Molecular weight PVAC

Often a chain-transfer agent is added to vinyl acetate polymerizations, whether emulsion, suspension, solution, or bulk, to control the polymer molecular weight. Aldehydes, thiols, carbon tetrachloride, etc, have been added. Some emulsion procedures call for the recipe to include a quantity of preformed PVAc emulsion and sometimes antifoamers must be added (see Foams). 

The presence of hydrolyzable long chain branches in PVAc was established by McDowell and Kenyon206 in 1940. They observed a reduction in molecular weight obtained on successively hydrolyzing and reacetylating samples of PVAc. Only branches to the acetate methyl will be lost on hydrolysis of the polymer i.e. on conversion of PVAc to PVA. 

For one of the samples, PVAc - Lot 1, the number average molecular weight distribution is plotted in Figure 2. Also shown is the instantaneous number average number of LCB points per molecule, plotted as a function of number average molecular weight. Note that increased rapidly with increasing num-... 

Figure 18. Plot of Branching Index as a Function of Molecular Weight for a Branched PVAc (Aldrich 18250-8 Lot 3).

Molecular Weight Distribution from Gel Permeation Chromatography Intrinsic viscosity [n] = 444 1/kg for the branched PVAc B41... 

Our primary calibration curve obtained with narrow MWD linear polystyrene samples could now be used to determine the molecular weight distribution of branched PVAc (B ) by means of eqs. (39), (40) and (17). The calibration curve for branched PVAc was not linear but showed a slight upturn at the low elution volume range as shown in Fig. 9. The calibration curve in Fig. 9 could be represented by... 

The molecular weight distribution obtained from SEC analysis was also shown in Fig. 8. In order to check the effect of the estimated exponent a(-0.55) on molecular weight distribution for Ei branched PVAc, we used another a(-0.58) value to compute a new calibration curve as shown in Fig. 9. The two calibration curves almost overlapped with each other. The results are listed in Table 2. In both cases, we obtained the same weight-average molecular weight and the polydispersity index (M /M ). Thus, we could confirm that in using a two-point (Bq and %l) estimate for a, we have not introduced an appreciable error in the determination of molecular weight distribution of branched PVAc. 

Figure 8. Molecular weight distribution of branched PVAc (B...) as determined by laser light scattering (LLS) and size exclusion chromatography (SEC)/viscometry. A comparison of results by the two methods is listed in Table 2.

The rate and/or the degree of hydrolysis of PVAc is independent of the molecular weight of the starting material. 

However, predicting the molecular weight of the hydrolysis product is another matter. Most commercial PVAc is somewhat cross-linked. The cross-... 

Fig. 41. Molecular weight dependence of Dz of linear PVAc (O), and of branched PVAc synthesized in latex particles ( )188,189)...

Fig. 42. Molecular weight dependence of

PC PE PES PET PF PFA PI PMMA PP PPO PS PSO PTFE PTMT PU PVA PVAC PVC PVDC PVDF PVF TFE SAN SI TP TPX UF UHMWPE UPVC Polycarbonate Polyethylene Polyether sulfone Polyethylene terephthalate Phenol-formaldehyde Polyfluoro alkoxy Polyimide Polymethyl methacrylate Polypropylene Polyphenylene oxide Polystyrene Polysulfone Polytetrafluoroethylene Polytetramethylene terephthalate (thermoplastic polyester) Polyurethane Polyvinyl alcohol Polyvinyl acetate Polyvinyl chloride Polyvinyl idene chloride Polyvinylidene fluoride Polyvinyl fluoride Polytelrafluoroethylene Styrene-acrylonitrile Silicone Thermoplastic Elastomers Polymethylpentene Urea formaldehyde Ultrahigh-molecular-weight polyethylene Unplasticized polyvinyl chloride... 

Poly(vinyl acetate), PVAc, of molecular weights (M ) 6.1x1 01, 1.9xl05, and 6.0xl05, was randomly labeled by ester exchange with 2,2,5,5-tetramethyl-3-pyrrolin-l-oxyl-3-carboxylic acid to give a spin labeled polymer containing typically 1-10 nitroxides per polymer molecule. Polystyrene was prepared by emulsion... 

Figure 6. Effect of molecular weight of PVAc on Ti02 in the presence of CHCh at saturation coverage (A) no solvent or surface (B) 61,000 (C) 194,000 (D)...

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