Polymer Synthesis1. Introduction

2. Polycondensation

3. Radical Polymerization

4. Ionic Polymerization 4a. Anionic Polymerization

4b. Cationic Polymerization

5. Ring Opening Metathesis Polymerization

G. H. Mehl

Objectives of the Lecture Course

Introduction to Polymer Chemistry

Introduction and discussion of several methodologies for the synthesis of macromolecules with a focus on linear polymers.

Some Useful Textbooks

J.M. Cowie, Polymers: Chemistry and Physics of modern materials, Glasgow, Blackie Academic, 1991.

H.G. Elias, An introduction into Polymers Science, VCH, Weinheim, 1997.

M. P. Stevens, Polymer Chemistry, An introduction, OUP, 1999.1. Synthesis of Polymers

There are a number different methods of preparing polymers from suitable monomers, these are step-growth polymerisation, addition polymerisation and insertion polymerisation.

Condensation or Step-growth Polymerization Step growth polymerisation is usually used for monomers with functional groups such as -OH, -COOH etc. It is usually a succession of non-catalysed, chemical condensation reactions associated with the elimination of low-molar-mass side-products, eg., water.

Addition Polymerization : Usually addition polymerizations involve the polymerisation of olefinic monomers. Polymerisation occurs via a chain reaction. The monomers are converted into polymers by opening of the double bond with a free radical or ionic initiator. The product, then, unlike that obtained from step-growth polymerisation, has the same chemical composition as the starting monomer.

Insertion Polymerization :

Usually addition polymerizations involve the polymerisation of olefinic monomers. Polymerisation occurs via an insertion of a monomer at the end of the growing chain, mediatied by a catalyst. The catalyst stays at the end of the growing chain. Polymers synthesised by insertion polymerisation are typically characterised by a very high stereoregularity. An example for such a polymerisaiton techniques is the Zie ler-Natta 01 merisation. Another exam Ie on which we will

contentrate in this course is the "Metathesis" polymerisation.

Natural Polymers: generally have more complex structures than synthetic polymers and are not discussed further here.(See Polypeptides and Proteins and Nucleic Acids courses).

Elastomers can be either man made or natural, and are therefore shown as a common subgroup. This term is used to describe elastically deformable, solid polymers.

Basic Definitions

A polymer is a large molecule constructed from many smaller structural units called monomers. covalently bonded together in any conceivable pattern.

A repeat unit must possess two or more bonding sites in order to form a polymer.

When only one species or building block is used to form a polymer, the product is called a homopolymer.

If the polymer chains are composed of two types of monomer unit, the material is known as a copolymer; for three monomer units it is called a terpolymer, etc.

2. Polycondensation

Monofunctional molecules lead to

Bifunctional molecules are required to synthesise linear chains

Examples: Polyesters Polyamides Polycarbonates Polycondensationthe number average degree of polymerization

The number of molecules present is determined by counting the number of end groups

Definition: The number average degree of polymerisation

Total number of molecules Originally present (monomers

Definition: p is the fraction of the functional groups that have reacted.Or: P is the probability the one such group taken at

random has reacted. i. e. the concentration of one of the functional groups,c, present after a fraction p has been reacted, is related to the initial concentration of such groups The number of molecules (N, No) can simply be converted to concentrations (c, co) so that:

The Number Average Molecular Weight, is simply:

Mo refers to the molecular mass of the monomers The Number Average Degree of Polymerisation as a Function of Conversion

Ramifications:

High molecular weight is only achieved at very high degrees of conversion.

At 90% conversion (p = 0.90) number average degree of polymerization is only 10 !

Equivalent to number average molecular weight of ~1000 g/mole.

At 95% conversion (p = 0.95) number average degree of polymerization is only 20 !

Equivalent to number average molecular weight of ~ 2000 g/mole.

Need to have conversions of 99.5% to obtain molecular weights in the range of 20,000 g/mole

An industrial nightmare!

Polvamides

(i) Preparation by direct amidation of diacids.

n HOOC-CH2CH2CH2CH2-COOH + n H2N-(CH2)6-NH2 Adipic acid Hexamethylenediamine

(1, 6-hexane-dicarboxylic acid) (1,6-diaminohexane) Heat - 2n H20

(HN-(CH2)6-NH-OC-CH2CH2CH2CH2-CO)n Nylon _66 initially prepared at Du Pont: the polymer repeat unit contains six carbon atoms from the diamine monomer and six carbon atoms from the diacid monomer.

(ii) Polvcondensation of diacid chlorides with diaminesn CIOC-CH2CH2CH2CH2-COCI +2n Et3N Adipoyl chloride CHCI3 + -2n HCIn H2N-(CH2)6-NH2

CH2 CH2CH2-CO)n (HN-(CH2)6 NH OC CH2 (alternative)(iii) Preparation by amidation of dicarboxvlic esters.

(iv) Preparation from monomer salts

Reason for this method: Precisely equivalent amounts of the monomers are required.This is often difficult on an industrial scale.

So a solution to this problem for "nylons" is to convert the acid and amine (which is a base) to salts, which will precipitate out of solution in the form of a 1: 1 complex. This is the desired stoichiometry.PolyestersPolyesters are synthesised by the typical esterification reactions, which can be generalised as follows, where N is a nucleophile:

Preparation from anhydrides: Poly(ethylene maleate)

Preparation from esters: Poly(ethylene terephthalate), Dacron, Mylar

Preparation from acid chlorides: Poly (1,4-butylene isophthalate)

PolycarbonatesPolycarbonates are polyesters derived from the reactions between carbonic acid (or its derivatives) with dihydroxy compounds (diols).

(i) Preparation by the Schotten-Baumann reaction

(ii) Preparation by transesterification

Lexan is a commercially available polycarbonate. It has a high impact strength and very good thermal and mechanical stability below 250 oC and is almost unaffected by water and many inorganic and organic solvents.

3. Radical PolymerizationUsually, many low molecular weight alkenes undergo rapid polymerization reactions when treated with small amounts of a radical initiator.

For example, the polymerization of ethylene is carried out at high pressure (1000 to 3000 atm) and high temperature (100 to 250C) with a radical catalyst, eg, benzoyl peroxide.

Step 1: Initiation

Step 2: Propagation

Step 3: Termination

Thermodynamic considerations for the free radical polymerization

Chain growth

Activation energy for chain growth much lower than for initiation.

Growth velocity less temperature dependent than initiation

Resonance effect: R more electronegative than H

Substituents are incorporated in 1,3 position (head-to-tail linkage):

Initiators Radical Polymerization for (i) Thermal Decomposition

Usually applied to organic peroxides or azo compounds, eg benzoyl peroxide.

(ii) PhotolysisUsually applies to metal iodides, metal alkyls, and azo compounds, eg, a,a'-azobisisobutyronitrile AIBN is decomposed by radiation of A. = 360 nm

(iii) Redox Reactions

Radicals are produced in a redox reaction in solution, eg, the reaction between the ferrous ion and hydrogen peroxide in solution produces hydroxyl radicals.

Alkyl hydroperoxides can also be used in place of hydrogen peroxide, in addition a similar reaction is obtained when Cerium (IV) sulphate oxidises an alcohol, eg,

(iv) Persulphates

Ce3+ + H+ + RC(OH)H

Persulphates are often used in emulsion polymerizations.

Decomposition occurs in the aqueous phase, and the resulting radical diffuses into the hydrophobic droplets containing the monomer.

(v) lonising Radiation

a, , y and x-rays can be used to initiate the polymerization reaction. Ejection of an electron is followed by dissociation and electron capture to produce a radical. Inhibitors and RetardersRetarders: Chain transfer reagents can lower the average length of the polmer chain. Some chain transfer reagents produce radicals with low activities. If the re-initiation process is slow, the polymerization rate decreases because there is a build up of radicals leading to increased termination by coupling. When this occurs the reagent responsible is called a retarder. For example, nitrobenzene acts in this way in the polymerisation of styrene.

Inhibitors: In extreme cases an added reagent can suppress polymerization completely by reacting with the initial radical species and converting them all efficiently into unreactive materials. This process is called inhibition. The difference between retardation and inhibition, however, is one of degree.

The stabilized radical diphenyl picryl hydrayl (DPPH) is an excellent inhibitor, and is extensively used as a radical scavenger because its stoichiometry of reaction is 1: 1.

Chain Branching During Radical Polymerization (i) Short Chain Branching: chain branching often occurs in radical polymerization processes. Short chain branching arises when the radical end of a growing chain bends back on the polymer chain created and abstracts a hydrogen from it. In effect this transfers the active site of polymerization to the internal region of the growing polymer, leaving a short, non-reacting chain (a branched chain) at the end of the polymer backbone, now in a different direction. Thus uncontrolled chain branching leads to higher viscosity and a less homogeneous polymer.Chain Branching During Radical Polymerization

(ii) Long Chain Branching: In long chain branching abstraction of hydrogen is not intramolecular but intermolecular, ie, abstraction takes place by the reaction of the end of one polymer chain with the middle of another chain.

Chain branching is common in radical polymerization: short chain branching is about 50 times more likely to occur than long chain branching. Chain branching occurs in all kinds of polymers other than polyethylene, eg, polystyrene, polypropylene, etc. Chain Branching in PolyethyleneLow Density Polyethylene LDPE

As seen earlier, the radical polymerization of ethylene leads to chain branching and as a consequence to the formation of a low density polymer. Low density polyethylene is partially crystalline (50-60%) and melts around 115 DC. LDPE was the first plastic with an annual production exceeding 1 billion Ib (in 1959), by 1979 this had reached 7.9 billion Ib/year.

High Density Polyethylene HDPEHigh density polyethylene tends to have a large proportion of linear chains (ie, contains less than one side chain per 200 carbon atoms in the main chain). HDPE is highly crystalline (90%) with a melting point above 127 oC. HDPE can be produced in several ways, including radical polymerization of ethylene at extremely high pressures, coordination polymerization of ethylene (eg, with a catalyst such as TiCI4 dispersed in a colloidal suspension with a solvent such as heptane) and polymerization on supported metal-oxide catalysts. In 1957 5 billion Ib/year had been reached in production.

Linear Low Density Polyethylene LLDPE

Linear low density polyethylene is a copolymer of ethylene with an a- olefin such as butene, hexene or octene. The addition of an a-olefin is designed to simulate the short chain branching and density of conventional branched polyethylene without the occurence of long chain branching. This produces polymers with good melt-flow and physical properties. LLDPE has been one of the fastest growing new plastics in recent years (1 billion Ib produced in 1982. The major use of this plastic is in waste bags.

Atom Transfer Radical Polymerisation

Compared to the free radical polymeriation the concentration of free radical at a given time is small, thus less side reactions are observed.

Stable Free Radical Polymerisation

This is a very mild polymerisaiton technique, which makes use of TEMPO as a protective group for the growing chain.

4. Ionic Polymerization

Ionic polymerization is more complex than free-radical polymerization: whereas free radical polymerization is non-specific, the type of ionic polymerization procedure and catalysts depend on the nature of the substituent (R) on the vinyl (ethenyl) monomer.

Cationic initiation is therefore usually limited to the polymerization of monomers where the R group is electron-donating, which helps stabilise the delocation of the positive charge through the p orbitals of the double bond.

Anionic initiation, requires the R group to be electron withdrawing in order to promote the formation of a stable carbanion (ie, -M and -I effects help stabilise the negative charge).

M is a Monomer Unit. As these ions are associated with a counter-ion or gegen-ion the solvent has important effects on the polymerization procedure.

Ionic Polymerization(ii) Chain PropagationChain propagation depends on:

(a) Ion Separation

The separation between the ionic part of the propagating polymer chain and the corresponding counter ion (Gegen-ion) determines the stereochemistry of reaction of a polymerising monomer.

(b) Nature of the Solvent

Polar and highly solvating media are required in order to dissolve the ionic catalysts and propagating polymer chains. However, many common polar solvents cannot be used because they would react with, or otherwise neutralise, the ionic initiator, eg, solvents incorporating hydroxyl and sometimes carbonyl groups (alcohols and ketones) . Solvents of low dielectric constant should be used since this results in a closer association of the counter-ion and the chain end as an ion pair. However, even in low polarity solvents such as tetrahydrofuran (THF) or dichloromethane (DCM) ion pair separation varies greatly.

(c) Nature of the counter-ion

The nature of the counter-ion can effect the stereochemistry of the polymer product as well as influencing the rate of polymerization.

4a Anionic Polymerization

Involves the polymerization of monomers that have strong electron- withdrawing groups, eg, acrylonitrile, vinyl chloride, methyl methacrylate, styrene etc. The reactions can be initiated by methods (b) and (c) as shown in the sheet on ionic polymerization, eg, for mechanism (b)

The gegen-ion may be inorganic or organic and typical initiators include KNH2, n-BuLi, and Grignard reagents such as alkyl magnesium bromides. If the monomer has only a weak electron-withdrawing group then a strong base initiator is required, eg, butyllithium; for strong electron-withdrawing groups only a weak base initiator is required, eg, a Grignard reagent. Initiation mechanism (c) requires the direct transfer of an electron from the donor to the monomer in order to form a radical anion. This can be achieved by using an alkali metal eg.,

Anionic polymerizations are similar to cationic reactions;

(i) They are generally rapid at low temperatures, eg, the above reaction is achieved in liquid ammonia at 198K.

(ii) They are slower and less sensitive to changes in temperature than cationic polymerization.

(iii) Reaction rates are dependent on the dielectric constant of the solvent, the resonance stability of the reactive anion, the electronegativity of the initiator and the solubility of the gegen-ion.

(iv) There is no formal termination step, but the reaction is very sensitive to small traces of impurities, eg water, O2, CO2 etc. This latter group of compounds can be used as termination reagents.

Anionic Polymerization of Styrene

Assuming steady state conditions, the concentration of propagating species is:

and the length of the polymer chains depends on the number of initiations

Thus The activation energy for transfer is larger than for propagation, and so the chain length decreases with increasing temperature.

Living PolymerizationsIn ionic polymerization reactions, eg., the polymerisation of styrene with potassium amide as shown previously, there is no automatic termination step. Therefore, if the impurities that are responsible for termination are suppressed or somehow excluded from the polymerisation reaction, then the reactive carbanion end of the growing polymer will remain reactive ("live") even when all of the monomer initially present in the reaction vessle had been consumed in the reaction.

Consequently, if more of the same or a different monomer is added to the reaction, the polymerization process could continue and the polymer chains would continue to propagate. This is an efficient way of controlling polymer molecular weight and synthesising block copolymers.

These active polycarbanions were first called "living polymers" by Szwarc. One of the first living polymer systems found was the polymerization of styrene initiated with sodium naphthalene.

(i) Initiation

The initiation process is achieved by adding sodium to a solution of naphthalene in an inert solvent, such as tetrahydrofuran (THF). The sodium forms an addition compound, and by transferring an electron, produces a green naphthalene anion radical.

Living Polymerizations

(ii) PropagationAddition of styrene results in electron transfer from the naphthyl radical to the monomer to form a red styryl radical anion, eg.,

It is possible that the dianion forms and addition then occurs at both ends of the growing polymer chain, eg.,

Thus, the absence of a termination and a transfer reaction means that the chains will remain active almost indefinitely. The utility of the approach can be demonstrated by adding more styrene in order to produce higher molecular-weight polystyrene or by adding a different reactive monomer to form a block copolymer. Anionic Ring Opening PolymerizationsRing opening polymerizations of oxiranes, thiiranes and thietanes can be initiated by anionic and cationic processes, but lactams and lactones are easier opened by anionic methods. eg, preparation of polyethylene oxide

Lactones can be used to prepare polyesters, but as seen earlier, ring size is important, eg, the 5-membered ring y-butyrolactone will not polymerize whereas the six membered ring 8-valerolactone will.

Lactams will also undergo ring opening and polymerization to produce polyamides, eg, nylon-6 can be prepared from the water catalysed reaction of caprolactam.

Typically this reaction is better controlled by using a two component catalyst system. The lactam is reacted with a base to produce an activated monomer, which then reacts with a promoter such as acyl lactam, this in turn initiates the ring opening growth of the linear polymer. Obviously, the preparation of nylons this way is of commercial importance. 4b. Cationic Polymerization (i) Initiation

(ii) PropagationChain growth takes place through the repeated addition of a monomer in a head-to-tail manner to the ion with retention of the ionic character throughout

Cationic Polymerization (iii) Termination

Termination of cationic polymerization reactions are less well-defined than in free-radical processes. Two possibilities exist as follows:

(a) Unimolecular rearrangement of the ion pair

Hydrogen abstraction occurs from the growing chain to regenerate the catalyst-eo-catalyst complex. Covalent combination of the active centre with a catalyst-eo-catalyst complex fragment may occur giving two inactive species. The kinetic chain is terminated and the initiator complex is reduced a more effective route to reaction termination.

(b) Bimolecular transfer reaction with the monomer

Reformation of the monomer-initiator complex, ensuring that the kinetic chain is not terminated by the reaction.

Cationic PolymerizationInitiation For initiation of the type: M+ + I" (M+l: i is typically a strong Lewis Acid. Electrophilic initiators are drawn from three groups:

(i) Classical protonic acids or acid surfaces - HCI, H2S04, HCI04 (ii) Lewis acids or Friedel-Crafts catalysts - BF3' AICI3, TiCI4, (iii) Carbenium ion saltsLewis Acids are the most important initiators, however, they are not particularly active and therefore require a co-catalyst to act as a proton donor, eg,

Initiation then becomes a two step mechanism, as follows:

The use of a co-catalyst is difficult to demonstrate in practice, however, consider the polymerization of isobutylene. The reaction does not proceed under anhydrous conditions using BF3 as the catalyst, but with trace amounts of water present reaction proceeds rapidly, eg,

The complex reacts with the monomer to produce a carbenium ion chain carrier which exists as an ion pair with [BF30H] .

Stable CarbonationsMany uncertainties with the catalyst cocatalysts described previously (Friedel Crafts catalysts) can be removed by using stable, well-defined initiators:

eg, triphenyl methyl and tropylium salts of general formula

Initiation then occurs by one of the following methods:

(i) Direct Addition

(ii) Hvdride Extraction

(iii) Electron Transfer

Cationic polymerization

Example of a cationic polymerisation, using boron trifloride (BF3) as initiator.

Cationic Ring Opening PolymerizationCyclic momomers such as lactones, lactams, cyclic amines and cyclic ethers can undergo ring opening and subsequent polymerization with cationic initiators. Ring opening depends on ring size, and consequently ring strain, eg, tetrahydropyran and 1 A-dioxane do not ring open because they have stable six membered ring structures, but ethylene oxide, trimethylene oxide, and tetrahydrofuran (which have smaller ring sizes) and hexamethylene oxide (which has a larger ring size than six) all can be induced to polymerize, eg, one mechanism involves the primary interaction of the catalyst system with the monomer to form the oxonium ion which acts as the initiating species.Propagation then occurs in a predominantly SN2 type of substitution reaction.

Alternatively ring cleavage can be achieved with the catalyst to form an ionic species, this attacks another monomer producing ring opening and regeneration of the active site.

The reaction can produce commercially useful polymers, but because of the low ceiling temperatures and the tendency of the polymer to unzip, end-capping may be required, eg, polyformaldehyde (an engineering plastic) can be prepared from 1,3,5-trioxane using boron trifluoride etherate as the initiator, and stabilising with acetylation of the terminal group.

Other Polymerizable Ring Systems

Ring Opening Metathesis Polymerisation (ROMP)The reaction involves a cyclic olefin and a metal-alkylidene as a catalyst. The polymer is formed via a cyclobutane. The double bonds are rearranged (greek: meta = change; tithemi = place). A "new" olefin that is generated remains attached to the catalyst as part of a growing polymer chain.

Propagation

The driving force for the ROMP reaction is the relief of ring strain. Therefore, the second step shown above is essentially irreversible. Strained cyclic olefins such as those shown below have sufficient ring strain to make this process possible. Monomers based on norbornene derivatives are especially popular, as they can be readily synthesized from Diels-Alder reactions with cyclopentadiene. Only the unsubstituted bonds are ring-opened (it is very difficult to metathesize or ROMP tri-and tetrasubstituted olefins).

Catalysts

A wide variety of catalyst has been explored. If the catalyst is too active it can start to metathesize unstrained olefinic bonds in the polymer chain, leading to chain scission

Chain Scission

Termination (with aldehyde, controlled)The ROMP technique is a living polymerisation and polymers with very high molecular weights can be achieved. For a controlled termination of the reaction aldehydes can be used.

The product is an "end-capped" polymer and the catalyst is deactivated as a "metal oxo" group.

Model Questions.

In the following there are some questions, which might be of use in evaluating your knowledge of the subject matter of the course. It should take you only a few minutes to complete answer the questions. The correct answers are included.

Which is the correct statement? The intermediate A, shown below is:

a. a reaction intermediate in the free radical polymerization

b. a reaction intermediate in the atom transfer radical polymerization

c. a reaction intermediate in the stable radical polymerization

d. a byproduct in the initiation step in the anionic polymerization

e. none of the above. Which is the correct statement? Polymethacrylates can be synthesized

(a) only by using a radical polymerisation method

(b) only by using an anionic polymerisation method

(c) by a polyesterification technique

(d) by a cationic polymerisation

(e) By either a radical polymerization or an anionic polymerization.

Which is the correct statement? An anionic polymerization of styrene

(a) is best carried in a protic solvent

(b) can be carried out with SbF 5 used as initiator

(c) requires that the initiator is activated by traces of water(d) can proceed as a reaction in THF at 90C

(e) can proceed as a reaction in THF below 0 C.

Which is the correct statement? A Metathesis polymerization

(a) is initiated by butyl-lithium in aprotic media

(b) proceeds via a four membered cyclic transition state.

(c) is a polymerisation with BF 3 as a co-catalyst

(d) yields polyesters in high yields

(e) is a mild reaction in the gasphase.

Anwers:

Which is the correct statement? The intermediate A, shown below is:

(a) a reaction intermediate in the free radical polymerization A free radical polymerization involves free radicals in the growing chain. There is none in structure A, this is the wrong answer.

(b) a reaction intermediate in the atom transfer radical polymerization This is the right answer

(c) a reaction intermediate in the stable radical polymerization A stable free radical polymerization involves the formation of a protected growing chain, by the reaction of the growing chain with and stable organic radical eg TEMPO. This is not the case. This is the wrong answer.

(d) a byproduct in the initiation step in the anionic polymerization There are no "byproducts" in the initiation step in the anionic polymerization, unless you would count the counter-ion as a byproduct .However generous you are with thisassingment" A " does not fit, so this is the wrong answer.

(e) none of the above.This is the wrong answer.

Which is the correct statement? Po1ymethacry1ates can be synthesized

(a) only by using a radical po1ymerisation method Methacrylates can be polymerized using radical polymerization techniques, however other methods are possible too, so this is the wrong answer.

(b) only by using an anionic po1ymerisation method This is the wrong answer as polymethacryalets can be polymerized using radical polymerisaiton techniques.

(c) by a po1yesterification technique This is the wrong answer.

(d) by a cationic po1ymerisation This is the wrong answer as cationic polymerisation requires the presence of electron donating groups next to the cation. (e) by either a radical polymerisation or an anionic polymerisation.

This is the right answer.

Which is the correct statement? An anionic polymerisation of styrene

a. is best carried in a protic solvent This is obviously wrong as a protic solvent would react immediately with the anion and terminate the reaction.

b. can be carried out with SbF 5 used as initiator SbFs is used as a co-initiator in the cationic polymerization. This is the wrong answer.

c. requires that the initiator is activated by traces of water Water would react immediately with the anions and quench the reaction. This is the wrong answer.

d. can proceed as a reaction in THF at 90C The boiling point of THF is 67C. This is the wrong answer.

e. can proceed as a reaction in THF below 0 C.

This is the right answer. Anionic polymerizations are often carried out in THF at temperatures mostly well below -30C.

Which is the correct statement? A Metathesis polymerization

(a) is initiated by butyl-lithium in aprotic media This is the wrong answer. Butyl-lithium initiates anionic polymerizations.

(b) proceeds via a four membered cyclic transition state.

This is the right answer.

(c) is a polymerisation with BF 3 as a co-catalyst BF3 is used as a co- catalyst in the cationic polymerizations. So, this is the wrong answer.

(d) yields polyesters in high yields This is the wrong answer. The methathesis polymerization is not a condensation reaction.

(f) is a mild reaction in the gas phase.

This is the wrong answer. Organic polymers are not prepared in the gas phase.

_1301747647.psd