Suppose you are given a new polymer made by a typical chain-growth polymerizatio

Question

Suppose you are given a new polymer made by a typical chain-growth polymerization in solution with free-radical initiation and are told that it's structure -(-CH2 - CH__X -)n- * don't select X= H or CH3 1. How would you go about confirming its complete structure, ie., its detailed micro-structure, considering all possible microstructrural elements at the monomer triad lever (X is not a proton or CH3)? 2. Confident in your microstructural analysis, what would you do next to begin establishing its structure-property relationships, and how would you accomplish this? 3. Describe those properties (solution and bulk) that could most likely be connected to the microstructure of your polymer and why and how these structure-property relation could be established? 4) Suggest alternative methods of polymerization to obtain -(-CH2-CH__X)n-, with a higher molecular weight, a narrower molecular weight distribution, reduced branching caused by chain-transfer, and produce a more regular microstructure. Be sure to explain how and why each alternative polymerization could lead to these changes in your polymer

Explanation / Answer

1.) Now that you have prepared your polymer it's important to perform its physical characterization to understand it's structure, the chain length, molecular weight, crystallinity, thermal stability etc. In order to do this, a variety of techniques can be used. Starting with electron microscopy: You can use scanning electron microscopy or transmission electron microscope to observe how the polymer beads (or whatever shape you have) have turned out. You can check to see if they have the appropriate shape (morphology) and the correct size (in nanometers or micrometers). After that you can subject your polymer to x-ray scattering or x-ray diffraction to test the crystallinity of the polymer. The diffraction pattern can be studied to extract information about the crystaline or amorphous nature of the polymer. Gel permeation chromatography can be used to figure out the molecular weight and DLS (Dynamic Light Scattering) to determine the polydispersity of the polymer. FTIR and NMR techniques can give you the detail chemical composition of the polymer. (FTIR will only give information about the major functional groups, whereas NMR will give detailed information about its overall structure and it's tacticity.) Raman spectroscopy is another way to obtain characteristic information about a specific molecule/compound and can be useful in finding out the structure. AFM (Atomic Force microscopy) is a good analytical technique that will give you the topographical layout of your polymer, although it must be mentioned that it's a destructive techniques meaning that the sample can't be reused again. Thermogravimetry can be performed to find out the thermal stability of the polymer, while differential scanning colorimetry (DSC) will help you determine the melting point and the glass transition temperature. You can also check the chirality and refractive index of your polymer.

2.) The structure and chemical composition of the polymer can have a major effect on it's properties. High molecular weight polymers often exhibit low solubility. The chemical groups in the polymers can decide the polarity of the polymer molecule which will in turn decide the solubility of the molecule. Using the above mentioned techniques you can also determine if the polymer has cross linking or not. Presence or absence of cross-linking will affect the solubility and strength of polymers.

3.) Now that we have all the physical and chemical information about our polymer, we can begin to predict what properties could be connected to our polymer. For example: If our polymer is a homogenous polymer, the chances of crystallinity are very high since the polymer tends to coil inside and start folding on itself. Performing XRD will prove if our claims are right. If on the other hand, we have heteropolymer on our hands, the chances of crystallinity are a lot less and hence, might be amorphous. A combined information obtained from NMR, FTIR, thermogravimetry can allow us to predict the tacticity of the polymer. For example: If the polymer shows high degree of crystallinity we know that it's willing to pack/fold in on itself. Which means that our polymer is probably isotactic. NMR will allow you to determine a detail structure of your polymer and hence, you already know if your polymer if branched or linear. Glass transition temperature and cooling curves will further help to deduce if your polymer is linear or branched.

4.) Atom Transfer Radical Polymerization is a technique used if we want to synthesize a polymer with a specific molecular weight and narrow molecular weight distribution. I uses a transition metal ion because it has the ability to exist in two different oxidation states. The higher metal oxidation ion reacts with a halide ion which acts as an activator. The initiation process occurs within a specific time frame if an appropriate alkyl halide activator is used and hence we get a narrow molecular weight distribution.

Reversible Addition/Fragmentation Chain Transfer is another method of synthesis. It's basically a spin on the traditional free radical polymerization except in this case, we have a chain transfer agent. In this polymerization, the propogating radicals are deactivated and hence the living chains are dormant. Hence, the molecular weight increases in direct proportion to the conversion rate. It allows you to have a narrow molecular weight distribution.

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