Lecture 11

Question 1

What is special about block copolymers

Answer 1

Block copolymers can self-assemble to form a di-block solid or a tri-block solid as shown in the digital notes. Furthermore, in amphiphilic solutions, block copolymers can also self-assemble to form many different structures

Note that AB block polymers of immiscible A and b blocks will phase-separate leading to self-organization of the blocks which depend on their respective mole fractions. This is also depicted within the notes

Question 2

How can the self-organization of immiscible block polymer be used as an application?

Answer 2

You can use the self-organization and play with the mole fractions to end up having rubbery spheres dispersed in a brittle material, effectively rubber-toughening the brittle polymer!

Question 3

What are the properties of block polymers described by?

Answer 3

The properties of a block polymer are defined by the mix (not average!) of the properties of its building polymers. Where within the block polymer different properties/functions will appear at different parts of the chain. This is depicted with the notes.

Question 4

What are the applications of copolymers?

Answer 4

1. Compatibilizer
2. Surfactant
3. Thermoplastic elastomers
4. porous material
5. Nanolithography
6. drug delivery

Question 5

How are copolymers used as compatibilizers?

Answer 5

Block copolymers can stabilize a polymer blend consisting of two immiscible polymers by acting as a compatibilizer. One segment of the block copolymer has an affinity for one of the polymers in the blend, while the other segment has an affinity for the second polymer. This causes the block copolymer to localize at the interface between the two immiscible polymers, reducing interfacial tension and preventing phase separation, thus stabilizing the blend.

This is depicted in digital notes

Question 6

How are copolymers used as surfactants?

Answer 6

Same concept as compatibilizers, except we are stabilizing the interface between two solvents or a solvent and a particle.

This is depicted in digital notes

Question 7

How are copolymers used in polyurethane elastomers?

Answer 7

Thermoplastic elastomers (TPEs) are a significant application of copolymers. TPEs are a class of materials that combine the elastic properties of rubber with the processability of thermoplastics. Many TPEs are copolymers or blends of copolymers that consist of both hard and soft segments, which give them their unique combination of elasticity and thermoplastic behavior.

In a typical TPE, the soft segments provide flexibility and elasticity (like rubber), while the hard segments contribute strength and stability (like plastic). When heated, the cross-links formed by one polymer segment are sheared allowing the TPEs to be moulded or extruded like thermoplastics, but when cooled, the crosslinks reform and they return to their elastomeric, rubber-like state. This ability to be reprocessed multiple times without significant degradation is a key advantage of TPEs.

Note that the materials (when cooled) are held by physical interactions (H-bonding)

The example shown in the slides (shown in digital notes) gives an example of TPE as polyurethane. Its applications range from toothbrushes to diapers to strong rubbery foils that can deform greatly and to plastic golf puts

Question 8

How are copolymers used to make porous material?

Answer 8

Copolymers that self-assemble in amphiphilic solutions from cylindrical structures, these structures can then have silicate polymerize around them forming a silica material filled with cylindrical copolymer.

These cylindrical copolymers can then be removed by using a solvent, leaving behind Nano- or mesoporous materials. Chain lengths determine pore sizes!

This is depicted in the digital notes,

Question 9

How are copolymers used in nanolithography?

Answer 9

The depiction in the digital notes is very straightforward. Basically, you can then form templates or just make patterns of the block copolymer

Question 10

What is the biggest problem faced in drug delivery?

Answer 10

Most drugs are considered to be hydrophobic compounds. They are either made from protein complexes or acid-base equilibrium. This hydrophilicity will cause solubilization problems

Additionally, when the drug is in the blood it can degrade via metabolization or attack by the immune system. This will reduce the efficiency of such drugs and will cause the use of larger doses which can have large side effects. Therefore we need to find ways to transport drugs efficiently without solubilizing or metabolizing

Question 11

What are the two ways to improve drug delivery?

Answer 11

1. Protection via shield
2. Micellar drug delivery
3. Nanoparticle drug delivery - Enhanced oermeability and retention effect (EPR)

All of these are application of block copolymers

Question 12

How can we improve drug delivery via protection by a shield?

Answer 12

A shield can be wrapped around the drug to effectively protect it from metabolic “attack”. This shield can be made to be hydrophilic improving the solubility of the drug.

An example of a shield is PEG (coupled with another polymer to make a block copolymer) which is a hydrophobic and biocompatible polymer. The PEG will be linked to the drug via a linker which will break upon a change in the environment (pH, enzyme, …) allowing for the release of the drug.

The PEG chain protects and solubilizes the active drug compound, leading to higher concentrations of drug reaching their target and more effective circulation times!

Another example is PK1 - anti anti-cancer drug which is made via radical polymerization. The DOXO group is hydrophilic and will act to solubilize the drug. This is also a copolymer

All of this is depicted within the notes

Question 13

How can we improve drug delivery via Micellar drug delivery?

Answer 13

Block copolymers, particularly amphiphilic block copolymers, can self-assemble in aqueous environments to form micelles. These micelles have a hydrophobic core and a hydrophilic shell:

Hydrophobic core: Encapsulates hydrophobic drugs like paclitaxel or poorly water-soluble drugs.

Hydrophilic shell: Helps in stabilizing the structure in the bloodstream, improving circulation time.

Note that the moment the micelle is in the blood, it is likely that it will fall apart due to the dilution in the blood, therefore for this application we must pick a micelle with a really small CMC

This is depicted in the notes

Question 14

How can we improve drug delivery via nanoparticles?

Answer 14

This follows the principle of Enhanced oermeability and retention effect (EPR):

Basically, a tumorous cell will have very leaky walls that drug particles can easily go in and out of. In healthy cells, the walls aren’t leaky but it is still possible for the drug particles to go into the healthy cell. These drug particles are then removed from the healthy cell using the lymphatic system. The fact that the drug particles on their own can go into the healthy cell, will cause major side effects on a person. Also, an increase in the dose of the drug is required since some of the drug will be lost in the healthy cells. again not ideal.

To fix this the drugs have been placed in nanoparticles made from block copolymers. These nanoparticles are used to encapsulate and deliver both hydrophobic and hydrophilic drugs,

When the nanoparticles enter the blood, due to their bigger size they will not enter the healthy cell, instead, they will enter through the leaky walls of the tumour cells. Once they are inside the nanoparticles will then clump up causing further resistance to enter the healthy cell. This is a huge improvement.

Once inside, we can use the fact that the tumour cells have lower pH to allow the nanomaterial to release the drug at said pHs!

This is depicted in notes!

Question 15

What are the two mechanisms for the synthesis of block copolymer?

Answer 15

1. Selective coupling of two functional homopolymer chains: increasingly difficult with increasing chain lengths due to the polymer chains become more entangled, which reduces their mobility and makes it harder for the reactive end groups to meet. Moreover, longer polymer chains tend to experience increased steric hindrance. The larger size of the chains physically obstructs access to the functional end groups, further reducing the likelihood of successful coupling.
2. Synthesis of block copolymers by sequential monomer addition: the chain resulting after polymerization of the first monomer needs to be able to initiate the polymerization of the second monomer. This is achieved by having a living chain growth polymerization!

Question 16

How is a living chain growth polymerization made?

Answer 16

It is made by eliminating the irreversible chain-stopping events (Termination and chain transfer). Aswell as making initiation much faster than propagation allowing for same-length chains to be made!

Question 17

What is the difference between making copolymers using Living chain growth polymerization or using free radical polymerization

Answer 17

By using living chain growth polymerization we achieve more narrow MMDs (PDI ≈ 1), with the WMDs being very similar to their MMDs. For free radical polymerization this is not the case and the polymer formed is nowhere near monodispersed.

This is depicted in the notes

Question 18

What are the different living chain growth polymerization?

Answer 18

1. Ionic polymerizations: anionic and cation
2. Coordination polymerization: Note can give a living system but will give very large PDI
3. Living Radical Polymerization

This is depicted in the notes (Read what written for coordination)

Question 19

Note about ionic polymerization

Answer 19

Whenever an ion is made from a neutral compound we will always need a counter ion for it!

For anionic we can easily use a metal ion as it will not form covalent bonds with the carbon atom allowing for the formation of a living system

For cationic this isn’t as nice as we need to find a negative ion that won’t form bonds with the carbon atoms (Not very possible). So we use big initiators with a delocalized negative charge. Note even then charge termination can occur

Question 20

What is the typical monomer used for anionic polymerization?

Answer 20

Look at the digital notes

Question 21

What are the normal steps of anionic polymerization using alkyl lithium initiators?

Answer 21

This is depicted in notes. Note that alkyl lithium is very effective for the initiation of 1,3-dienes and styrenes!

Question 22

How is an anionic system made living?

Answer 22

The anionic system is made living by removing any laible protons that can initiate termination!

Therefore if the anionic polymerization system is made in super-pure conditions, no reaction with the propagating chains will occur as the C-H bonds are too strong, meaning propagating will end when no more monomer is present.

This is the only true living system as it has no irreversible termination whatsoever and fast initiation

Question 23

What is the equation that represents the degree of polymerization in a living anionic polymerization system?

Answer 23

This is found in digital notes

From this equation, we see that the molar mass can be controlled via the initial concentration of the monomer, the initial concentration of the initiator, and the conversion!

Question 24

What is MMD distribution like for living anionic polymerization systems?

Answer 24

it follows the Poisson distribution, which is very narrow. This results in a very narrow PDI and therefore the approximation of PDI ≈ 1 + 1/xn can be made

Question 25

What are the advantages and disadvantages of Living anionic polymerization?

Answer 25

Advantage: PDI very low (in principle no dead material!)

Disadvantages:
1. Extremely clean conditions

2. Limited number of solvents
3. Limited number of functional groups (no OH, COOH) since we can’t have liable protons to cause termination
4. Limited number of functional monomers (in principle)

Question 26

What is the application of living anionic polymerization systems to the synthesis of copolymers?

Answer 26

Straight forward: It is depicted in notes. Might be nice for you to draw the mechanism thou!

Question 27

What are examples of Anionic ring-opening polymerization?

Answer 27

Depicted in notes

Keep in mind that we can also make a living anionic ring-opening polymerization using the same concepts discussed. Again would be nice if you drew the mechanism

Question 28

What are the typical monomers used in cationic polymerization?

Answer 28

Shown in the notes.

Note that 1,1-dialkyl olefins and vinyl ethers only polymerize via cationic polymerization as it is stated in table 5.1! VERY IMPORTANT TABLE

Question 29

What is the initiators of cationic polymerization?

Answer 29

Cationic active centres are created by reaction of monomer with electrophiles (e.g. R+).

Protonic acids such as sulphuric acid (H2SO4) and perchloric acid (HClO4) can be used as initiators and act by the addition of a proton (H+) to monomer. However, hydrogen halide acids (e.g. HCl or HBr) are not suitable as initiators because the halide counter-ion rapidly combines with the carbocationic active centre to form a stable covalent bond.

Lewis acids such as boron trifluoride (BF3), aluminium chloride (AlCl3) and tin tetrachloride (SnCl4) are the most important ‘initiators’ but must be used in conjunction with a so-called co-catalyst which can be water, but more commonly is an organic halide. (The reactions are depicted in the notes)

For the BF3/H2O system, the second step of initiation for a monomer of general structure
CH2=CR1R2 involves both electrons from the π-bond joining with the H+ ion to create a new H–CH2 bond, thereby leaving an empty orbital (i.e. a positive charge) on the carbon bearing the substituents with which the counter-ion then becomes associated. (This is again depicted in the notes)

note that The term ‘co-catalyst’ (as used above) is common in the literature on cationic polymerization, but nevertheless is poor. It is clear from the chemistry of initiation that the ‘co-catalyst’ is in fact the initiator and that the Lewis acid behaves as an activator which also provides a more stable counter-ion.

Also, Iodine can be an initiator as shown in the slides

Question 30

What is the propagation in cationic polymerization?

Answer 30

For reasons of carbocation stability, propagation proceeds predominantly via successive head-to-tail additions of monomer to the active centre. (This is shown in the notes)

Question 31

What is the nature of termination and chain transfer for cationic polymerization?

Answer 31

Unlike in radical polymerization, for which the active centres bear no charge, termination by reaction together of two propagating chains is not possible for ionic polymerizations due to charge
repulsion. Instead, the growth of individual chains terminates most commonly by either unimolecular
rearrangement of the ion pair in which a penultimate C–H bond breaks to release H+ and generate a
terminal C=C bond in the dead molecule of polymer (as shown in notes) or chain transfer. Chain transfer to monomer often makes a significant contribution and involves
abstraction of an H-atom from the penultimate C–H bond by monomer, resulting in the formation of a monomeric carbocation (as shown in notes)

In either case, the resulting electrophile becomes associated with the counter-ion A– and starts the growth of a new polymer chain with little effect on the concentration of actively propagating chains. The terminal C=C bond in the dead polymer chain is of low reactivity due to steric hindrance to reaction arising from 1,2-disubstitution (and the presence of three substituents if both R1 and R2 ≠H) and so does not easily participate in the polymerization (i.e. it tends to remain unreacted).

Chain transfer to solvent and reactive impurities (e.g. H2O) can further limit the degree of polymerization, so the choice of solvent and the purity of the reactants is of great importance

Question 32

What is isobutyl, its applications and its initiation for cationic polymerization?

Answer 32

Depicted in the notes

Question 33

What is the butyl rubber process

Answer 33

It is depicted in the notes.

Note that it happens are very low temperatures

Question 34

What is the cationic synthesis of vinyl ether?

Answer 34

It is depicted in the notes.

Note that it has a lot of side reactions, like wayyyyy to many

Question 35

How can we make a living cationic polymerization system?

Answer 35

Basically what you do is have an initiator that will form a covalent bond with the carbon atom in the monomer, then add an activator that will attach to the initiator that forms the covalent bond. This activator will effectively pull the initiator causing the bond to become polarized forming a partial positive charge on the carbon.

This will activate the carbon monomer and will allow polymerization to continue with the initiator-activator complex going to the next monomer chain as depicted in the notes.

Question 36

What are the cationic open-ring polymerizations?

Answer 36

Keep in mind that we can also make a living cation ring-opening polymerization using the same concepts discussed. Again would be nice if you drew the mechanism

Question 37

What are the different possible living radical polymerizations?

Answer 37

1. Nitroxide-mediated polymerization (NMP)
2. Atom transfer radical polymerization (ATRP)
3. Reversible Addition-Fragmentation chain Transfer (RAFT)

Question 38

What are the equations for the degree of polymerization in NMP, ATRP, and RAFT?

Answer 38

This is depicted in notes along with the possible structures that can be made via these controlled radical polymerizations.

Two important things to remember:

1. For all radical-controlled polymerizations the Xn is a linear function of conversion
2. Many structures can be made via this controlled polymerization with all of them yielding very low PDI!

Question 39

Generally, how is radical polymerization made into a living system? (Very long but its important read)

Answer 39

before I explain I want to stress again that the only real living system is the anionic one.

The basic principle underlying all so-called living/controlled radical polymerizations is to suppress termination to the extent that it becomes insignificant by reversibly trapping and temporarily deactivating the chain radicals. Although the activation deactivation cycle is rapid, the chain radicals
that are free can still propagate (but also can undergo all other possible reactions, though termination has a much-reduced probability). From this simple picture, it is easy to see why reversible deactivation radical polymerization the name for these types of radical polymerization.

The various types of reversible-deactivation radical polymerization employ one or other of two general strategies which are illustrated in Figure 4.11 and will be described here in generic terms.

The first is shown in Figure 4.11(a) (For NMP and ATRP) and involves rapid, reversible end-capping of the chain radical by a chain-capping species, where the equilibrium lies far to the left. In this way, [M* ] is reduced massively, causing an enormous reduction in the rate of termination (because this depends upon [M*]^2).

Since the rate of propagation equation still is applicable and [M*] is very low, a feature of these types of reversible deactivation radical polymerizations is that they normally proceed at much lower rates than conventional free-radical polymerizations. A further interesting feature of reversible-deactivation radical polymerizations that proceed via Strategy 1 of Figure 4.11 is that, in principle, they should not suffer from autoacceleration (see Section 4.3.4) because both the rate of polymerization and the number average degree of polymerization are independent of kt.

The second strategy (For RAFT) is shown in Figure 4.11(b) and makes use of highly-efficient chain transfer reactions in which a free chain radical displaces a trapped chain radical from an end-capped species and in the process becomes end-capped. In this case, it is the very high efficiency of the exchange process and the much higher number of trapping agent molecules present compared to the total number of primary radicals produced from the initiator that makes termination negligible. Hence, [M*] can have values similar to those for conventional free-radical polymerizations. In this case, the normal rate of polymerization equation applies, so rate autoacceleration can occur but usually is mitigated because the polymerizations often are designed to produce polymers with lower molar masses.

Both types of reversible deactivation radical polymerization are quasi living because, although
at any instant in time most of the chain radicals are in the end-capped dormant state, when released
they can undergo propagation, chain transfer and termination as in a conventional free-radical polymerization, though as indicated above, termination has a much lower probability.

Whilst reversible end-capping enables the chain radicals to be protected from bimolecular termination (i.e. kept
‘alive’), in order to produce polymers with narrow molar mass distributions, it is essential that the
rates at which chain radicals are released and recaptured by the trapping agent are both high. Thus,
the most important criterion for living-like control in reversible-deactivation radical polymerizations is rapid, reversible exchange between the active and dormant states. Each chain then grows
with approximately equal probability in very short bursts of activity and the Poisson distribution
of molar mass can be achieved, as in a true living polymerization.

Because termination is suppressed almost completely in reversible-deactivation radical polymerizations and exchange between the active and dormant states is rapid, they show the characteristic features
of true living polymerizations, namely: (i) degree of polymerization increases linearly with conversion
and the molar mass distribution is narrow and (ii) chains can be
grown first with one monomer and then another to produce block copolymers.

When these features are considered together with the wide range of functional groups that are tolerant of radicals, it is not surprising that there has been an explosion of academic and industrial interest in synthesis of well-defined polymers from a wide range of monomers using methods of reversible-deactivation radical polymerization. This remains a rapidly developing area of polymer chemistry

Finally in order to keep the system living we must have a much larger amount of dormant chains in comparison to dead chains

Question 40

What are the common initiators used in NMP?

Answer 40

Shown in the notes

Question 41

What is the mechanism for NMP?

Answer 41

It is depicted within the notes

Note that we will be tested on the mechanism. SO WRITE IT ON WHITEBOARD

Question 42

What are the general aspects and the structures formed from NMP?

Answer 42

it is again depicted within the notes

Question 43

What are ATRP initiation steps, Intiators, overall mechanisms, and the structures made by it?

Answer 43

All depicted in notes

Question 44

What is RAFT (Detailed explanation)?

Answer 44

RAFT polymerization is, in essence, a normal free-radical polymerization to which is added a
highly active dithioester transfer agent (the RAFT agent) that fragments during the chain-transfer
process to release a new radical and generate a new dithioester species via the addition-fragmentation mechanism shown in the notes.

In order for this chain-transfer reaction to be facile, the reactivities of the two radicals (R* and A) must be similar and the group Z must activate the exchange process. If R is a chain radical, it becomes trapped by addition to the RAFT agent and that chain remains dormant until it is released in a further addition-fragmentation chaintransfer process when another chain radical adds to the dithioester end-group. By using a RAFT agent that is very highly active in chain transfer at a level that greatly exceeds the total number of primary radicals generated from the initiator during the course of the whole polymerization, addition-fragmentation chain-transfer to dithioester groups becomes the dominant process by which propagating chains are both generated and captured, thereby making bimolecular termination an event of low probability. Since capture of a chain radical by a dithioester group is reversible and fast, at any instant in time, most chains are in the dormant dithioester form.

Question 45

What are possible RAFT agents?

Answer 45

This is depicted in notes

Question 46

What is the full RAFT mechanism?

Answer 46

This is depicted in notes. Again we will be tested on this so do on the whiteboard

Question 47

What are some RAFT general aspects?

Answer 47

This is depicted in notes

Question 48

What are some RAFT structures?

Answer 48

This is depicted in notes

Note that, unlike in NMP and ATRP, the RAFT agent isn’t in the final product!