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The association behavior of a triblock ethylene oxide (E) and butylene oxide (B) copolymer with a very long middle E block (B12E260B12) in aqueous solution has been investigated by means of laser light scattering. The flower-like closed association behavior of BEB-type triblock copolymers in aqueous solution can be altered to form open, branched, associated structures because of the very long E blocks. The B12E260B12 triblock forms flower-like micelles in a closed association process with a critical micelle concentration of 0.056, 0.040, and 0.033 mg/mL even at fairly low temperatures of 10.0, 20.0, and 25.0°C, respectively. At room temperature, B12E260B12 can form large secondary associated structures which coexist with the flower-like micelles at concentrations of only 1-2% (w/v). The secondary associates increase in both amount and size with increasing concentration and increasing temperature, producing a highly viscous fluid even for a 4% aqueous solution of B12E260B12.

have been synthesized and purified, and their association properties in aqueous solution studied by surface tension and static and dynamic light scattering. A 50 : 50 wt.% mixture of the two copolymers in dilute aqueous solution formed mixed micelles in which the triblock copolymer molecules were predominantly in the looped conformation. At higher concentrations (e.g. 100 g dm-3, the mixed micelles themselves associated to form micellar clusters which were bridged by triblock copolymers in the extended conformation. The results include micelle and cluster association numbers.The relationship between the association properties of block copolymers in solution and block architecture is of practical importance and academic interest. With water as solvent, we have systematically explored the effect of block architecture on the association of oxyethylene/oxybutylene (E/B) copolymers (Em B, , Em B, Em and B, Em B,)1-3 and oxyethylene/ oxypropylene (E/P) copolymers (Em P, Em and P, Em P,)." For E/B copolymers of high E content ( > 60 wt.%), comparing the properties of copolymers of identical chain length and overall composition in solution at a given temperature, the rank orders are as follows. Critical micelle concentration (c.m.c.) : Em B, < B, Em B, = Em B, Em; association number: Em B, > B, Em B, > Em B, Em. The placing of the c.m.c.s of the Em B, Em copolymers relative to B, Em B, copolymers depends on E-block length.3 The ranking order of clouding temperature for e.g. a 1 wt.% soiution is: EmB, > EmB,Em >

Introduction.Commercially available triblock copolymers of ethylene oxide (E) and propylene oxide (P), usually in the form of E m P n E m , known as Pluronic polyols, have found diverse industrial applications. 1 In recent years, a great many fundamental studies have been devoted to the self-assembly and the structure characteristics of supramolecular particles, mostly in the form of polymeric micelles, for the EPE-type triblock copolymers in aqueous solution. 2 Of particular interest are the properties of such water-soluble triblock copolymers in a solvent which is selectively good for the middle block. If the selective solvent is water, the self-assembly behavior of P n E m P n triblock copolymers, known as reverse Pluronics or Pluronic R, has been reported. 3 Many PEP-type triblock copolymers will probably form flower-like micelles in water with an entropy-driven closed association mechanism. However, it is anticipated that when the middle E block becomes sufficiently long, the two end P blocks may not necessarily return to the same micellar core. Then, an extended network structure may result. The relationship between the selfassembly of block copolymers in solution and chain architecture can be extended to other types of longerchain block copolymers, such as poly(tert-butylstyrene)-block-polystyrene-block-poly(tert-butylstyrene) (P(t-BS)-PS-P(t-BS)) in N,N-dimethylacetamide (DMA), which is a good solvent for the middle PS block 4 and poly-(caprolactone)-block-poly(ethylene oxide)-block-poly-(caprolactone). 5 Butylene oxide (B) is more hydrophobic than propylene oxide. Thus, a study of the micellization behavior of B n E m B n triblocks becomes of interest. Experimental Procedures. B 5 E 91 B 5 and B 6 E 46 B 6 were received from Dow Chemical Co., while B 12 E 260 B 12 was prepared by the Manchester Polymer Center. In order to avoid the anomalous micellization behavior before the onset of critical micelle formation, 6 the B n E m B n triblocks were purified by hexane extraction. 7 The light scattering measurements and sample preparation were the same as described elsewhere. 6 The copolymer solutions at high concentrations (e.g., 10% (w/w)) were prepared by first dissolving the B n E m B n copolymer in water at low temperatures (e.g., 5°C) in order to ensure complete solute dissolution. The copolymer solution was then filtered, since the presence of dust could interfere with light scattering experiments. The filtered solution was then brought to the desired temperature for a sufficiently long time to ensure that the association process has reached a dynamic equilibrium among the species (unimers, micelles, etc.) present. The CONTIN method was used to analyze the intensityintensity time correlation function G (2) (τ). 8 All the

Many plant species have two modes of reproduction: sexual and asexual. Both modes of reproduction have often been viewed as adaptations to temporally or spatially variable environments. The plant should adjust partitioning to match changes in the estimated success of the two reproductive modes. Perennial plants showed that favorable habitats in soil nutrients or water content tend to promote clonal growth over sexual reproduction. In contrast, under high light-quantity conditions, clonal plants tend to allocate more biomass to sexual reproduction and less to clonal propagation. On the other hand, plants with chasmogamous and cleistogamous flowers provides with a greater tendency of the opportunity to ensure some seed set in any stressful environmental conditions such as low light, low soil nutrients, or low soil moisture. It is considered that vegetative reproduction has high competitive ability and is the major means to expand established population of perennial plants, whereas cleistogamous reproduction is insurance to persist in stressful sites due to being strong. Chasmogamous reproduction mainly enhances established and new population. Therefore, the functions of sexual and asexual propagules of perennial or annual plants differ from each other. These traits of propagule thus determine its success at a particular region of any environmental gradients. Eventually, if environmental resources or stress levels change in either space or time, species composition will probably also change. The reason based on which the plants differ with respect to favored reproduction modes in each environmental condition, may be involved in their specific realized niche.

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