Living Carbocationic Polymerization of Isobutylene by Epoxide/Lewis Acid Systems: The Mechanism of Initiation
Abstract (Summary)The objective of the work presented in this dissertation was to generate a fundamental understanding of the synthesis of star-branched polyisobutylenes (PIBs) arising from hexaepoxysqualene (HES)/Lewis acid (LA) initiating systems, using BCl3 and TiCl4 as LAs. The understanding of initiation and propagation mechanisms by HES is crucial to control the number of arms and arm lengths of star PIBs expected from this initiator. The initiation by monofunctional epoxides, such as ?-methylstyrene epoxide (MSE), 1,2-epoxy-2,4,4-trimethylpentane (TMPO-1), and 2,3-epoxy-2,4,4-trimethylpentane (TMPO-2), was investigated. In situ FTIR spectroscopy, which was highly utilized in this research, provided valuable information. Most importantly, the ability to identify the head group by in situ FTIR of growing PIB chains initiated by an epoxide, i.e. the –C–O–LA complex, contributed significantly to the understanding of initiation of IB polymerization. This technique allowed the monitoring of the rate of initiation with the multifunctional epoxy initiator. Previous research showed that TiCl4 gave 40% initiating efficiency in conjunction with the aromatic epoxy initiator MSE, whereas the aliphatic initiators TMPO-1 and TMPO-2 gave only 3 and 10% efficiency, respectively. In this research it was found that BCl3 is more efficient with the aliphatic initiator, TMPO-1, yielding an asymmetric telechelic PIB carrying an ?-primary OH and an ?-tertiary Cl functional group with 70% initiator efficiency, while MSE gave only 1-4% efficiency. The TMPO-2/BCl3 system gave 20% initiator efficiency. The various initiation mechanisms were discussed. IB polymerization was successfully initiated by HES in the presence of excess BCl3 and monitored by in situ FTIR spectroscopy. The gradual increase of the IR band assigned to the –C–O–BCl2 group demonstrated that slow initiation was occurring. Chain extension with the HES/BCl3 initiated PIB was achieved leading to high molecular weight PIBs in the presence of TiCl4. It was also demonstrated for the first time that BCl3 invariably leads to ?-proton expulsion, leading to chain transfer in IB polymerization. In addition, 1,2-epoxycyclohexane and epoxycyclohexyl-functional siloxanes, i.e. epoxycyclohexylisobutyl polyhedral oligomeric silsesquioxane (POSS) and bis[3,4-(epoxycyclohexyl)ethyl]tetramethyldisiloxane, were found to be initiators for the polymerization of IB. 1,2-epoxycyclohexane/TiCl4 was an efficient initiating system for the IB polymerization yielding up to 45% initiator efficiency. It was proposed that initiation of IB polymerization involves an SN2 reaction between IB and TiCl4-coordinated epoxide.
School:The University of Akron
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:06/09/2009