Lecture 9 Flashcards
What is important when it comes to copolymers?
Copolymers comprise more than one type of repeat unit and their properties are controlled by the nature and proportions of the repeat units together with their sequence distribution along the chain. Thus, it is important to understand how to control copolymer composition and repeat unit sequence distribution.
What is the simplest form of copolymerization?
Simultaneous polymerization of a mixture of monomers is the simplest form of copolymerization and is used to prepare statistical copolymers whose properties are intermediate to those of the corresponding homopolymers. A very high proportion of commercial polymers are copolymers
produced in this way and even many commercial ‘homopolymers’ are in fact copolymers with small proportions of other types of repeat units included to modify their properties.
What happens to the properties of the monomers in graft, block, alternating, and random?
For block and graft polymers: The homopolymer blocks usually are immiscible, a consequence of which is that block and graft copolymers show properties characteristic of the homopolymer constituents as well as unique properties resulting from the chemical linking of the homopolymer blocks. (Basically the combination of properties from each homopolymer)
For alternating and random: The properties of the monomers are averaged and are represented by the Foxes equation.
All of this is depicted in notes from the lecture slides!
What is an important thing to note in chain copolymerization?
differences in monomer reactivity in statistical chain copolymerization affect the sequence distribution of the different repeat units in the copolymer molecules
formed. The most reactive monomer will be incorporated preferentially into the copolymer chains but, because high molar mass copolymer molecules are initiated, propagate and terminate in short timescales, even at low overall conversions of the comonomers, the high molar mass copolymer molecules formed can have compositions which differ significantly from the composition of the initial comonomer mixture.
What is the Copolymer composition equation? (Note that the final equation is what should be noted and the derivation isn’t required)
Theory (IMP!!!):
In order to predict the composition of the copolymer formed at a particular instant in time during a binary chain copolymerization, it is necessary to construct a kinetics model of the reaction. The simplest model will be analysed here and is the terminal model which assumes that the reactivity of an active centre depends only upon the terminal monomer unit on which it is located. It is further
assumed that the amount of monomer consumed in reactions other than propagation is negligible and that copolymer molecules of high molar mass are formed. Thus, for the copolymerization of monomer A with monomer B, only two types of active centres need to be considered:
Go to written notes
Flash card 6 likeeeee
Equation 9.5 gives the molar ratio of A-type to B-type repeat units in the copolymer formed at any instant (i.e. a very small interval of time) during the copolymerization when the monomer concentrations are [A] and [B]. Often it is more convenient to express compositions as mole fractions. The mole fraction fA of monomer A in the comonomer mixture is [A]/([A]+[B]) and that of monomer
B is fB =1−fA. The mole fraction FA of A-type repeat units in the copolymer formed at a particular instant in time is d[A]/(d[A]+d[B]) and that of B-type repeat units is FB =1−FA. Addition of unity to
both sides of Equation 9.5 allows it to be rearranged in terms of FA (or FB), fA and fB. The following copolymer composition equations are obtained:
(Back to written notes)
What are the reactivity ratios?
The reactivity ratios are the ratios of the homopropagation to the cross-propagation rate coefficients for the active centres derived from each respective monomer. Thus if rA >1 then ∼A* prefers to add monomer A (i.e. it prefers to homopolymerize), whereas if rA <1, ∼A* prefers to add monomer B and hence copolymerize. Similarly, rB describes the behaviour of monomer B.
Why are the reactivity ratios important?
Monomer reactivity ratios are important quantities since for a given instantaneous comonomer composition, they control the overall composition of the copolymer formed at that instant and also the sequence distribution of the different repeat units in the copolymer.
How does the monomer composition change with different pairs of rB and rA?
Plots of equation 7 for different rA and rB pairs are shown in Figure 9.1:
Random copolymers with FA = fA (for all values of fA) are formed when rA =rB =1, i.e. when the probability of adding monomer A is equal to the probability of adding monomer B for both types of active centres. There are very few copolymerizations which approximate to this condition and those that do involve the copolymerization of monomers with very similar structures (e.g. free-radical copolymerization of tetrafluoroethylene with chlorotrifluoroethylene).
More commonly, one monomer,
assumed here to be monomer A, is more reactive than the other, and both types of active centre prefer to add the more reactive monomer. In terms of reactivity ratios, this gives rise to rA >1 with
rB <1 (rArB ≠1). On this basis, it is easy to appreciate why there are no simple copolymerizations for which rA >1 with rB >1
The case of ideal polymerization is considered in flashcard 10
The higher the ratio rA/rB for rA >1, rB <1 copolymerizations, the longer are the continuous sequences of A-type repeat units and the shorter are the continuous sequences of B-type repeat
units in the copolymer molecules formed. When rA≫1 with rB≪1, there is a tendency towards consecutive homopolymerization of the two monomers. The molecules formed early in the reaction
have very long sequences of A-type repeat units with the occasional B-type repeat unit (i.e. they are essentially molecules of homopolymer A). Later in the reaction, when monomer A has been
consumed virtually completely, the very high concentration of residual monomer B leads to the formation of copolymer molecules that are essentially homopolymer B.
The case of Azeotropic copolymerization is considered in Flashcard 11
What is Azeotropic copolymerization?
Azeotropic copolymerization occurs when rA <1 with rB <1 and when rA >1 with rB >1, though the latter of these conditions rarely is observed in practice f. Substituting FA = fA =(fA)azeo into Equation 4
leads to
(fA)azeo = (1-rB)/(2-rB-rA)
As the product rArB decreases, there is an increasing tendency towards alternation in the additions of monomer molecules to the propagating chains. The extreme case of azeotropic copolymerization is
rA =rB =0 and always produces perfectly alternating copolymers, irrespective of the value of fA (i.e. FA =0.50 for 0<fA <1), because the homopropagation reactions do not occur.
What is ideal copolymerization?
Ideal copolymerization is a special case of rA >1, rB <1 (or rA <1, rB >1) copolymerization for which rArB =1. Under these conditions::
rA = 1/rB i.e., kAA/kAB = kBA/kBB
which means that the relative rates at which the two monomers are incorporated into the copolymer chains are the same for both types of active centres (i.e. ∼A* and ∼B*). Thus, even though one monomer is more reactive and FA ≠fA, the sequence distribution of the repeat units in the copolymer formed is random. Substitution of rB =rA^−1 into eq 7 gives a simplified copolymer composition equation for ideal copolymerization:
FA = rAfA/(rAfA +fB)
Ideal copolymerization occurs when there are no specific effects controlling either one or more of the four propagation reactions, since the relative rates of reaction of the two monomers then depend only upon their absolute relative reactivities. In many copolymerizations rA >1, rB <1 (or vice versa) with rArB ≠1 (usually rArB <1). Such copolymerizations give FA versus fA curves that are similar to those for ideal copolymerization but skewed towards copolymer compositions richer in the repeat units derived from the more reactive monomer.
What is copolymer composition drift?
For a given pair of comonomers, the value of FA for the copolymer formed early in the reaction is determined by the initial value of fA via Equation 4. For most copolymerizations, FA ≠fA and
one monomer is consumed preferentially causing fA to change as the overall monomer conversion increases. Since Equation 4 is applicable to each increment of conversion, the change in fA gives
rise to a variation in FA with conversion.
This is known as copolymer composition drift and leads to copolymers which consist of copolymer molecules with significantly different compositions. This broadening of the distribution of copolymer composition beyond that arising from the normal statistical variation of copolymer composition about FA at any specific value of fA, clearly becomes more significant as the overall monomer conversion increases.
In rA >1, rB <1 copolymerizations fA (and hence FA) decreases with conversion as monomer A is consumed preferentially. Eventually, monomer A is consumed virtually completely leaving mainly
unreacted monomer B (i.e. fA eventually approaches zero) and so, thereafter, essentially homopolymer of monomer B is formed.
In rA <1, rB <1 azeotropic copolymerization fA changes with conversion until it becomes equal to either zero or unity and the corresponding homopolymer is formed from then onwards. (For the unrealistic rA >1, rB >1 azeotropic condition fA would change with conversion until it is equal to
(fA)azeo and copolymer with FA =(fA)azeo would be formed thereafter.)
How do we control copolymer composition drift?
For many applications, the tolerance for copolymer composition drift is small and its control is essential. The strategies most commonly used for this purpose are as follows:
- The overall monomer conversion is limited (usually to ≤5%) in order to reduce the drift in fA.
- Additional quantities of the monomer that are consumed preferentially are fed to the reaction vessel at a controlled rate during the copolymerization in order to maintain fA constant.
- Starve-feeding of the comonomer mixture to the reaction vessel (i.e. feeding at rate below the potential rate of polymerization) in order to achieve very high instantaneous conversions (close to 100%), thereby ensuring that fA does not vary during the copolymerization.
Check digital notes for extra stuff not mentioned in the book
Literally just like 4 slides
