End Group Transformation

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. 

Table 9.24 Methods for End Group Transformation of Polymers Formed by ATRP by Addition or Addition-Fragmentation,...

Table 9.25 End Group Transformations for Polymers Formed by ATRP...

Fig. 5. Schematic of end group transformations that have been demonstrated for mono-layers with terminal ester groups. The terminal acid group has been used to couple amino acids to the surface while the branched structure that results from the Grignard reaction is expected to result in more stable, passivated surface.

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... 

Scheme 11.18 Synthesis of block copolymers by combination of CROP and NMRP using end-group transformation.

Matyjaszewski et reported the first end-group transformation of polystyrene prepared by ATRP. The halogen-functionalized polystyrene, where the halogen is Cl or Br, was transformed to an azide by reaction with trimethylsilyl azide in... 

Though typically a stepwise procedure was used in which the azide-terminatred polymer was isolated and puri ed before reaction with the alkyne species, an in-situ end group transformation process was also condncted in both the cases in which the azide-terminated polymers were reacted in situ with alkyne-functionahzed methacrylates. This one-pot synthesis is possible due to the orthogonal reactivity inherent in chck chemistry. 

High yielding end group transformations are particularly attractive because otherwise the cyclization reaction will be incomplete causing the presence of linear impurities. These linear polymers can be challenging to remove, and even small percentages of these non-cyclic impurities can have a detrimental effect on the bulk properties of cyclic polymers. 

Reviews focussing on end-functional polymers include those by Willcock and O Reilly, Moad et Other reviews that include significant sections on end-functional polymers and end-group transformation include our reviews of the RAFT process, that by Boyer etfll. " on biomedical applications, and that by Moad et al on optoelectronic applications. 

Scheme 31 Processes for RAFT end-group transformation (R = radical, [H] = Hatom donor, M = monomer, Co"=square planar cobalt complex). Adapted from Moad, G. Rizzardo, E. Thang, S. H. Polym. Int. 2011,60.9-25. ...

Block copolymers based on polymers formed by other mechanisms can be made by first preparing an end-funrtional prepolymer which is converted to a polymer with thiocarbo-nylthio groups by end-group transformation. This is then used as a macro-RAFT agent in preparation of the desired block copolymer (Scheme 34). We first exploited this methodology to prepare PEO-Wocfe-PSt from commerdally available hydroxy end-functional... 

A characteristic advantage of the TERP, SBRP, and BIRP is the versatility of the transformations that can be carried out on the polymer-end groups. Since polymer-end radicals are easily generated from the organoheteroatom dormant species, several polymer-end group transformations mediated by radical species have been developed. 

The selective co-end group transformation combined with the use of functional CTAs, such as Sb-7 and Sb-8, have led to the synthesis of structurally well-controlled telechelic poly-mers. For example, PSt 14, which possesses an alkene functionality at the a-polymer end prepared from Sb-7 and St, was heated with allylstannane 11 in the presence of AIBN at 80 °C to give telechelic polymer 15 as the sole product (Scheme 12(a)). Aerobic oxidation of an organostibanyl group to a hydroxyl group has been reported,and application of this procedure to 14 resulted in the selective formation of co-hydroxylated PSt 16 (Scheme 12(b)). The different functional groups at the a-polymer and co-polymer ends of these PSts should be useful for further selective synthetic transformations. 

---