Controlled polymerisation and industrial application of poly(2-chloro-1,3-butadiene)

Abstract

Poly(2-chloro-1,3-butadiene) (PCB, polychloroprene) has wide-ranging applications as neoprene rubber. Favourable chemical and physical properties in the material are attributed to a three-dimensional network of polymer chains, which is realised through cross-linking. Ethylene thiourea (ETU) and zinc oxide (ZnO) are the standard reagents which facilitate this in industrial processes. However, ETU is a suspected carcinogen and its usage is due tobecome severely restricted, so much so that the future production of neoprene rubber is at risk. Hence, an alternative, non-toxic cross-linking agent is required which can cross-linkPCB in the same fashion. The way in which the ETU/ZnO system functions must first beunderstood before a replacement can be proposed. Thus, mechanistic studies were initially undertaken with PCB oligomers in order to elucidate the reaction. To this end, a synthetic protocol was established for 2-chloro-1,3-butadiene (CB) and the monomer was subsequently adopted in numerous polymerisation reactions. Investigations into the reversible addition-fragmentation chain transfer (RAFT) polymerisation of CBproceeded to predefine low molecular weight PCB. A successful procedure was realised, employing 2-cyano-2-propylbenzodithioate (CPD) CTA and conditions which were able to furnish <1000 g/mol to 50,000 g/mol, low dispersity PCB in a controlled manner. This invention was novel in that PCB has historically been synthesised via conventional(uncontrolled) free radical techniques. PCB oligomers were adopted in cross-linking reactions with ETU and various model compounds, alone, and with ZnO, to aid the interpretation of the ETU/ZnO mechanism. Spectroscopic analyses and the observation of by-products revealed that three disparate reactions occur; ETU and ZnO were found to act both synergistically and independently of each other. A newly-proposed mechanism describes activation of the polymer chain by ZnOand subsequent reaction through sulfur. As a result of this discovery, alternative compounds have been tested and found capable of cross-linking PCB. In a second industrial study, the eradication of allergy-causing cross-linking additives for PCBlatex (gloves) was investigated. PCB latex films were generated under various conditions and the materials physically tested. A novel amine-dithiocarbamate complex, combined with a xanthogen polysulfide, afforded comparable properties in PCB latex and as such is apotential replacement system.