Control of the Molar Mass

Qnite obvionsly, the control of the molar mass of the product of these reactions is very important. Very-high-molar-mass material may be too difficult to process, whereas low-molar-mass polymer may not exhibit the properties desired in the end product, and one mnst be able to stop the reaction at the required value of p. Consequently, the reactions are particularly demanding with respect to the purity of the reagents, and acenrate control of the amount of each species in the mixture is cardinal. It is symptomatic of these critical requirements that only the following four types of reaction usefully produce linear polymers with M 25,000 g mol  

SchoUen-Baumann reaction This involves the use of an add chloride in an esterification or amidation for example, the so-called nylon rope trick reaction is an interfacial condensation between sebacoyl chloride and hexamethylenediamine, producing a polyamide known as nylon-6,10. 

The bifunctional acyl chloride is dissolved in a suitable solvent such as dichloromethane and placed in a beaker. An aqueous alkaline solution of 

Salt dehydration. Direct esterification requires high purity materials in equimolar amounts because esterifications rarely go beyond 98% completion in practice. To overcome this, hexamethylene diamine and a dibasic acid such as adipic acid can be reacted to produce a nylon salt, hexamethylene diammonium adipate. A solution of 0.5-mol diamine in a mixture of 95% ethanol (160 cm ) and distilled water (60 cm ) is added to 0.5-mol diacid dissolved in 600 cm of 95% ethanol over a period of 15 min. The mixture is stirred for 30 min during which time the nylon salt precipitates as a white crystalline solid. This can be recrystallized and should melt at 456 K. The pure salt can be converted into a polyamide by heating it under vacuum in a sealed tube, protected by wire gauze, at about 540 K in the presence of a small quantity of the diacid, e.g., 10 g salt to 0.55-g adipic acid is a suitable mixture. If a lower molar mass is desired, a monofunctional acid can replace the adipic acid and act as a chain terminator. 

Ester interchange An alternative reaction is a trans-esterification in the presence of a proton donating or weak base catalyst such as sodium methoxide, e.g.. 

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Several ligands were used with allyl-type and vinyl-type initiators, such as 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMDETA), NJJyN JJ jN -pentamethyldiethylenetriamine (PMDETA), or compound 7 in Scheme 66. Zeng et al. showed that the combination of initiator 1 in Scheme 66 with BA6TREN or initiator 4 in Scheme 66 with BA6TREN gave the best control of the molar mass for the ATRP of 2-(dimethylamino)ethyl methacrylate. These allylic macromonomers are then able to copolymerize with acrylamide. 

Vicente et al. [30] used the heat of reaction and the open-loop observers developed in Section 7.2.5.3 to determine the concentration of monomer and CTA and hence to infer the instantaneous number-average molar masses during emulsion homo- and copolymerization reactions. In addition, the authors used the inferred values for online control of the molar mass distributions of copolymers with predefined distributions. They demonstrated that polymer latexes with unimodal MMD with the minimum achievable polydispersity index in free-radical polymerization (PI = 2) and bimodal distributions could be easily produced in linear polymer systems [15, 30]. 

Control of the molar mass and MMD using LRP has important implications on the mechanical properties of the final polymer, making these techniques highly attractive for industrial applications. 

Anhydrides of super acids such as trifluoromethane sulfonic acid and fluorosulfonic acid yield two active growing chain ends (Scheme Successful control of the molar mass and functionality... 

Ultrasonic Mainly gases of low molar mass Hj, N2. COj, He. Ar. A quartz crystal transducer transmits a sound wave through a sample of gas with a similar crystal used as the receiver. The velocity of the sound wave is proportional to the square root of the molar mass of the sample. The phase shift of the sound signal is measured by comparison with a reference signal. Precise temperature control is required. 1-10 Gas chromatography... 

A comparison of the molar mass control efficiencies of ZnEt2, DIBAH and TIBA allowed for the establishment of the ranking DIBAH > ZnEt2 > TIBA [180,205]. The reaction mechanism which accounts for the control of molar mass by reversible transfer of (poly)butadienyl chains between Nd and Al on one hand and between Nd and Zn on the other hand was established for the catalyst system NdV/DIBAH/EASC. In addition, it was concluded that in this system the (poly)butadienyl chains are only active during the period in which they are attached to Nd. The (poly)butadienyl chains are dormant in the period during which they are attached to Al or Zn. In the context of these results it is not clear whether the irreversible transfer of polybutadienyl... 

The modulus is controlled by crosslinks for low molar mass strands between crosslinks G = for < Mg) and by entanglements for high molar mass strands between crosslinks (G = Gg for > Mg). The modulus becomes nearly independent of the molar mass of the network strands between crosslinks in the limit of very long strands. The straight lines with non-zero intercept in Figs 7.11 and 7.12 are Eq. (7.48). 

Already before reporting this combined inifer and living polymerization approach, Kennedy and coworkers developed a controlled isobutene polymerization method based on cumyl ester initiators (Scheme 8.6) with boron trichloride as activator and incremental monomer addition [28], The livingness of the polymerization was demonstrated by the linear increase of number-average molar mass and the constant number of polymer chains (A) with the amount of PIB obtained (wp, as measure for conversion) as well as the narrowing of the molar mass distribution with conversion (Fig. 8.1) [28]. 

The disadvantage of the mo.st freqently used condensation reactions ( bad control of average molar mass and molar mass distribution ) can be avoided by use of ring opening metathesis polymerization ( ROMP ) for the precursor monomers. So a very accurate control of the length of the polymer backbone is achievable 2. 

At an early stage, the addition of a core molecule By(y > 2) was explored (Scheme 24.1), mainly to provide a better control over the molar mass but also to control the resulting geometric shape [21]. When using a core molecule, the resultant products should no longer exhibit the focal unit A, though this will limit the molar mass build-up such that the molar mass will be controlled by the ratio ByiAB ,. The most prominent example of this is the polycondensation of 2,2-bis(hydroxymethyl)propionic acid in the presence of l,l,l-tris(hydroxymethyl)propane. In this case, control over the reaction is achieved by the successive addition of monomer portions this process is used for the industrial synthesis of Boltorn . However, the complete incorporation of the core molecule into each hb macromolecule, in aU molar mass fractions, is not trivial and cannot always be achieved. 

Figure 1.67 shows a schematic of a simple, but effective set-up for cryoscopy, the method for the measurement of the freezing point lowering. Cryoscopy is perhaps the easiest of the molar mass determinations. The main prerequisites are a good temperature control and uniformity, corrections for the common supercooling observed on crystallization, and the usual extrapolation to infinite dilution. The thermodynamic equations are derived in Sect. 2.2.5, together with the equations needed for the ebulliometry. 

The remarkable selectivity of the ROMP were again verified not only was the conversion complete (100%), but a relatively good control of polymacromonomer molar mass was also achieved, Table 4. 

The single most important characteristic of a polymer chain is the molar mass and it is important for the synthetic polymer chemist to be able to prepare polymer molecules with controlled size. The molar mass of the molecule in Figure 1(a) is 431 times the molar mass of a styrene repeat unit (104gmol ) plus the molar mass of the butyl (57 g mol ) and hydrogen (1 gmol ) end groups, that is, it is equal to 44882gmol While the... 

The spontaneous formation of Au particles at room temperature in air-saturated aqueous solutions of poly(ethylene glycol)s was investigated using optical, potentiometric and conductivity techniques. The kinetic information is consistent with a mechanism in which Au(III) complexes bind through ion-pairs to pseudocrown ether structures of the polymers. Reduction of the metal centers follows through their reactions with the oxyethylene groups that form these cavities. The particle size of the metal crystallites is controlled by the molar mass of the polymers. Agglomerates of small Au particles are formed as final products when polymers of low molar mass are used in the synthesis. 

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