (back to written notes)

Question 22

Non-linear radical polymerizations

Answer 22

In our course we will consider radical copolymerizations that involve comonomers which have more than one C=C bond. Such copolymerizations lead to the formation of non-linear polymers and are used in the production of a wide range of materials from crosslinked particles for gel permeation chromatography to crosslinked matrices for fibre-reinforced composites

Question 23

What are crosslinking polymers?

Answer 23

a monomer that has more than one C=C bond, leads to the formation of branched
and network polymers. An example of this is shown in figure 4.19 (in notes) where copolymerization of a mono-olefin with a crosslinking monomer occurs.

The junction points in the resulting non-linear polymer are provided by the
linking groups between the C=C bonds in the crosslinking monomer. In most cases, crosslinked. polymers are required and can be formed by using only a low level of the crosslinking monomer.

Because crosslinked polymers are intractable, they need to be produced directly in the form required for their application and so it is usual to carry out crosslinking polymerizations in the
absence of a solvent (e.g. by bulk, suspension or emulsion polymerization). In some cases, highly branched (soluble) polymers are required (e.g. viscosity modifiers) and can be produced using crosslinking monomers if a chain transfer agent is included at high enough level to prevent formation of a network.

Question 24

What controls the degree of crosslinking?

Answer 24

The degree of crosslinking is controlled by the mole fraction of the crosslinking monomer, the differences in reactivity of the various types of C=C bond in the monomers employed and the overall conversion.

Question 25

What are examples of some common cross-liking monomers?

Answer 25

The chemical structures of some common crosslinking monomers are shown in Figure 4.20 and described below:

Divinylbenzene is widely used to synthesize crosslinked polystyrene beads, which are applied in gel permeation chromatography and other fields like ion exchange resins and solid-phase peptide synthesis. The beads are often modified chemically to introduce functional groups to the polystyrene’s phenyl rings. However, commercial divinylbenzene is not ideal due to being a mixture of isomers, including about 20% of ethylstyrene with only one C=C bond. The reactivity of divinylbenzene’s second C=C bond diminishes after the first reacts, leading to inconsistent crosslinking.

Ethylene glycol dimethacrylate is a better alternative since its C=C bonds are far enough apart not to interfere with each other, making it popular in acrylate and methacrylate polymerizations and hydrogel production (e.g. contact lenses).

Bisphenol-A glycidyl methacrylate (BIS-GMA) is crucial in dental fillings, while methylene bisacrylamide is widely used in polyacrylamide gels and hydrogel synthesis.

Crosslinking monomers of
higher functionality (such as (vi) and (vii) in Figure 4.20) are used when higher degrees of crosslinking are required. Allyl methacrylate ((iv) in Figure 4.20) is an example of a graftlinking monomer,
the characteristic feature of which is the presence of C=C bonds with very different reactivities; in allyl methacrylate, the methacrylate C=C bond is far more reactive than the allyl C=C bond, which has very low reactivity. Graftlinking monomers are inefficient crosslinkers because relatively few of the low reactivity C=C bonds polymerize; however, the reactive C=C bond copolymerizes effectively to leave many unreacted pendant allyl C=C bonds on the polymer chains, some of which do
react with newly added monomer. This is used to provide grafting at interfaces between different polymers, e.g. to provide strong interfaces between phases in core-shell particles produced by emulsion polymerization

Question 26

What happens in the Branching and Crosslinking during Polymerization of 1,3-Dienes?

Answer 26

When 1,3-diene monomers (CH₂=CR—CH=CH₂) polymerize, they form unsaturated polymers with one C=C bond per monomer unit (similar to Figure 4.21). As the polymerization progresses, unreacted monomers and unsaturated polymer coexist, and the chances increase that the C=C bonds in the polymer will react with unreacted monomers, leading to branching and eventually crosslinking. Most repeat units in the polymer are cis-1,4 or trans-1,4, which are less reactive due to bulky chain substituents on the C=C bond. However, the 1,2 (and 3,4) repeat units, though fewer in number, have more reactive pendent C=C bonds and contribute significantly to branching and crosslinking.

In addition to copolymerization, chain transfer to polymer makes a significant contribution due to the ease of H-abstraction from any of the four labile allylic C–H bonds present in each repeat unit, with an example shown in notes

Question 27

How can you control the branching and crosslinking arising from copolymerization?

Answer 27

The branching and crosslinking arising from copolymerization and chain transfer to polymer needs to be controlled, which is achieved in two ways. Inclusion of a highly active transfer agent (usually a mercaptan) limits the extent to which chains propagate through more than one polymer C=C bond because chain transfer to the transfer agent reduces the probability of a propagating chain growing long enough to react with more than a few polymer C=C bonds. The other important control is simply to restrict the monomer conversion to values low enough (typically around 30%) to limit the formation of crosslinked polymer.

Question 28

Free-radical ring-opening polymerization

Answer 28

For efficient ring-opening, the cyclic monomers should have a terminal
C=C bond that acts as the site for initial attack, sufficient ring strain to promote ring-opening, and the ability to form thermodynamically stable C=C or C=X double bonds, where X is a heteroatom.
Ring-opening competes with ‘normal’ free-radical polymerization of the C=C bond and so high temperatures (e.g. 80–150 °C) typically are used to promote the ring-opening mechanism. Peroxide
initiators tend to be used since they have more appropriate half-lives than azo initiators at these temperatures. Ring-opening also is promoted by Z groups which stabilize the propagating radical (e.g. Ph, CN, CO2R).

Question 29

What are the mechanisms of free-radical ring-opening polymerization?

Answer 29

The mechanisms of the two main types of free-radical ring-opening polymerization are shown generically in Figure 7.3 (in the notes):

Ring-opening polymerization of cyclic ketene acetals (type I monomers) is most important and produces polyesters, the extent of ring-opening versus ‘normal’ C=C bond polymerization depending upon monomer structure and polymerization temperature, as is exemplified by the data in Table 7.2 (shown in notes). Thus, free-radical ring-opening polymerization is a convenient method for synthesis of
aliphatic polyesters and is particularly effective for monomers with x=4 or 5. However, copolymers are formed when both modes of propagation occur, e.g. polymerization of the monomer with x=2
at lower temperatures produces (a structure shown in notes). where p and q are the mole fractions of the two types of repeat unit (from Table 7.2, at 60 °C
p≈q≈ 0.5).

Polyamides also can be produced in this way (as shown in notes). The polymerizationgives 100% ring-opening product at 80 °C, i.e. ring-opening is more favourable than for cyclic
ketene acetals.

However, polythioesters are not produced cleanly as shown in notes. The polymerization gives just 45% ring-opening product at 160 °C and only 15% at 120 °C. The differences have been explained on the basis of the order of thermodynamic stability of the link produced (amide > ester > thioester).

Ring-opening polymerization of type II monomers is less important. Two examples are given in the notes to illustrate the kinds of polymers that can be produced.

In both polymerizations, 100% ring-opening product is produced, principally due to substituentgroup stabilization of the propagating radicals, which are, respectively, shown in note notes.