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Synthetic Auxiliary

Introduction

Polymer synthesis, also called polymerization, is the process by which monomers (small molecules) are covalently bonded to form a (usually long) polymer chain or network. Various synthetic auxiliaries are needed in the synthesis of polymers.

Our auxiliaries in polymer synthesis mainly refer to photoinitiators, chain extenders, nucleating agents and polymerization inhibitors.

Photoinitiator

Photoinitiators are molecules that absorb photons upon irradiation with light and form reactive species out of the excited state, which initiate consecutive reactions. The initiating species may be radicals, cations, or anions.

A photo initiator should exhibit several properties, from which the most important are: (i) high absorption at the exposure wavelength and high molar extinction coefficient, (ii) high quantum yield of formation of initiating species, and (iii) high reactivity of the radical towards the monomer.

Photo initiators can be divided into two classes radical photo initiators and cationic photo initiators. Radical photo initiators can react differently under UV-irradiation. Depending on their reaction they are classified as Norrish Type I or Norrish Type II Photo initiators.

Norrish photoinitiatorFig 1. Norrish photoinitiator

Cationic photoinitiatorFig 2. Cationic photoinitiator

Chain Extender

Chain extenders are low molecular weight diols or diamines that react with diisocyanates to build polyurethane molecular weight and increase the block length of the hard segment. Much like the diisocyanates, chain extenders can be either aliphatic or aromatic.

Chain extender in polyurethane-based phase change materialFig 3. Chain extender in polyurethane-based phase change material

Nucleating Agent

A nucleating agent has a surface charge that is opposite to the partial charge of the chemical moiety of the polymer. It accelerates its rate of crystallization, and a melting point is greater than the melting point of the melt processible polymer.

The effects of three kinds of nucleating agents, including talc, SB and Ion., Na+ on the crystallization kinetics of PET were studied by using DSC. The results indicate that three nucleating agents can increase the crystallization rate of PET, and SB has the most excellent nucleating effect for the crystallization of PET with the same content of nucleating agent. The crystallization mode of PET might shift from three-dimensional growth to two-dimensional growth by the addition of the nucleating agents.

The determination of the equilibrium melting temperatureFig 4. The determination of the equilibrium melting temperature

Polymerization Inhibitor

Polymerization inhibitors are industrial additives which are usually used to prevent spontaneous polymerization. Without the addition of a small amount of polymerization inhibitor, the reactive monomer will polymerize during storage, transportation, etc. Polymerization inhibitors slow or stop the polymerization process by reacting with growth chain radicals or initiators.

Reference:

  1. X. L. Jiang, S. J. Luo, K. Sun, X. D. Chen. (2007). Effect of nucleating agents on crystallization kinetics of PET[J]. eXPRESS Polymer Letters Vol.1, No.4 (2007) 245–251.
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