Mikto-arm star

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). 

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

The computed results in Fig. 36 capture the observed phase behaviour of mikto arm-star copolymer melts. The effect of the molecular asymmetry seems to saturate for n > 3, and phase boundaries do not change significantly as a function of 0 or higher asymmetry. 

There are two gereral routes to mikto-arm star polymers. The first method makes use of the stepwise addition of living polymers to multifunctional chloro-silane compounds [59-62], The Athens group uses the sequential addition of living polymers to multifunctional chlorosilane compounds under tight stoichiometric control [63, 64],... 

The properties of these polymers have similarities with mikto-arm star block copolymers. 

Finally, Lecomte and coworkers reported the synthesis of mikto-arm star-shaped aliphatic polyesters by implementing a strategy based on click chemistry (Fig. 36) [162]. Firstly, the polymerization of sCL was initiated by a diol bearing an alkyne function. The chain-ends were protected from any further undesired reaction by the esterification reaction with acetyl chloride. The alkyne was then reacted with 3-azidopropan-l-ol. The hydroxyl function located at the middle of the chain was then used to initiate the ROP of sCL and y-bromo-s-caprolactone. Finally, pendant bromides were reacted successfully with sodium azide and then with N, N-dimethylprop-2-yn-l-amine to obtain pendant amines. Under acidic conditions, pendant amines were protonated and the polymer turned out to exhibit amphiphilic properties. 

The product contains dormant functionality in the core. Tliis can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP/° ATR " ). 

Multiarm polymers (11) can be prepared that still have the reactive functional groups (Z) close to the core. As these are still active, they can be used as sites to initiate the growth of more arms by adding either the same monomer used to prepare (11) or a second monomer to prodnce mikto-arm star polymers, in which the arms have different chemical structures. Thus, an active ended poly(t-butyl acrylate), prepared by ATRP, can be coupled with divinyl benzene to form a multiann star polymer. This structure can be converted to a mikto-arm star polymer by reacting the living ends still present with n-butyl acrylate, and so propagate poly(n-butyl acrylate) chains from the core outward. 

Babin J, Taton D, Brinkmann M, Lecoimnandoux S (2008) Synthesis and self-assembly in bulk of linear and mikto-arm star block copolymers based on polystyrene and poly(glutamic acid). Macromolecules 41 1384-1392... 

In addition, 1,1-diphenylalkyllithium sites in the living linked polymer product (100) can initiate polymerization of a second monomer (M ) to generate a heteroarm (or mikto-arm), star-branched polymer, 102 [243, 244). 

Figure 20.4 AFM image in phase mode of the self-assembled lamellar structure of PS-b-PGA block copolymers. (Reprinted with permission from J. Babin, D. Taton, M. Brinkmann and S. Lecommandoux, Synthesis and self-assembly in bulk of linear and mikto-arm star block copolymers based on polystyrene and poly(glutamic acid), Macromolecules, 41, 4, 1384-1392, 2008. 2008 American Chemical Society.)...

Star polyethers have been synthesized by a core-first method. AB2- and A2B-type mikto-arm star copolymers consisting of aromatic polyether arms as the A segment and polystyrene arms as the B segment were synthesized by using orthogonal trifunctional initiators (Scheme 32) [73]. 

Fig. 4 TEM images of CHCI3 cast of (a) AB-type diblock copolymer, (b) AB2-type mikto-arm star copolymer, (c) A2B-type mikto-arm star copolymer, and (d) (AB)3-type star block copolymer. The samples were stained with RUO4 prior to TEM measurement...

As mentioned above, stoichiometric control allows the preparation of tri-, tetra-and penta-adducts. With the upper limit of the number of grafts being six, it becomes possible to add, respectively, three, two or one additional PS or PI arms. This has been used to prepare asymmetric stars (PSa)6.>,C )(PSb) or mikto-arm stars (PS)s. Cso( PI)n by reacting such adducts of lower functionality with an excess of PSIi that has a different molar mass or with polyisoprenyllithium (Scheme 5.12) [73]. 

Scheme 5.12 A six-arm asymmetric star (PS,),C6o(PSb)6, or mikto-arm star (PSa) ,C6o(PI)6- obtained by saturation of a polystyrene star (PSa),Cso with an excess of PSbLi or PI Li. The number of arms of the non-saturated star (x < 6) is controlled by adjusting the stoichiometry PSa/Ceo-...

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