Molecular weight copolymers

This polymeric oxocarbenium salt readily initiates the cationic ring opening polymerization of oxolane to produce a polystyrene-polyTHF block copolymer. Molecular weight control is provided, polydispersity is narrow and compositional heterogeneity is small59). 

Advanced computational models are also developed to understand the formation of polymer microstructure and polymer morphology. Nonuniform compositional distribution in olefin copolymers can affect the chain solubility of highly crystalline polymers. When such compositional nonuniformity is present, hydrodynamic volume distribution measured by size exclusion chromatography does not match the exact copolymer molecular weight distribution. Therefore, it is necessary to calculate the hydrodynamic volume distribution from a copolymer kinetic model and to relate it to the copolymer molecular weight distribution. The finite molecular weight moment techniques that were developed for free radical homo- and co-polymerization processes can be used for such calculations [1,14,15]. 

Table I. PTBMA Containing Block Copolymers Molecular Weight Control...

In the following sections, synthesis of the anionic polymers, copolymer molecular weight, limiting viscosity number, electrolyte effects, solution shear thinning, screen factor, polymer radius of gyration, and solution aging will be discussed and data on the copolymers presented. 

Limiting Viscosity Number. Limiting viscosity numbers for the polymers in distilled water are given in Table 3 Limiting viscosity number increases with increasing copolymer molecular weight. 

Limited silica fines stabilization data indicated that increasing copolymer molecular weight from 100,000 to 1,000,000 daltons had, if anything, a negative effect on silica fines stabilization. At a molecular weight of 1,000,000 daltons, this copolymer appeared to be more effective in stabilizing silica fines than silica/kaolinite, calcite, or hematite fines. However, the results may be due in part to the larger particle size and lower surface area of the silica fines (see Table II). 

When the total polymer response, is known as a function of retention volume, the molecular weight distributlon can be obtained in the usual manner with the appropriate molecular weight calibration curve. The molecular weight calibration curve can be obtained (a) by using the Runyon (65) copolymer molecular weight scale approach, or (b) by using a hydrodynamic volume approach if the Mark-Houwink constants for the polymer of interest are known or can be determined, or (c) by using a hydrodynamic volume approach in conjunction with an on-line viscosity detector. 

With this continued growth in variety of plastics materials, selection and matching of optimum combinations between materials and applications becomes a major problem. With at least 50 types of commercial plastics already available, each in a variety of copolymers, molecular weights, and different manufacturers, there are already too many for rational manual choice of the optimum material for any specific application and the situation is growing worse at an alarming rate. 

Small size (10-100 nm) is one ofthe most interesting features of polymeric micelles. Besides allowing the extravasation of the carriers, it permits the sterilization of the preparation to be done simply by Lltration and minimizes the risks of embolism in capillaries, contrary to larger drug carriers (Kwon and Okano, 1996). Micellar size seldom exceeds 100 nm, but depends on several factors including copolymer molecular weight, relative proportion of hydrophilic and hydrophobic chains, and aggregation number (Yokoyama etal., 1990 Trubetskoy and Torchilin, 1996 Shin etal., 1998). 

Hexenylsilane was introduced as a co-monomer into the organotitanium-mediated polymerization of ethylene to produce silane-terminated ethylene-5-hexenylsilane copolymers. High activities and narrow polydispersities were observed in the polymerization-chain transfer process. Ethylene-5-hexenylsilane copolymer molecular weights... 

Figure 2. Effect of carbonyl group content on the yield of copolymer, molecular weight, and apparent number of grafted chains in the pMMA-grafted dialdehyde-cellulose. Conditions cellulose, 0.3 g H2(), 10 mL MMA, 2 mL at 50°C for 1 h. Key (reaction tube) O, quartz d.Pyrex.

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

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