Precision Vinyl Acetate/Ethylene (VAE) Copolymers by ROMP of Acetoxy-Substituted Cyclic Alkenes Jihua Zhang, Megan E. Matta, Henry Martinez, and Marc A. Hillmyer* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States * S Supporting Information ABSTRACT: Precision linear vinyl acetate/ethylene (VAE) copolymers containing acetoxy groups on precisely every eighth backbone carbon were synthesized by ring-opening metathesis polymerization (ROMP) of racemic 3-acetoxy cyclooctene (3AcCOE) followed by hydrogenation. The use of enantiomerically pure 3AcCOE resulted in an optically active polyalkenamer that afforded isotactic precision VAE materials after hydrogenation. Both of these VAE polymers are semicrystalline (by differential scanning calorimetry and wide- angle X-ray scattering) due to their high degrees of regioregularity and the isotactic VAE samples exhibited a higher apparent degree of crystallinity and melting point compared to the atactic version. In contrast, analogous linear VAE copolymers derived from ROMP-hydrogenation of racemic 4- or 5- acetoxy cyclooctenes were regio-irregular and completely amorphous. The ROMP-hydrogenation of 3-acetoxy cycloheptene also affords precision linear VAE copolymers with acetoxy groups on every seventh carbon, but this polymer was noncrystalline. Mechanical characterization showed that the precision 3AcCOE-derived VAE samples possess improved mechanical properties compared to the compositionally similar commercial VAE copolymers produced by radical copolymerization. P recision control of polymer structure is important for both tailoring ultimate properties and gaining a complete understanding the structure-property relationships. Biomacro- molecules often possess high degrees of specificity and structural complexity not commonly found in synthetic polymers. In synthetic polymers the control of size (molar mass) and architecture has been demonstrated in various approaches during the past several decades. 1 Polymer micro- structures (i.e., comonomer sequence and stereochemistry), on the other hand, are more challenging to regulate. 2 The control over repeating unit sequence has been achieved mainly through polymerization of prestructured monomers with built-in sequence, 3 coupling of sequence-specific short oligomers, 4 and particular copolymerization processes in which monomers exhibit different reactivity 5 or where external templates are applied. 6 One of the notable examples of the first approach is the preparation of precision polyethylene by acyclic diene metathesis (ADMET) polymerization 7 of symmetrically sub- stituted α,ω-dienes. 3g-i Alternating copolymerization of 1,3- butadiene with other vinyl monomer (e.g., methyl methacry- late) and polymerization of substituted 1,3-butadiene were reported to eventually result in periodic ethylene copolymers. 3c Use of stereoselective catalysts is by far the most popular strategy to control backbone stereochemistry which includes E-Z isomerism and tacticity. 8 However, the complete control over both the sequence and stereochemistry has only been realized in limited cases. 9 Recently, we discovered that ring-opening metathesis polymerization (ROMP) 10 of cis-cyclooctene (COE) substi- tuted with hydrocarbon groups at the 3-position (3RCOE) using the well-defined Grubbs second (G2) or third (G3) generation catalyst leads to highly regio- and stereoregular polyalkylenamers. 11 The polymers possess high levels (>95%) of head-to-tail (HT) repeating unit connectivity and E double bond configurations. Hydrogenation of these polyalkylenamers afforded model linear low density polyethylene (LLDPE) with precisely positioned branches. In a recent computational study, 12 we proposed that the rate-limiting step for the polymerization of COE and its derivatives by Grubbs type catalysts is determined by the barrier for breakdown of the metallacyclobutane intermediate formed between the catalyst and the monomer. Consequently, the regio- and stereo- chemistry seen in the ROMP of 3RCOEs is mainly due to significant repulsive interaction between the substituent and the N-heterocyclic carbene (NHC) mesityl ligand. These reports suggest that any 3-substituted COE having a sufficiently bulky functional group could also lead to regio- and stereoregular polymers. 13 Furthermore, the use of enantiomeri- cally pure 3-substituted COEs in such polymerization would enable the synthesis of isotactic, all-E, and sequence specific polymers. To test this hypothesis we focused on the acetoxy- substituted system 14 (Figure 1) because of the convenient accessibility of the monomers, the introduction of a functional group and the practical value of the final polymers, as they are Received: January 14, 2013 Revised: March 11, 2013 Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A dx.doi.org/10.1021/ma400092z | Macromolecules XXXX, XXX, XXX-XXX