Oligomer structure molecular weight

Polymer or oligomer Structure Molecular weight Yield (%) ... 

The structure of MAO is complex and has been investigated by cryoscopic measurements hydrolysis reactions infrared, ultraviolet, and NMR spectroscopies and other methods (77). Equilibria are attained between the oligomers. The molecular weight, determined cryoscopically with samples in benzene, is between 1000 and 1500 g/mol. The work of Sinn (77) and Barron et al. (78, 79) has provided details of the structure of MAO and t-butylalumoxane. Among the different oligomers, the units shown in Scheme 1 are important. 

Free Radical Cure UV Adhesives. As with any adhesive, formulation variables are critical to the processing and performance characteristics. Variables such as oligomer selection, modifiers and additives, monomer structure, molecular weight, and glass transition temperature directly affect application and performance properties. 

Supercritical-fluid-fractionation techniques have been explored to separate the aminopropyl-terminated oligomers into fractions of narrow poly-dispersity. The fractions have been fully characterized in terms of their structure, molecular weight, molecular weight distribution, and functionality. The results of selected studies are presented in this section. 

Flavocytochrome 62 is composed of a single polypeptide chain with a molecular weight, as estimated by SDS-polyacrylamide gel electrophoresis, of 57,500 in the case of the S. cerevisiae enzyme (28) and 58,000 for the enzyme from H. anomala (29). The native enzymes behave as oligomers with molecular weights around 230,000 (29-31), suggesting that each is a homotetramer. This has been confirmed for S. cerevisiae flavocytochrome 62 by X-ray crystallography, which shows the subunits to be related by a four-fold axis of symmetry (Fig. 3) (23-25). The tetrameric structure is important for activity because dissociation of the H. anomala enzyme at low ionic strength yields inactive monomers that can be reassociated with recovery of activity (32). 

The properties that determine whether a product can be labelled as a pressure sensitive adhesive are tack, peel and creep. Tack is the property related to bond formation. Peel defines the tension or force necessary to remove the adhesive tape. Creep is the property describing the flow characteristics of the PSA. Formulators need to be aware of any factors that can directly affect these three properties such as temperature, aging, film thickness, cure rate and post-cure parameters. There are also several formulation variables - these include oligomer selection, tackifier addition, monomer structure, molecular weight and glass transition - that directly impact tack, peel and creep. 

The monomeric unit for the oUgomers is mainly 6.14, i.e., [Al OjMCg]. Because of the Lewis acid and Lewis base characters of aluminum and oxygen, respectively, such units can join together to form oligomers of molecular weights -1200 to -1600. Structure 6.15 shows how two 6.14 units could combine through acid-base interactions. 

In these reactions the system is tuned, for example by adjustment of the reaction temperature and time and modification of the catalyst structure to maximize the quantity of the desired dimers produced, and to minimize the production of higher molecular weight oligomers and polymers. In other reactions it is the opposite... 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

Synthesis of hydrolytically stable siloxane-urethanes by the melt reaction of organo-hydroxy terminated siloxane oligomers with various diisocyanates have been reported i97,i98) -yhg polymers obtained by this route are reported to be soluble in cresol and displayed rubber-like properties. However the molecular weights obtained were not very high. A later report56) described the use of hydroxybutyl terminated disiloxanes in the synthesis of poly(urethane-siloxanes). No data on the characterization of the copolymers have been given. However, from our independent kinetic and synthetic studies on the same system 199), unfortunately, it is clear that these types of materials do not result in well defined multiphase copolymers. The use of low molecular weight hydroxypropyl-terminated siloxanes in the synthesis of siloxane-urethane type structures has also been reported 198). 

Multiblock polyethylene-polydimethylsiloxane copolymers were obtained by the reaction of silane terminated PDMS and hydroxyl terminated polyethylene oligomers in the presence of stannous octoate as the catalyst 254). The reactions were conducted in refluxing xylene for 24 hours. PDMS block size was kept constant at 3,200 g/mole, whereas polyethylene segment molecular weights were varied between 1,200 and 6,500 g/mole. Thermal analysis and dynamic mechanical studies of the copolymers showed the formation of two-phase structures with crystalline polyethylene segments. 

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