Star copolymers

For ihe case of NMP and RAFT, there exist two basic ways of growing star copolymers (this discussion also applies to block and graft copolymer synthesis). 

Graft copolymers made by living polymerization processes are often called polymer brushes because of the uniformity in graft length that is possible. The basic approaches to graft copolymers also have some analogies with those used in making block and star copolymers. 

Spot tests, 245 Spray elastomers, 204 Star-branched polymers, 187 Star copolymers, 7... 

A star copolymer (SCP) of PCLA was synthesized by Younes and coworkers. This kind of SCP PCLA elastomer was also synthesized in two steps. First, the small molecular SCP was produced by ring-opening polymerization of s-caprolactone (s-CL) with glycerol as initiator and stannous 2-ethyUiexanoate as catalyst. Second, the living SCP was further reacted with different ratios of a cross-linking monomer, such as 2,2-bis(s-CL-4-yl)-propane (BCP) and s-CL. The SCP elastomers had very low glass transition temperature (—32°C). It was reported that the SCPs were soft and weak with physical properties similar to those of natural bioelastomers such as elastin. A logarithmic decrease in each tensile property with time was observed in this SCP PCLA. 

The living ALi chains polymerize a small amount of DVB leading to the formation of a star molecule bearing within its core (microgel nodule of DVB) a number of active sites, which is theoretically equal to the number of incorporated A arms. Subsequent addition of monomer B yields the /z-star copolymer. 

In miktoarm-star copolymers (or heteroarm star copolymers ), the unlike blocks are connected at one junction point, as shown in Fig. 34. As in linear-block copolymers, exactly two blocks are linked. Hence, it seems tempting to approximate those copolymers as a set of diblocks with the free ends of one block component joined together in a cluster [111]. 

However, the central junction of miktoarm-star copolymers exhibits more crowding compared to linear diblocks. The penalty for blocks bending at the interface will be higher (Fig. 34a,c), resulting in a shift of the phase boundaries and different spacings compared to their linear analogues of similar composition. 

Recently, Grason and Kamien calculated the phase diagrams in the weak and strong segregation limit for AB miktoarm-star copolymers using both... 

Fig. 35 Phase diagrams AB miktoarm-star copolymers for n = 2, n = 3, n = 4 and n = 5. mean-field critical point through which system can transition from disordered state to Lam phase via continuous, second-order phase transition. All other phase transitions are first-order. From [112]. Copyright 2004 American Chemical Society...

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. 

Fig. 37 Morphology of 4-miktoarm-star copolymers of PS(P2MP)3 type, a TEM images, b SAXS data. From [113]. Copyright 2003 American Chemical Society...

Fig. 38 Milner s diagram for morphologies of A-b-B copolymers as reported in literature [111]. Observed morphologies of linear and miktoarm-star copolymers of BS-b-P2MP system are represented. Lamellar structure, cylinders of P2MP in PS matrix spheres of P2MP in PS matrix, biphasic structure of lamellar/double gyroid microdomains. From [113]. Copyright 2003 American Chemical Society...

In the same study the coexistence of two morphologies (L and G) exhibited by a four-miktoarm-star copolymer was reported. This biphasic structure... 

The influence of the architecture on the phase behaviour of symmetric miktoarm stars AnBn (Fig. 34c) was investigated by Grayer et al. [119]. Symmetric miktoarm-star copolymers PS-arm-P2VP having a mean func-... 

In conclusion, it can be suggested that the lamellar spacing of miktoarm-star copolymers is controlled by two parameters (i) the molecular weight of the corresponding AB diblock, which also controls the segregation strength, and (ii) the functionality of the central core. 

Fig. 43 Plot of normalized lamellar long periods, Dn/Rg,m of (PS) -arm-(PI)M miktoarm-star copolymers (n = 1, 2, 4 and 16) divided by corresponding diblocks of same series, Di/Rg,u against respective star functionality, n. Normalization factor PgiM represents unperturbed radius of gyration of diblock consisting of one PS block, one PI block and average number of bonds linking these two arms through core. From [121]. Copyright 2003 American Chemical Society...

Despite the fact that Milner s theory was originally developed for miktoarm-star copolymers, it can also be adopted for more complex branched structures. This empirical concept termed constituting-block copolymers approximates the architecture of branched molecules to be composed of an array of A2B and A2B2 miktoarms. This approach is capable of predicting the morphology of architectures as complex as centipedes or barbed wires, as shown in a very recent publication [125]. 

Fig. 6 AFM topographic images (a-d, i, j) and cross sections (e, f, k, I) of a miktoarm PS-P2VP star copolymer adsorbed on mica from chloroform (a-c, e), from THF (d, f) and from acidic water (HC1, pH = 2) in salt free (i, k) and in the presence of 1 mM Na3P04 (j, I). Schematic representation of the solution conformations and conformations in adsorbed state of the PS-P2VP in chloroform (g), THF (h), in water at pH = 2 before (n) and after adsorption (m) respectively (PS arms in red, P2VP ones in blue). Reprinted with permission from [116]. Copyright (2003) American Chemical Society...

Similar onion type micelles were obtained by the combination of PS-P2VP heteroarm-star copolymers with a P2VP-PEO diblock copolymer, as reported recently by Tsitsilianis et al. [269]. 

The second technique makes extensive use of multifunctional diphenylethy-lene compounds to produce e.g. A2B2 star copolymers with A and B polystyrene and polybutadiene chains, respectively [66],... 

Linear-dendritic star copolymers [5], most frequently obtained via processes in which dendrimers function as multifunctional initiator cores for the poly-... 

Fig. 1. a Star polymer, b Comb polymer, c Brush, d Miktoarm star copolymer, e Star copolymer, f Star chain center-adsorbed in a plane, g Dendrimer... 

Block copolymers have peculiar characteristics due to the coexistence of two or several different parts of different chemical compositions within a chain. They can undergo microphase separation transitions from a homogenous phase to a variety of spatially periodic structures [176]. A distinction should be made between star copolymers, where each arm is composed by two or more blocks, and miktoarm polymers, formed by homopolymer arms of different chemical compositions. Floudas et al. [177] recently performed an extensive study of four-... 

Source-based copolymer names [6] are given first, followed by an explanation in parentheses before the proposed formula. Structure-based names [5] are given for star copolymers in the notes to examples 4.4-E6 and 4.4-E7... 

The system of naming graft copolymers is also applicable, in principle, to star copolymers , where chains having different constitutional or configurational features are linked through a central moiety. 

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