Lab 1- ATRP - CHEM 5384 Polymer Characterization II Lab 1 Atom Transfer Radical Polymerization of styrene Mengfang Li Introduction In this experiment we

Lab 1- ATRP - CHEM 5384 Polymer Characterization II Lab 1...

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CHEM 5384 Polymer Characterization II Lab 1: Atom Transfer Radical Polymerization of styrene Mengfang Li 01/29/2012
Introduction: In this experiment we were introduced the method to control a radical process to prepare well-defined and functionalizable polymers. Over 100 billion pounds of synthetic polymers were produced each year in United alone. Chain polymerization accounts 80% of these and by far free radical polymerization is the most commonly used reaction method in this category[1]. Radicals are very reactive and add quickly across the double bond to form polymers. This process will terminate if the chain end reacts with another radical. This can be observed by plotting the molecular weight as a function of conversion of monomer into polymer. As shown in Figure 1 for conventional method, high molecular weights are obtained at low conversion and remain at approximately same level throughout the reaction. The initiator (eg. BPO or AIBN) decomposes slowly compared to the rate of polymerization[1]. The radical species have a very short life time and are continuously being generated and terminated. One problem of conventional method is the polydispersity (PDI) is usually 2 or higher. Another problem with conventional method is that radical chain ends react with each other to terminate chains with alkane or alkenes. This makes chain end functionalization or extension very difficult. Atom transfer radical polymerization (ATRP) using copper complex was developed to eliminate the classical mode of terminations. In this method, almost all polymer chains start growing at same time. The polymerization rate is controlled by exchange of halogen atom between a metal catalyst and propagating radical [2]. This, combined with fast initiation from a small organic halide, RX, with a sufficiently reactive CX bond, leads to a polymer sample of fairly uniform molecular weight chains, which retain the halogen end group, X.As a result, the all chains are about the same length at all times during the reaction. Since carbon-centered radicals and catalyst can be easily oxidized by oxygen, the purge of reactants with nitrogen is required before the reaction. One distinct advantage of ATRP is that it produces well defined polymer sample with low PDI[2]. Based on the controlling the reaction kinetics, the molecular weight increases steadily with conversion. Another advantage is that all chains have a halogen atom on the end which means copolymerization or end/side functionalization is possible.
Figure 1. Molecular weight versus function of % monomer conversion for conventional and radical polymerizations.

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