Dormant chain

The number-average degree of polymerization for a living cationic polymerization is defined as the concentration of monomer consumed divided by the total concentration of all propagating chains (dormant and active)... 

These pioneer studies laid dormant until 1977 and, influenced by Kondo and colleagues [59] reports on the synthesis of po]y(vinylsulfonium yiide) with a trivaient sulfur attached directly to the polymer chain, poly[ethyl-vinylsulfonium bis-(methoxycarbonyl) methylide] (Scheme 25) was prepared by irradiation of a benzene... 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

An issue in living radical copolymerization is that the conditions for dormant chain activation can vary substantially according to the particular propagating radical. The problem may be mitigated by two factors. 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

Polymers formed by ATRP should retain a halogen (typically bromine) on the dormant chain end and this is confirmed by analysis for many polymerizations. 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Recent kinetic studies of this polymerization 14) revealed that some parasitic reactions cause termination and induction periods in the overall process. Their nature is not known yet. It is tentatively suggested that the activated polymers react with the dormant ones yielding some destruction products, although the nucleophile capable of activating the still available dormant chains is regenerated. Alternatively it is possible that the intermediate 3 is labile and may decompose before collapsing into 4 with regeneration of the nucleophile. Whatever the cause of these side reactions, one should stress that the conversion of the monomer into polymer is almost quantitative. 

The mechanism of anionic polymerization of styrene and its derivatives is well known and documented, and does not require reviewing. Polymerization initiated in hydrocarbon solvents by lithium alkyls yields dimeric dormant polymers, (P, Li)2, in equilibrium with the active monomeric chains, P, Li, i.e. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Scheme 1 Activation of dormant chains by exchange with the growing chain from a live cata- AU9 lyst...

Our approach to polymer chain growth modeling is based on population balances for the various polymer species participating in and resulting from chain growth and transfer [34], The kinetics scheme is written below in mathematical fashion and is a precursor to the derivation of population balances. Monomer units are represented as M, and growing polymer chains are represented by the symbol Pn, where n is the number of repeat units attached to the active catalyst. Dormant polymer is represented by An where n is the number of repeat units attached to the CTA. Dead polymer chains, which arise from chain termination events such as hydrogenolysis... 

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