Functional stars

According to Benoit and Hadziioannou [148] the partial structure factors of a f functional star polymer with diblock arms of A- B-type in the Gaussian approximation are given by... 

Functional stars are small molecules compared with dendrimers. Some stars have simple and some dendritic structures. Their functional groups are placed at arms extremities or distributed along the arms. 

In general, wider polydispersity in functionality or group reactivity makes the gel time shorter and critical conversion lower. This conclusion is based on the results of theoretical and experimental studies concerning the effect of distributions in functionality and in group reactivity on crosslinking of functional stars [31-33],... 

FIGURE 6.2 Representation of multivariate data by icons, faces, and music for human cluster analysis and classification in a demo example with mass spectra. Mass spectra have first been transformed by modulo-14 summation (see Section 7.4.4) and from the resulting 14 variables, 8 variables with maximum variance have been selected and scaled to integer values between 1 and 5. A, typical pattern for aromatic hydrocarbons B, typical pattern for alkanes C, typical pattern for alkenes 1 and 2, unknowns (2-methyl-heptane and meta-xylene). The 5x8 data matrix has been used to draw faces (by function faces in the R-library Tea-chingDemos ), segment icons (by R-function stars ), and to create small melodies (Varmuza 1986). Both unknowns can be easily assigned to the correct class by all three representations. 

Freed et al. [42,43], among others [44,45] have performed RG perturbation calculations of conformational properties of star chains. The results are mainly valid for low functionality stars. A general conclusion of these calculations is that the EV dependence of the mean size can be expressed as the contribution of two terms. One of them contains much of the chain length dependence but does not depend on the polymer architecture. The other term changes with different architectures but varies weakly with EV. Kosmas et al. [5] have also performed similar perturbation calculations for combs with branching points of different functionalities (that they denoted as brushes). Ohno and Binder [46] also employed RG calculations to evaluate the form of the bead density and center-to-end distance distribution of stars in the bulk and adsorbed in a surface. These calculations are consistent with their scaling theory [27]. 

RG calculations in the EV regime have been performed by Ereed and Douglas for linear and uniform star chains [69]. The results are expected to be valid only for low functionality stars and, in fact, the comparison with existing experimental data is only reasonable for small number of arms, while the theoretical results for f 6 clearly exceed the experimental data [49]. 

Fig. 8a,b. Off-lattice representations of a three-functional star a Bead and Rod model b Bead and Spring model... 

Fig. 22. KMHS relationships for the fractions of end-linked 3-arm star-branched polystyrene molecules and of linear polystyrene fractions. The data refer to three samples of different in the pregel state and one from the sol fraction of a gel. The curves for the branched macromolecules coincide within experimental error in the high molar mass region. The deviations at lowM result from a different amount of non-reacted end-functionalized stars. The exponents of the end-linked and linear PS chains are a =0.42 0.02 while that of linear chains is 0.70 0.01 [95,120,123,124]. Reprinted with permission from [95]. Copyright [1997] American Society...

Turunen MPK, Korhonen H, Tuominen J, Seppala JV (2001) Synthesis, characterization and crosslinking of functional star-shaped poly(E-caprolactone). Polym Int 51 92-100... 

Although the core-first method is the simplest, success depends on initiator preparation and quantitative initiation under living conditions. This method is of limited use in anionic polymerization because of the generally poor solubility of multifunctional initiators in hydrocarbon solvents [12]. Solubilities of multifunctional initiators are less of an issue in cationic polymerizations, and tri- and tetrafunctional initiators have been used to prepare well-defined three- and four-arm star polymers by this method [7] Except for two reports on the synthesis of hexa-arm polystyrene [27] and hexa-arm polyoxazoHne [26], there is a dearth of information in regard to well-defined multifunctional initiators for the preparation of higher functionality stars. 

The growing B arms have anionic sites at their outer ends thus providing the possibility of reacting with electrophilic compounds or other monomers towards the preparation of end-functionalized stars or star-block copolymers. This method can be carried out in inert atmosphere, avoiding the use of the highly demanding and time consuming vacuum technique. It was first reported by Okay and Funke [11] and by Eschwey and Burchard [12] and developed by Rempp and collaborators [13-16]. Scheme 3 illustrates the DVB method. 

A larger reduction in the mean square unperturbed dimensions can be achieved by rearrangement of the n + 1 atoms into an /-functional star-branched polymer. In this architecture, the macro molecule contains / branches (f> 2), each with n/f bonds, that emanate from a common atom. The same terminology is frequently used for branched macromolecules with very large/if the branches emanate from a collection of atoms that are constrained to remain close together, so that the origin of all of the branches is clustered in a volume much smaller than (s2)m. The influence of the star-branched architecture on the mean square dimensions is traditionally designated by a factor g that is defined as ... 

Deviations from spherical symmetry are largest for the linear chain, and smallest for the tetrafunctional star. The macrocycle occupies a position intermediate between the tri-and tetra-functional star. 

By the use of the polymer-linking method with 20a, a variety of starshaped poly(vinyl ethers) have been synthesized (Scheme 12) [208-212]. A focus of these syntheses is to introduce polar functional groups, such as hydroxyl and carboxyl, into the multiarmed architectures. These functionalized star polymers include star block (23a,23b) [209,210], heteroarm (24) [211], and core-functionalized (25) [212] star polymers. Scheme 12 also shows the route for the amphiphilic star block polymers (23b) where each arm consists of an AB-block copolymer of 1BVE and HOVE [209] or a vinyl ether with a pendant carboxyl group [210], Thus, this is an expanded version of triarmed and tetraarmed amphiphilic block copolymers obtained by the multifunctional initiation (Section VI.B.2) and the multifunctional termination (Section VI.B.3). Note that, as in the previously discussed cases, the hydrophilic arm segments may be placed either the inner or the outer layers of the arms. 

Subsequently, this concept has been extended to more highly functionalized star-initiators as well as polymeric initiators for the synthesis of graft structures. It should however, be appreciated that the polymerization process is still radical in nature and while radical—radical coupling reactions are decreased they are not eliminated and so as the number of initiating sites per molecule increase, the prob-... 

The review roughly consists of two parts, the scope and design of initiating systems followed by precision polymer synthesis. The former will treat the scope of metal catalysts, initiators, and monomers along with polymerization mechanisms. The latter will focus on the precision synthesis of various polymers with controlled structures and interesting properties or functions, such as block, end-functionalized, star, and other architecturally well-defined polymers. Other reviews are also available, comprehensive,31,40-44 and relatively short,45-52 dealing with the recent developments of the field. 

L-24 as a ligand, up to 85—90% yield. The linking reaction of a poly(tBA) with a bromide terminal was also possible with divinylbenzene, whereas the other two divinyl compounds led to side reactions.328 The yield of star polymers can be increased up to 95% with the use of additives. The a-end-functionalized linear polymers afford surface-functionalized star polymers with various functional groups such as alcohols, amines, epoxides, and nitriles. 

The polyfunctional initiator method provides the possibility to prepare end-functionalized stars by deactivating the living branches by suitable electrophilic terminating agents. Polystyrene (PS) and PEO stars having end hydroxyl groups were prepared by this method.1519... 

These methods have also been employed for the synthesis of functionalized star polymers. Recent applications will be presented in the following sections. 

PI)sPS(PI)5 copolymers (pom-pom shaped) were synthesized in a way similar to the preparation of H-shaped copolymers101 (Scheme 75). A hexafunc-tional silane was reacted with PILi in a ratio SiCl Li = 6 5, giving the five-arm star having a SiCl group at the central point. These functional stars were reacted in a second step with a difunctional PS, giving the desired pom-pom copolymers. 

The birthrate function- Stars form and die continuously in galaxies, therefore a recipe for star formation is necessary. We define the stellar birthrate function as the number of stars formed in the time interval dt and in the mass range dm as ... 

Several other efforts for synthesizing high-functionality star polymers have been... 

Functionalized star polymers can be prepared by the reaction of end-fiinctionalized living polymers with suitable ddorosilanes whi act as linking agents. Three and twelve arm PI stars (21) and three arm star PBd with all ends functionalized whli dimethylamine groups (22) were synthesized following this procedure. An example is given in scheme 2. 

Neutral and charged multiarm star polymers possessing homopolymeric arms functional groups can be introduced either along the arms (in-chain functionalized stars) or at the end of each arm (end-functionahzed star polymers). 

---