A Low Temperature Alkoxyamine Designed for Use in Nitroxide-Mediated Miniemulsion Polymerization

UoM administered thesis: Phd

  • Authors:
  • Bencha Thongnuanchan

Abstract

The basis for this research project is based on the discovery in previous research that 2,2',5-trimethyl-3-(1-phenylethoxy)-4-tert-butyl-3-azahexane (Styryl-TITNO) is able to control bulk polymerization of styrene at temperatures as low as 60 °C. The principle objective of this project was to evaluate the feasibility of using Styryl-TITNO to control radical solution and miniemulsion polymerizations at temperatures below 100 °C.Styryl-TITNO was shown to effect solution polymerizations of both n-butyl acrylate (BA) and styrene below 100 °C. Polymerization temperature was shown to be a crucial parameter for achieving control in Styryl-TITNO-mediated polymerizations. Good control of the number-average molecular weight (Mn) and molecular weight dispersity for the polymerization of BA was observed at 90 °C. However, a lower temperature of 70 °C is required for good control of styrene polymerization. Living characteristics of polymer chains were demonstrated by a sequential chain extension of TITNO-terminated PBA with styrene at 90 °C to form poly(n-butyl acrylate)-block-poly[(n-butyl acrylate)-co-styrene] block copolymers. An improvement in livingness in these reactions was observed when the second P(BA/styrene) block was formed at 70 °C after the first PBA block was produced at 90 °C. Kinetics studies facilitated determination of the activation-deactivation equilibrium constant (K), which for styrene polymerization at 90 °C (K = 4.2 x 10-9 mol L-1 at 90 °C and 3.1 x 10-9 mol L-1 at 70 °C) is nearly an order of magnitude higher than that for BA polymerization at the same temperature (K = 8.5 x 10-11 mol L-1). This is the reason why BA polymerization shows better control than styrene polymerization at 90 °C. The activation energy (Ea) for thermolysis of Stryl-TITNO is 104.1 kJ mol-1, which is relatively low compared to the literature values of Ea for various styryl alkoxymines. This explains why Styryl-TITNO is able to effect polymerization at temperatures as low as 70 °C.The studies of Styryl-TITNO-mediated miniemulsion polymerizations at 90 °C indicate that accumulation of free TITNO• in the particles is an issue for use of Styryl-TITNO in miniemulsion polymerizations. The use of L-ascorbic acid (L-AA) and L-ascorbic acid 6-palmitate (L-AAP) as nitroxide scavengers to reduce the level of free TITNO• in the polymerization was investigated. The best result was observed for the polymerization of BA in the presence of 5.36 mol% of L-AAP relative to Styryl-TITNO, which attained 48% conversion after 5 h. The chain extension of isolated TITNO-terminated PBA, TITNO-PBA, was used to examine the livingness of polymer chains before the rate of polymerization was severely retarded. The livingness of TITNO-PBA was evidenced by a shift of the starting PBA molecular weight distribution towards higher molecular weight, which provides solid evidence that the majority of polymer chains remained living. Thus, it can be concluded with certainty that the accumulation of free TITNO• was mainly responsible for the suppression of polymerization in miniemulsion polymerizations mediated by Styryl-TITNO.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • Peter Lovell (Supervisor)
Award date31 Jul 2012