Non-covalent Intermolecular Interactions in Polymer Design: Segmented Copolymers to Non-viral Gene Delivery Vectors

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Virginia Tech

Non-covalent intermolecular interactions play a large role in determining the properties of a given system, from segmented copolymers to interactions of functionalized polymers with non-viral nucleic acids delivery vehicles. The ability to control the intermolecular interactions of a given system allow for tailoring of that system to yield a desired outcome, whether it is a copolymers mechanical properties or the colloidal stability of a pDNA-delivery vector complex. Each chemical system relies on one or more types of intermolecular interaction such as hydrogen bonding, cooperative À-À stacking, electrostatic interactions, van der waals forces, metal-ligand coordination, or hydrophobic/solvophobic effects. The following research describes the tailoring of specific intermolecular interactions aimed at altering the physical properties of segmented copolymers and non-viral gene delivery vectors.

Amide containing segmented copolymers relies heavily on hydrogen bonding intermolecular interactions for physical crosslinking to impart the necessary microphase separated morphology responsible for a copolymers physical properties. Amide containing hard segments are composed of various chemical structures from crystalline aramids to amorphous alkyl amides with each structure possessing unique intermolecular interactions. Variations to either of the copolymer segments alters the copolymers physical properties allowing for tuning of a copolymers properties for a particular application. The synthetic strategies, structure-property relationships, and physical properties of amide containing segmented copolymers are thoroughly reported in the literature. Each class of segmented copolymer that contain amide hydrogen bonding groups exhibits a wide range of tunable properties desirable for many applications. The segmented copolymers discussed here include poly(ether-block-amide)s, poly(ether ester amide)s, poly(ester amide)s, poly(oxamide)s, PDMS polyamides, and polyamides containing urethane, urea, or imide groups.

The structure-property relationships (SPR) of poly(oxamide) segmented copolymers is not well understood with only one report currently found in literature. The effects of oxamide spacing in the hard segment and molecular weight of the soft segments in PDMS poly(oxamide) segmented copolymers demonstrated the changes in physical properties associated with minor structural variations. The optically clear PDMS poly(oxamide) copolymers possessed good mechanical properties after bulk polymerization of ethyl oxalate terminated PDMS oligomers with alkyl diamines or varied length. FTIR spectroscopy experiments revealed an ordered hydrogen bonding carbonyl stretching band for each copolymer and as the spacing between oxamide groups increased, the temperature at which the hard segment order was disrupted decreased. The increased spacing between oxamide groups also led to a decrease in the flow temperature observed with dynamic mechanical analysis. Copolymer tensile properties decrease with increased oxamide spacing as well as the hysteresis. The structure-property investigations of PDMS poly(oxamide) segmented copolymers showed that the shortest oxamide spacing resulted in materials with optimal mechanical properties.

A new class of non-chain extended segmented copolymers that contained both urea and oxamide hydrogen bonding groups in the hard segment were synthesized. PDMS poly(urea oxamide) (PDMS-UOx) copolymers displayed thermoplastic elastomer behavior with enhanced physical properties compared to PDMS polyurea (PDMS-U) controls. Synthesis of a difunctional oxamic hydrazide terminated PDMS oligomer through a two-step end capping procedure with diethyl oxalate and hydrazine proved highly efficient. Solution polymerization of the oxamic hydrazide PDMS oligomers with HMDI afforded the desired PDMS-UOx segmented copolymer, which yielded optically clear, tough elastomeric films. Dynamic mechanical analysis showed a large temperature insensitive rubbery plateau that extended up to 186 ÚC for PDMS-UOx copolymers and demonstrated increased rubbery plateau ranges of up to 120 ÚC when compared to the respective PDMS-U control. The increase in thermomechanical properties with the presence of oxamide groups in the hard segment was due to the increased hydrogen bonding, which resulted in a higher degree of microphase separation. DMA, SAXS, and AFM confirmed better phase separation of the PDMS-UOx copolymers compared to PDMS-U controls and DSC and WAXD verified the amorphous character of PDMS-UOx. Oxamide incorporation showed a profound effect on the physical properties of PDMS-UOx copolymers compared to the controls and demonstrated promise for potential commercial applications.

Two novel segmented copolymers based on a poly(propylene glycol) (PPG) that contained two or three oxamide groups in the hard segment were synthesized. Synthesis of non-chain extended PPG poly(trioxamide) (PPG-TriOx) and PPG poly(urea oxamide) (PPG-UOx) segmented copolymers utilized the two-step end-capping procedure with diethyl oxalate and hydrazine then subsequent polymerization with oxalyl chloride or HMDI, respectively. The physical properties of the PPG-TriOx and PPG-UOx copolymers were compared to those of PPG poly(urea) (PPG-U) and poly(oxamide) (PPG-Ox) copolymers. FTIR studies suggested the presence of an ordered hydrogen bonded hard segment for PGG-TriOx and PPG-Ox copolymers with PPG-TriOx possessing a lower energy ordered hydrogen bonding structure. PPG-UOx copolymers exhibited a larger rubbery plateau and higher moduli compared to PPG-U copolymers and also a dramatic increase in the tensile properties with the increased hydrogen bonding. The described copolymers provided a good example of the utility of this new step-growth polymerization chemistry for producing segmented copolymers with strong hydrogen bonding capabilities.

Non-viral nucleic acid delivery has become a hot field in the past 15 years due to increased safety, compared to viral vectors, and ability to synthetically alter the material properties. Altering a synthetic non-viral delivery vector allows for custom tailoring of a delivery vector for various therapeutic applications depending on the target disease. The types of non-viral delivery vectors are diverse, however the lack of understanding of the endocytic mechanisms, endosomal escape, and nucleic acid trafficking is not well understood. This lack of understanding into these complex processes limits the effective design of non-viral nucleic acid delivery vehicles to take advantage of the cellular machinery, as in the case of viral vectors.

Mechanisms for cellular internalization of polymer-nucleic acid complexes are important for the future design of nucleic acid delivery vehicles. It is well known that the mammalian cell surface is covered with glycosaminoglycans (GAG) that carry a negative charge. In an effort to probe the effect of GAG charge density on the affinity of cationic poly(glcoamidoamine) (PGAA)-pDNA complexes, quartz crystal microbalance was employed to measure the mass of GAGs that associated with a polyplex monolayer. Affinity of six different GAGs that varied in the charge density were measured for polyplexes formed with poly(galactaramidopentaethylenetetramine) (G4) cationic polymers and pDNA. Results showed that the affinity of GAGs for G4 polyplexes was not completely dependent on the electrostatic interactions indicating that other factors contribute to the GAG-polyplex interactions. The results provided some insight into the interactions of polyplexes with cell surface GAGs and the role they play in cellular internalization.

Two adamantane terminated polymers were investigated to study the non-covalent inclusion complexation with click cluster non-viral nucleic acid delivery vehicles for passive targeting of the click cluster-pDNA complexes (polyplex). Incorporation of adamantyl terminated poly(ethylene glycol) (Ad-PEG) and poly(2-deoxy-2-methacrylamido glucopyranose) (Ad-pMAG) polymers into the polyplex formulation revealed increased colloidal stability under physiological salt concentrations. Ad-pMAG polyplexes resulted in lower cellular uptake for HeLa cells and not two glioblastoma cell lines indicating the pMAG corona imparts some cell line specificity to the polyplexes. Ad-pMAG provided favorable biological properties when incorporated into the polyplexes as well as increased polyplex physical properties.

poly(amide), oxamide, segmented copolymer, trioxamide, poly(urea), poly(urea oxamide), non-viral gene delivery, click cluster