Chain transfer ring-opening polymerization

Heterochain polymers produced by ring-opening polymerization contain the hetero-atoms in the main chain as well as in the monomer and the polymer chain competes with the monomer for the reaction with the propagating species. This competition leads to polymer transfer and back-biting reactions during the polymerization. Heterochain polymers are also susceptible to depolymerization by the ionic active species which are easily formed during processing. 

Ring-opening polymerization of cyclic siloxanes with cationic initiators allows the possibility of introducing stable end groups by the use of suitable chain transfer agents. Thus, polysiloxanes with trimethylsilyl end groups are formed when the cationic polymerization of octamethylcyclotetrasiloxane is carried out in the presence of hexamethyldisiloxane as transfer agent ... 

RAFT Reversible addition-fragmentation chain transfer ROP Ring-opening polymerization... 

The synthetic procedure of PE-fo-PCL using hydroxyl terminated polyethylene was reported [39]. Terminally hydroxylated polyethylene was prepared during a metallocene-catalyzed polymerization using controlled chain transfer reaction with alkylaluminum compounds. PE-fo-PCL block copolymer was synthesized from terminally hydroxylated PE and e-caprolactone (e-CL) using Sn(Oct)2 as a catalyst for ring opening polymerization. 

Thus, chain transfer to polymer is a general phenomenon in cationic ring-opening polymerization. Consequences will be discussed in more detail in Section II.C. 

As discussed in Section II.C, in any system in cationic ring-opening polymerization, a reaction of active species with polymer repeating unit may lead to chain transfer to polymer or termination (if the resulting branched or cyclic onium ions are not active), whereas recombination with counterion leads to termination in the case of irreversible reaction. The later reaction may be avoided by the proper choice of counterion. As the onium ions are generally inherently stable there is no other termination reaction, provided that impurities that may act as terminating agents are eliminated. 

As discussed already, termination of ring-opening polymerization may proceed by (a) irreversible recombination with counterion and (b) irreversible chain transfer to polymer. Other sources of termination are also possible, depending on the system (c) reaction with other components of the system, solvent or impurities and (d) different reactions of more reactive species existing in equilibrium with stable onium species. 

The results show that the presence of bulky substituent on a polymer chain may effectively inhibit the termination proceeding by this mechanism. The results presented at this point may be summarized as follows chain transfer to polymer is a general feature of cationic ring-opening polymerization although for different systems the contribution of this reaction may vary only in some systems this process results in termination (These systems involve, e.g., cyclic amines (3- and 4-membered) and cyclic sulfides (3- and 4-membered) and the contribution of the reaction is reduced for substituted chains. 

Even in thoroughly purified systems, impurities can not be completely eliminated. The most common impurity, water, is detrimental for cationic ring-opening polymerization, although it acts rather as a chain transfer agent and not a terminating agent, because usually the proton formed can initiate the new chain ... 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

In cationic ring-opening polymerization, there are not too many examples of the systems in which ratios of kplk, are known. In the polymerization of substituted aziridines and substituted thietanes the ratios of rate constants of chain transfer to polymer to the rate constants of propagation have been measured and at least the value obtained for polymerization of N-/-butylaziridine (1.2-104) [260], indeed indicates the living character... 

As shown earlier in this chapter, there are several systems in cationic ring-opening polymerization, in which quantitative initiation may be achieved and there is essentially no transfer and/or termination. Thus, all the chains have the initiator moiety as the head group and active centers at the growing chain end. These cationic active centers may be deactivated with suitable nucleophiles, leading to specific structure of the terminal end group. 

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