Question

Suggest two methods for making the PStyrene-PButadiene-PStyrene (polystyrene-polybutadiene-polystyrene)
triblock copolymer in a batch reactor using n-BuLi as an initiator.

Answer 1

These polymers are known as thermoplastic elastomers (TPE). The polymerization is carried out in a semi-batch reactor by sequential addition of monomers. Styrene-butadiene-styrene (SBS) block polymers are produced by first polymerizing styrene to form the “S” block followed by addition of half of butadiene to form the half “B” block. Then a di-functional coupling agent such as iodine is added to link the living polymer chains and form the tri-block polymer.

The ionic kinetic scheme in Polymers Plus can handle anionic, cationic and group transfer polymerization. A subset of the ionic kinetic scheme shown below is used to model semi-batch copolymerization of styrene and butadiene using butyllithium as an initiator.

Initiator Dissociation : (BuLi)6 6 BuLi

Chain Initiation : BuLi + Mj -> P11,j

Propagation : Pln,k+ Mj -> P1n+d j,

Association : Pln,k + Plm,k -> P2 n+m,

Coupling : Pln,k + I2 -> P3n,k

Pln + P3m -> P4 n+m,

Pi n,k is a growing polymer with an index ‘i’. The index ‘i’ keeps track of different types of active species (e.g., free-ions, ion-pairs, dormant esters, coupled polymer, associated polymer). The subscript ‘n’ and ‘k’ represent the chain length and the end segment, respectively. The built-in kinetic scheme in Polymers Plus has several other reactions, e.g., activation and initiation reactions, exchange reactions, chain transfer reactions and termination reactions that are not shown in the above kinetic scheme.

living anionic polymerization: A living polymerization is a polymerization that takes place without any termination reactions. This means that once all the monomer in your beaker is used up, and has been turned into polymer, the polymer chain ends are still active. This means that if you put more monomer in the beaker, it would add to the polymer ends and make the polymers bigger.

polymerizing the monomer styrene with an anionic initiator like butyl lithium.

this polystyrene chain is living, so if we add a second monomer to it, it'll add to the polymer. So we'll add some of the monomer butadiene.

That gives us a living styrene-butadiene block copolymer.

The next step is obvious: just add more styrene monomer, and get a styrene-butadiene-styrene triblock copolymer.

The anionic living chain kicks off a chlorine atom from the silane, and we get a chlorosilane end-capped polymer.Now our polymer is no longer living! It's good because we can do something with this chlorosilane end-capped polymer. You see, if we take living polystyrene homopolymer, it will react with the chlorosilane end-capped polymer, just like the styrene-butadiene copolymer reacted with the dichlorodimethylsilane.