Molecular architecture sequence distributions

The magnitude of T, also depends on several factors. For example, any intrinsic modification to the composition of the polymer chain as a result of changing the sequence or distribution of the different domains, or extrinsic factors such as salt additions, that modifies the tendency of water molecules to take part in hydrophilic vs. hydrophobic hydration will alter the clathrate structure and modify the T,. This effect is a result of both the mean polarity of the polymer chain and the molecular architecture and distribution of the different amino acid domains in the ELRs [27]. Tt can also be affected by the polymer concentration and by other factors, such as changes in pH or the oxidation state of a side chain or functional group, the... 

The characterization of copolymers must distinguish copolymers from polymer blends and the various types of copolymers from each other (97,98). In addition, the exact molecular stmcture, architecture, purity, supermolecular stmcture, and sequence distribution must be determined. 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

Polymers are found in such a large variety of products that they have shaped modern life. The extraordinary versatility of polymers in terms of end-use properties is due to the variety and complexity of the microstructure of the polymeric material. The polymeric material includes both the polymer and the additives with which it is compounded. The microstructure of the polymeric material is determined by the molecular and morphological characteristics of the polymer itself, the way in which the polymer is processed and the additives used for compounding (Figure 1.1). The molecular characteristics of the polymer include chemical composition, monomer sequence distribution (MSD), molecular weight distribution (MWD), polymer architecture, chain configuration and morphology. 

We devoted special attention to characterize all the polymers that were prepared for this study. The characterization techniques listed in Table 4 were used for determining (1) comonomer composition, butadiene microstructure, and sequence distribution of the monomer units (2) molecular weight, molecular weight distribution, and chain architecture ... 

Our focus in this chapter is on model block copolymers those with well-defined molecular architectures, i.e., where the block sequences (AB vs. ABA vs. ABC. . . ) are practically identical across the ensemble of chains, and where the individual blocks possess narrow chain length distributions. Throughout this chapter, block copolymer chemistries will be denoted as A/B , and particular diblock copolymers as A/B n/m , where A is the abbreviation for the monomer comprising the crystallizable block (e.g., CL for s-caprolactone), B is the monomer comprising the amorphous block (e.g., S for styrene), and n and m are the crystallizable and amorphous block molecular weights, in kg/mol (rounded to the nearest kg/mol). This notation immediately connotes the approximate volume fraction of A block, and hence suggests the likely melt morphology. 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

One can describe polymer samples in a similar way. First of aU, a polymer chain possesses semi-flexibility, that characterizes the intra-chain interactions for the most stable conformation persisting along the chain axis. Secondly, a polymer chain also holds complex inter-chain interactions. These two intrinsic characteristic factors dictate the basic physical behaviors of the same species of polymers. Besides these two intrinsic factors, each individual polymer sample possesses certain extrinsic characteristic factors, i.e., molecular weights and their distributions, topological architectures, and sequence irregularities. These extrinsic characteristic factors are also important in determining the physical behaviors of the polymer samples. 

As seen in Scheme 5.1, preparation of the IG polymer in this synthesis involved the ROP of cCL and subsequent chain-end modification. Conversion of the terminal hydroxyl group to two hydroxyl functions enabled further ROP to the 2G polymer. The IG polymer synthesized by this procedure was a 6-arm star-branched PcCL. The target dendrimer-like star-branched polymer was obtained as a 2G polymer by the second iteration and possessed a minimum architectural unit. One more repetition of the synthetic sequence involving the two reaction steps resulted in a 3G dendrimer-like star-branched PaCL. The 3G polymer possessed six branches at the core and two branches at the junctions in both the 2G- and 3G-based layers, composed of 42 arm segments (6 (IG) + 12 (2G) + 24 (3G) = 42). The observed M value was 96 000 g/mol, close to the theoretical value, and the molecular-weight distribution was not narrow, but an acceptable value of 1.14. 

---