Polymer Synthesis
1. 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-growthpolymerisation, addition polymerisation and insertionpolymerisation.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
Polycondensation
the 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 <Xn
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, <Mn 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 diamines
n CIOC-CH2CH2CH2CH2-COCI
+2n Et3N
Adipoyl chloride CHCI3
ـ‹ــــــــــــــــــ +
-2n HCI
n H2N-(CH2)6-NH2
CH2CH2CH2-CO)n (HN-(CH2)6 – NH OC – CH2
(alternative)
(iii) Preparation by amidation of dicarboxvlicesters.
(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.
Polyesters
Polyesters 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)
Polycarbonates
Polycarbonates 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 Polymerization
Usually, 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 250°C) 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) Photolysis
Usually 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, ß, 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 Retarders
Retarders: 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 BranchingDuring 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 BranchingDuring 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 Polyethylene
Low 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 HDPE
High 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 Propagation
Chain 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.
LivingPolymerizations
In 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) Propagation
Addition 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 OpeningPolymerizations
Ring 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) Propagation
Chain 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