No abstract

Poly(N,N-dimethylacrylamide) (PDMA) brushes were successfully grown from polyvinyl chloride (uPVC) sheets via well-controlled surface-initiated atom transfer radical polymerization (SI−ATRP). An ATRP initiator containing a chloropropionate moiety was chemically tethered onto the surface of PVC via a novel wet chemical modification. Negatively charged sulfate groups were introduced to facilitate polymerization. By incorporating a chemically cleavable group into the initiator, molecular weight, polydispersity and graft density of a series of PDMA brushes synthesized on the flat surface were unambiguously characterized for the first time by gel permeation chromatography. ATR−FTIR, contact angle, SEM and AFM were used to characterize the PDMA grafted surfaces. Reaction conditions such as monomer concentration, reaction time, copper(II) concentration and salt additives were varied to systematically investigate their effects on molecular weight and graft density of the PDMA grafted from PVC. Molecular weights of grafted PDMA brushes varied from ca. 20 000 to 2 170 000 Da, while graft density ranged from 0.08 to 1.13 chains/nm2. Polydispersity of grafted PDMA brushes was controlled between 1.20 and 1.60 by Cu(II) complex addition. Kinetic studies revealed that the surface initiation was a slow process and graft density increased during the reaction. The brush uniformity increased with increasing reaction time. Reinitiation of the obtained PDMA brushes was demonstrated, suggesting that the polymerization is “living”. The successful growth of a PDMA-b-poly(N-isopropylacrylamide) (PNIPAM) copolymer brush was verified by GPC and AFM.

methyl]acrylamide) (PDMDOMA), a novel thermo-responsive polymer containing pendant dioxolane groups was synthesized via atom transfer radical polymerization (ATRP). Water soluble PDMDOMAs with controlled molecular weight and narrow molecular weight distribution were obtained. GPC-MALLS and MALDI-TOF-MS analysis verified the controlled nature of polymerization. It was found that an aqueous solution of PDMDOMA has a lower critical solution temperature (LCST) around 23 °C. The LCST of PDMDOMA was finely tuned over a wide temperature range by the partial hydrolysis of the acid labile dioxolane side group to form diol moieties (PDMDOMA diols). Unlike the traditional way of controlling LCST by copolymerization, the advantage of this method is that a series of thermo-responsive polymers with different LCST can be prepared from a single batch of polymer with comparable molecular weight profiles. The LCST of the resulting PDMDOMA diols increased almost linearly up to 28 mol % of diol in the copolymer and the LCST disappeared above 43 mol % diol content. The diol moiety generated during the hydrolysis was further oxidized to create aldehyde functionalities along the polymer backbone (PDMDOMA-aldehyde). The NMR analysis indicates that the aldehyde groups in the polymer exist in equilibrium with their covalent hydrates in water. The presence and reactivity of aldehyde groups on the PDMDOMA-aldehyde was verified by reaction with propylamine and aniline. The LCST of PDMDOMA-aldehyde did not change significantly compared to the precursor diol polymer. However, the propylamine or aniline derivatives showed a dramatic decrease in the LSCT possibly due to an increase in the hydrophobic character. The LCST of PDMDOMA-propylamine and PDMDOMA-aniline derivatives depends on the composition and nature of the attached groups. The structure of PDMDOMA and its derivatives were fully characterized by 1 H, 13 C, and 2D HMQC NMR, GPC-MALLS, and MALDI-TOF-MS.

The performance of biomaterials in contact with biological systems can be greatly affected by hydrophobic interactions at the interface between the biomaterial surface and surrounding biomolecules. Polymer brushes can function as a protective layer, preventing such interfacial hydrophobic interactions. In this paper, a systematic study of the barrier properties of a hydrophilic polymer brush is made by investigating the influence of graft density and its chemical nature (hydrophilicity/hydrophobicity) on hydrophobic interactions with the surface. To achieve this, a series of novel thermoresponsive poly-N-[(2,2-dimethyl-1,3dioxolane)methyl]acrylamide (PDMDOMA) polymer brushes were grown from silicon wafers via surfaceinitiated atom transfer radical polymerization. Without changing graft density or degree of polymerization, the hydrophilicity of the PDMDOMA brushes was manipulated by partial or complete hydrolysis of the pendent dioxolane moieties. A lower critical solution temperature (LCST) was observed at 22-24 °C, below which the PDMDOMA brush was found to be in a hydrated state (amphiphilic), while at temperatures above the LCST, the PDMDOMA brush formed a collapsed, more hydrophobic structure. A physical method was developed to analyze the ability of these brushes to act as a barrier against hydrophobic interactions based on AFM force-distance measurements. The adhesive forces between the Si 3 N 4 tip and the silicon wafer surface upon (a) modification with ATRP initiator, (b) grafting of PDMDOMA brushes, and (c) partial and complete hydrolysis of PDMDOMA were investigated. Hydrophobic interactions decreased after each modification, while graft density and the degree of hydrolysis increased the barrier function of the surface layer. In particular, when graft density was above 0.22 chains/nm 2 , the barrier capacity completely counteracted the hydrophobic interactions, as evidenced from the disappearance of the adhesive force in AFM measurements. Further studies revealed that the barrier property as assessed by AFM correlated well with the wettability of the surfaces.

Copolymers of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and one of two dioxolane-containing monomers, (2,2-dimethyl-1,3-dioxolane)methyl acrylate (DDMA) and (2,2-dimethyl-1,3-dioxolane)methyl acrylamide (DDMAA), were successfully synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT copolymerization was performed in dimethylformamide (DMF) at 70°C for 24 h using 4,4′-azobis(4-cyanovaleric acid) as initiator and N-(sodium ethane sulfonic acid)-2-((thiobenzyl)sulfanyl)proprionamide (CTA 1), 4-cyano-4-((thiobenzoyl)sulfanyl)pentanoic acid (CTA 2), or S,S′-bis(R,R′-dimethyl-R′′-acetic acid)trithiocarbonate (CTA 3) as the chain transfer agent (CTA). Control over molecular weight and composition was achieved by altering the CTA concentration and the monomer feed ratio respectively. The resulting copolymers had narrow molecular weight distributions (polydispersity indices typically between 1.2 and 1.3), while monomer conversions were typically 60%. Kinetic studies revealed that PEGMA was consumed at a higher rate than the comonomers over a given time. The molecular weight of the copolymer increased linearly with conversion, while a low polydispersity was maintained throughout. The copolymerization reactivity ratios were determined using the Mayo-Lewis method. After copolymerization, the dioxolane functional groups were deprotected to form 1,2-diol groups and subsequently oxidized with HIO 4 to form reactive aldehyde groups. Subsequent chemical modification of the dioxolane moieties to aldehyde groups showed no adverse effects in terms of degradation of the copolymer (specifically ester linkages). The advantage of the current synthesis over direct copolymerization of aldehyde-based monomers is the stability of the 1,2-diol moiety compared to the corresponding aldehyde copolymer. The availability of the aldehyde groups along the polymer backbone to form stable conjugates with amine containing molecules was confirmed via a reaction with the iron chelating drug desferrioxamine (DFO). Conjugation was achieved via an aldamine reaction, followed by a reduction of the resulting Schiff base to a secondary amine. Full characterization of the copolymers was performed using NMR spectroscopy and GPC-MALLS, while UV-vis absorption spectroscopy was used to determine the efficiency of DFO conjugation.