Molecular weight determination branched copolymers

The branching sites can be introduced onto the backbone either by postpolymerization reactions or by copolymerization of the main backbone monomer ) with a suitable comonomer, with the desired functional group (unprotected or in a protected form if this functional group interferes with the polymerization reaction). Branches of comb-shaped polymers are commonly prepared by anionic polymerization, and backbones with electrophilic functionalities such as anhydrides, esters, pyridine, or benzylic halide groups are employed.88 The actual average number of branches in the final copolymer can be found by the determination of the overall molecular weight of the copolymer and the known molecular weights of the backbone and the branches. 

One or more detectors is attached to the output of the columns. For routine analysis of linear homopolymers, this is most often a Differential Refractive Index (DRI) or a UV detector. For branched or copolymers, however, it is necessary to have at least two sequential detectors to determine molecular weight accurately. Branched polymers can be analyzed using a DRI detector coupled with a "molecular weight sensitive" detector such as an on-line viscometer (VIS) or a low-angle laser light scattering (LALLS) detector. 

Analysis of plastics is a complex task and involves preliminary tests, determination of nonmetallic and metallic elements, analysis of functional groups and double bonds, molecular weight determinations, chemical compositional analysis, sequence length distribution in copolymers, determination of tacti-city and branching in polymers, and analysis of additives. Because of the great variety in structures in commercially produced plastics, the number of methods that can be applied for their analysis is considerable. 

An alternative procedure has been utilized to prepare 1,1-diphenylethy-lene-functionalized poly(ethylene oxide) macromonomers as shown in Scheme27 [210, 211]. This macromonomer (81) reacted quantitatively with poly(styryl)lithium to form the corresponding living diblock copolymer adduct. After cooling to 5 °C, ferf-butyl methacrylate was added and polymerized for ca. 2h. After quenching with acidic methanol, the hetero, threearmed, star-branched, ABC-type block copolymer was isolated. The molecular weight determined by SEC (universal calibration M = 19,500 M /M =1.14) and by H NMR (M =17,300) were somewhat higher than the calculated value (M = 15,500) [211]. 

With the use of such a well defined, ultrahigh molecular weight E/B copolymer, a very sophisticated solid state NMR method has been developed, suitable to determine the partitioning of end groups and branches among the crystalline and amorphous phases [51], The method claims sensitivity of 2 per thousand carbon atoms. It was found that methyl and vinyl end groups are, to a considerable extent, incorporated in the crystal, whereas only a small fraction of methyl branches, and almost no ethyl branches, enter the crystalline phase. The ease of incorporation into the crystalline phase thus can be represented as follows ... 

Its actual molecular weight, as obtained from gel permeation chromatography, is My, = 1263 M = 1136. (29) Its melting teir ier-ature, under the conditions of the NMR measurements, was 108.8°C as determined by differential calorimetry. The low density (branched) polyethylenes studied here were commercial varieties whose molecular weights, distribution and side group concentrations have been reported. (30) The ethylene-butene-1 copolymers were a gift from the Exxon Chemical Corporation. 

The composition of the star-shaped block copolymer is easily determined by proton NMR analysis from this and the mean number average molecular weight (Mn) of the sequence PA, Mn of the polyether component can be calculated. The later is very similar to the value from membrane osometry. Hydroxyl end group of PA(P0)2 star-shaped block copolymers have been titrated and their mean number per copolymer (1.85) agrees with the presence of two polyoxirane branches. On the average, the polydispersity of the star-shaped block copolymers varies between 1.2 and 1.3 (Figure 6). 

A wide variety of polymers have been analyzed by gel-permeation, or size-exclusion, chromatography (sec) to determine molecular weight distribution of the polymer and additives (86—92). Some work has been completed on expanding this technique to determine branching in certain polymers (93). Combinations of sec with pyrolysis—gc systems have been used to show that the relative composition of polystyrene or acrylonitrile—polystyrene copolymer is independent of molecule size (94). Improvements in gpc include smaller cross-linked polystyrene beads having narrow particle size distributions, which allow higher column efficiency and new families of porous hydrophilic gels to be used for aqueous gpc (95). 

Gel Permeation Chromatography (GPC), also known as Size Exclusion Chromatography (SEC), is a technique used to determine the average molecular weight distribution of a polymer sample. Using the appropriate detectors and analysis procedure it is also possible to obtain qualitative information on long chain branching or determine the composition distribution of copolymers. 

The problem of determining the erage number of branches is theoretically simple, it is the ratio of the M of the star polymer to that of the uncoupled linear branches. In fact, the branched copolymers, even after a heavy purification, contain a residual amount of unused linear branches. Because these two materials have different molecular weights, they elute separately, and it is possible to analyze the star peak alone. The big issue is that this impurity (uncoupled linear branches) is weighed when making the solution and, accordingly, the real concentration of the star copolymer is not known exactly this information is very necessary when using GPC with mass detectors. 

The difference in grafted polystyrene and homopolystyrene molecular weights substantiates the proposition of branchy-branch formation. Due to this circumstance it is not possible to determine branch frequencies in the graft copolymer. 

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