Polymerization number average molecular

The addition of small, but specific, amounts of a monofunctional acid to the polymerization is often used to control molecular weights and catalyze reactions. The polymerization is controlled to produce a number-average molecular weight of 18,000—30,000, depending on the end use. 

The poly(styrene-b-isoprene) (P(S-b-IP)) and poly(-styrene-b-2-vinyl pyridine) (P(S-b-2VP)) block copolymers with narrow molecular weight distributions for blending with the microspheres were also synthesized using the additional anionic polymerization technique. The number-average molecular weights (Mns) and PS contents are also shown in Table 1. 

The equilibrium between monomer and living polymer is dynamic and therefore the molecular weight distribution of the polymer will change with time until the equilibrium distribution is reached. This is a peculiar process in which the amount of polymer present in the system, as well as its number average molecular weight is constant. This means also that, the number of polymeric... 

GPC-derived weight average molecular weights are often less prone to error than number average molecular weights. When termination is wholly hy disproportionation or chain transfer and chains are long (>10 units), classical kinetics predicts Xn = XJ2 (Section 5.2.1.3). It follows that Cit can be obtained from the slope of a plot of 21 Xw vs [T]0/[M]t>."4 "5 The errors introduced even when the dominant process for radical-radical termination is combination (e.g. S polymerization) are small as long as X n is small in relation to... 

The polymerization of 1,3,3-trimethyl-2,7-dioxabicyclo[2.2.1 Jheptane 35 was carried out in methylene chloride, toluene, and 1-nitropropane at temperatures between —78 and 0 °C32l Boron trifluoride etherate, triethyloxonium tetrafluoro-borate, antimony pentachloride, and iodine were used as initiators. Irrespective of the solvents and initiators employed, the products obtained at 0 °C were white powders with melting points of 50—55 °C, while those obtained at tower temperatures were sirups. The number average molecular weight of the unfractionated products ranged from 400 to 600. The molecular weight distribution of the oligomers prepared at 0 °C was broad, in contrast to the relatively narrow distribution of those obtained at -40 °C. 

Attempted polymerization of ds-7,9 dioxabicyclo[4.3.0]nonane 43 with phosphorus pentafluoride as initiator at temperatures ranging from -25 to 0 °C provided only a cyclic dimer 44 in high yield35. Under similar conditions trans-7,9-dioxabicyclo-[4.3.0]nonane 45 polymerized almost instantly to polymer 46 with number average molecular weight of several thousands, along with a small amount of a cyclic dimer 47. 

Figure 1 Is a flow sheet showing some significant aspects of the Iterative analysis. The first step In the program Is to Input data for about 50 physical, chemical and kinetic properties of the reactants. Each loop of this analysis Is conducted at a specified solution temperature T K. Some of the variables computed In each loop are the monomer conversion, polymer concentration, monomer and polymer volume fractions, effective polymer molecular weight, cumulative number average molecular weight, cumulative weight average molecular weight, solution viscosity, polymerization rate, ratio of polymerization rates between the current and previous steps, the total pressure and the partial pressures of the monomer, the solvent, and the nitrogen.

The number average molecular weight is required. This is obtained directly from measurements of a colligative property, such as the osmotic pressure, of dilute polymer solutions (see Chap. VII). It is often more convenient to establish an empirical correlation between the osmotic molecular weight and the dilute solution viscosity, i.e., the so-called intrinsic viscosity, and then to estimate molecular weights from measurements of the latter quantity on the products of polymerization. 

Figure 2. Relation between the intrinsic viscosity and the number-averaged molecular weight of trimethylsilylated alkoxide polymeric particles for Si(0C2H5) solutions with different r values. (Reproduced with permission from Ref. 3. Copyright 1984 North Holland Physics Publishing.)...

The number average molecular weights, Mq, of the polymers obtained in Table II ranged from 1300 to 3900 as determined by vapor nressure osmometry (VPO), and they were further characterized by - -H- and C-NMR spectrometry at ambient temperature and at 90°C, as well as by IR spectroscopy. The homopolymers of isopropenylferro-cene were found to have the expected structure, P, shown below, obtained by polymerization through the isopropenyl units, as indicated by spectroscopic characterization ... 

The rat constant k,, is a function of reaction temperature, functionality and environmental factors which include molecular diffusion. Steric hindrance may be significant with these chemically complex monomers. These functions likely are separable. To address functionality, consider the number average molecular weight of the polymeric phase... 

Conjugated boron polymers containing platimnn or palladium atom in the main chain were also prepared by hydroboration polymerization between tetrayne/ metal complex monomers and tripylborane (scheme 16).30 From gel permeation chromatographic analysis [THF, polystyrene (PSt) standards], the number-average molecular weights of the polymers obtained were found to be 9000. The polymers were soluble in common organic solvents such as THF, chloroform, and benzene. The absorption peaks due to tt-tt transition were observed around 390 nm in the UV-vis spectra of these polymers. The fluorescence emission spectra exhibited intense peaks at 490 nm in chloroform. 

Living" carbocationic polymerizations are most difficult to achieve mainly because of chain transfer to monomer and termination processes both of which frequently occur in carbocationic polymerizations. It has recently been demonstrated (JL) that "quasiliving" polymerization of a-methylstyrene (aMeSt) can be achieved by slow and continuous monomer addition and that the number-average molecular weight (Mn) of PaMeSt increases linearly with the weight of added monomer. A theory for quasiliving polymerizations has been developed (2). 

Lipase-catalyzed polymerization of divinyl adipate or divinyl sebacate with a, co-glycols with different chain length has been reported [40]. Lipases CA, MM, PC, and PF showed high catalytic activity toward the polymerization. A combination of divinyl adipate, 1,4-butanediol, and lipase PC afforded the polymer with number-average molecular weight (Mn) of 2.1 x 104. The yield of the polymer from divinyl sebacate was higher than that from divinyl adipate, whereas the opposite tendency was observed in the polymer molecular weight. 

PMBV was synthesized by a conventional radical polymerization. The monomer unit compositions of the PMBV were 0.64, 0.25, and 0.11 unit mole fractions for MPC, BMA, and VPBA, respectively. The number-averaged molecular weight and weight-averaged molecular weight were 6.2 x 104 and 6.5 x 104, respectively. This PMBV was completely water-soluble due to hydrophilic MPC units in the polymer. Figure 1 shows the chemical structure of PMBV. 

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