Monoterpenes, such as d-limonene and a-pinene, are a broad class of naturally occurring substances.  d-Limonene and a-pinene are less toxic, less volatile replacements for petroleum based solvents.  We have investigated the use of these monoterpenes as renewable solvents and chain transfer agents for metallocene polymerization of olefins, such as 1-hexene, ethylene and propylene.  Since monoterpenes are available in large quantities and do not require synthetic modification, they have wide-ranging potential as chain transfer agents.  The toxicity of monoterpenes is also much lower than chain transfer agents, such as vinyl chloride. 

NMR and gel-permeation chromatography (GPC) detected chain transfer for 1-hexene, ethylene and propylene polymerizations in monoterpenes.  The polymerization of olefins in d-limonene resulted in monomodal molecular weight distributions with decreased molecular weight values compared to polymerizations in toluene and hydrogenated d-limonene.  For example, the molecular weight value (M_w = 26000 g/mol; M_w/M_n = 1.8) for 1-hexene polymerizations in d-limonene after 1 h at 23° C was substantially less than similar polymerizations in toluene (M_w = 36000 g/mol; M_w/M_n = 2.0).  Polymerizations in a-pinene were more prone to oligomerization reactions than d-limonene and resulted in ~ 10 % oligomers (M_n ~ 1000 g/mol).  The resulting chain transfer to d-limonene during the polymerization of 1-hexene yielded a higher percentage of trisubstituted alkenes as detected by ¹H NMR.  Propylene polymerizations with the Et(Ind)₂ZrCl₂/MAO and Me₂Si(Ind)₂ZrCl₂/MAO catalyst systems in d-limonene gave increased molecular weights, larger [mmmm] pentads and higher T_m values compared to those run in toluene.  The [mmmm] pentads for isotactic polypropylene were characterized by ¹³C NMR and FTIR spectroscopy.  For the Et(Ind)₂ZrCl₂/MAO catalyst system, the mol fraction of [mmmm] pentads increased from 0.86 to 0.94 upon switching the solvent from toluene to d-limonene.   Similar increases in molecular weights and T_m values were observed for propylene polymerizations in a-pinene with Et(Ind)₂ZrCl₂/MAO. 

Receiving a GB grant has immensely benefited students, several research projects and the ability to establish collaborations.  Approximately seven undergraduate students, many who plan to attend graduate school in chemistry or materials science, have benefited from participation in this project.  These students have presented their work at numerous conferences.  Several students conducted full time research in the summer as a result of PRF funding and a SUMR grant.  Acquisition of a Wyatt light scattering detector has facilitated GPC characterization of polymerizations in renewable monoterpenes.  Over the last two years, this GPC system was a central instrument for two publications.  Additional research projects stemming from the initial experiments with d-limonene and a-pinene currently use the GPC system.  Fruitful collaborations with Dr. Kerry McMahon (Geneva College) and Dr. Carl LeBlond (Indiana University of Pennsylvania) have been established through funding from the SRF program.