Photoregulation of polymer conformation by photochromic moieties. II. Cationic and neutral moieties on an anionic polymer

Abstract— Photoregulation of the conformation of poly (methacrylic acid) (PMA) in the range 4·5 < pH > 6·5 has been achieved by means of the cationic photochromic ligand, p-phenyl-azophenyl trimethyl ammonium iodide (PTA), as well as by means of copolymerization of methacrylic acid (MA) with the neutral photochromic co-monomer, p-phenyl azomethacrylanilid (PM). At degrees of ionization α, corresponding to the conformational unfolding of PMA, the pK_app is 0·2 units higher when the ligand PTA is trans than when PTA is in the photostationary 80%“cis” state. Equilibrium dialysis shows that the binding of PTA is independent of the cis/trans ratio at high α (after the conformational unfolding), but at lower αtrans-PTA binds positively and cis-PTA negatively. In the transition region, the presence of non-polar trans-azo ligand is therefore held responsible for a shift of the conformational equilibrium toward the more compact, folded form, which is harder to ionize than the unfolded form. Because of the relation pK_app# pH + log [(1 –α)/α], the degree of ionization of PMA, and thus the polymer conformation, as reflected in the reduced viscosity, can be photochemically regulated in pH buffers. At pH # 5·5 (α# 0·20) a maximal relative shift of Δα/α= 20 per cent was found, with a corresponding relative shift in reduced viscosity of 20·5 per cent. In the photochromic copolymer a pK_app switch of similar magnitude was found. In this case the azo-moiety is permanently affixed to the PMA so that the photosensitivity of the pK_app in the range of the conformational unfolding is due to a shift in the conformational equilibrium induced by the presence of non-polar trans-azo and polar “cis” azo, respectively. In both cases only about 1 photochromic moiety per 100 monomer units is capable of appreciably shifting the conformational manifold of PMA and/or the degree of ionization in constant pH buffers, provided one stays close to the unfolding transition. Mention is made of the implications for photoregulation of the permeability of, and the potential across. model membranes made of photochromic PMA molecules.     

Physico-chemical study of ionic equilibria for poly-2 and poly-4-vinylpyridines in alcohol-water solutions

The ionic equilibria for poly-4-vinyl pyridine (P4VP) and poly-2-vinyl pyridine (P2VP) were studied by physico-chemical techniques such as potentiometry, viscosity and NMR-¹H. The mixture of ethanol (45 per cent w.p.) and water was used as solvent to obtain the total range of ionization (0–1). It was found that the dissociation constants of pyridine residue of polymers in the absence of electrostatic interaction (pK₀ = 3·3–3·9) are lower than for the monomer analogues 4-ethylpyridine and 2-ethylpyridine (5·02) and depend on ionic strength (NaCl).A sharp decrease of pK_app at the beginning of titration and increase of specific viscosity for P4VP and P2VP are both explained by electrostatic interactions between positive charges forming during titration of the macromolecules. Most probably, these interactions act through the organic part of the macromolecule. On the other hand, it is shown by NMR-¹H that sharp changes in pK_app and specific viscosity at the beginning of the titration are not associated with changes in the average conformation of the monomer unit in the polymer. This conformation can be destroyed only when the energy of electrostatic interactions is large enough and this occurs when the mean distance between positive charges is relatively short.     

Characterizing polymer macrostructures by identifying and locating microstructures along their chains with the kerr effect

In this brief report, we demonstrate that Kerr effect measurements, which determine the excess birefringence contributed by polymer solutes in dilute solutions observed under a strong electric field, are highly sensitive to and capable of determining their microstructures, as well as their locations along the macromolecular backbone. Specifically, using atactic triblock copolymers with the same overall composition of styrene (S) and p-bromostyrene (pBrS) units, but with two different block arrangements, that is, pBrS₉₀-b-S₁₂₀-b-pBrS₉₀ (I) and S₆₀-b-pBrS₁₈₀-b-S₆₀ (II), which are indistinguishable by NMR, we detected a dramatic difference in their molar Kerr constants (_mK), in agreement with those previously estimated. Although similar in magnitude, their Kerr constants differ in sign, with _mK(II) positive and _mK(I) negative. In addition, S/pBrS random and gradient copolymers synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization exhibit a heretofore unexpected enhanced enchainment of racemic (r) pBrS-pBrS diads. Comparison of their observed and calculated _mKs suggests that the gradient S/pBrS copolymers possess an unanticipated additional gradient in stereosequence that parallels their comonomer gradient, that is, as the concentration of pBrs units decreases from one end of the copolymer chain to the other, so does the content of r diads. This conclusion could only be reached by comparison of observed and calculated Kerr effects, which access the global properties of macromolecules, and not NMR, which is only sensitive to local polymer structural environments, but not to their locations on the copolymer chains. Molar Kerr constants are characteristic of entire polymer chains and are highly sensitive to their constituent microstructures and their distribution along the chain. They may be used to both identify constituent microstructures and locate them along the polymer chain, thereby enabling, for the first time, characterization of their complete macrostructures. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Control of Intramolecular Hydrogen Bonding in a Conformation-Switchable Helical-Spring Polymer by Solvent and Temperature

A substituted poly(phenylacetylene) derivative (PPA_HB) with two hydroxymethyl groups at the meta position of the side phenyl ring was examined as a conformation-switchable helical spring polymer that responds to solvent and heat stimuli in a precisely controlled manner. Intramolecular hydrogen bonds, which cause the helical structure of the polymer, were broken and re-formed by adjusting the hydrogen-bonding strength values (pK_HB) of various combinations of solvents or by varying the temperature. In this process, a reversible conformational change from cis–cisoid to cis–transoid, accompanied by a phase transition in the form of a helix-coil transformation occurred, with the polymer exhibiting critical changes of color fading and recovery in specific environments. These results demonstrate that PPA_HB can be used as either a pK_HB indicator or a thermometer. The color changes of the polymer solution are described in detail based on spectroscopic analyses and thermodynamic considerations.     

Effects of Nonionic, Cationic, and Mixed Nonionic-Cationic Surfactants on the Acid-Base Behavior of Phenyl Salicylate

The effects of Brij 35 micelles, CTABr micelles, and mixed Brij 35–CTABr micelles on the acid–base behavior of phenyl salicylate (PST) have been studied in aqueous solution containing 2% v/v acetonitrile. The apparent pK_b (pK^app_b) of PST is decreased by 1.5 pK units with the increase in [Brij 35] from 0 to 0.02 M which is attributed to micellar medium effect. The values of pK^app_b remain almost independent of [CTABr] within its range 0.01–0.03 M. The increase in [CTABr] from 0 to 0.03 M in aqueous solution containing 0.02 M Brij 35 has not resulted in a change in pK^app_b. This shows that the characeristic structural features of nonionic Brij 35 micelles remain essentially unchanged on addition of CTABr under the present experimental conditions.     

The Stabilities of the o-Carboxyphenylhydrazo-Ethylacetoacetate OF Cu,Ni, Co,Zn, Mn and Cd Chelate Compounds in Different Solvents

The dissociation constants for o-carboxyphenylhydrazoethylacetoacetate (o-CPHEA) ligand, as well as the stability constants for the divalent metal complexes of Cu, (II), Ni (II), Co (II), Zn (II)and Cd (II) ions, have been calculated pH-meterically in different solvents. The dissociation constans pK₁=4.10 and pK₂=10.55 of the insoluble organic ligand are calculated in aqueous medium. The effect of solvents, the relation between stabilities and both electronegativities and ionization potential are studied.     

Control of Intramolecular Hydrogen Bonding in a Conformation‐Switchable Helical‐Spring Polymer by Solvent and Temperature

A substituted poly(phenylacetylene) derivative (PPA_HB) with two hydroxymethyl groups at the meta position of the side phenyl ring was examined as a conformation‐switchable helical spring polymer that responds to solvent and heat stimuli in a precisely controlled manner. Intramolecular hydrogen bonds, which cause the helical structure of the polymer, were broken and re‐formed by adjusting the hydrogen‐bonding strength values (pK_HB) of various combinations of solvents or by varying the temperature. In this process, a reversible conformational change from cis–cisoid to cis–transoid, accompanied by a phase transition in the form of a helix‐coil transformation occurred, with the polymer exhibiting critical changes of color fading and recovery in specific environments. These results demonstrate that PPA_HB can be used as either a pK_HB indicator or a thermometer. The color changes of the polymer solution are described in detail based on spectroscopic analyses and thermodynamic considerations.     

Modulation of Electronic Mobility of a One‐Dimensional Coordination Polymeric Molecular Wire with Light

Metal ions often influence the photoswitching efficiency of a photochromic system. This article reports a one‐dimensional polymer having cyclic azobenzenes coordinated to silver ions that are bridged by nitrates. The coordination polymer (CP‐ 2 ) displays a photoresponsive behavior. The switching ability in the polymer form was faster compared to the parent azobenzene ligand without the metal ions. Azobenzenes are reported to be poorly conducting. Here, although the azobenzene ligand does not show significant electronic mobility, the coordination polymer (CP‐ 2 ) displays a modest conductivity. The conductance in the cis form of the polymer is significantly higher compared to the trans form. Upon exposure to visible light, the cis form undergoes photoisomerization to the trans form with a drastic drop in the electronic mobility. The trans form can be reverted to the cis form thermally or by using UV light. Thus, this system offers a reversible control of the conductivity using light.     

Deformational,swelling, and potentiometric behavior of ionized poly(methacrylic acid) gels. II. Experimental results

The deformational, swelling, and potentiometric behavior of poly(methacrylic acid) gels was measured as a function of the degree of crosslinking, ionic strength, and degree of ionization. The comparison of the stress–strain behavior with theoretical relations derived in the preceding part has shown that the relations are valid only if an increase is assumed in the number of monomeric units in the statistical chain segment with increasing degree of neutralization of the gel. This dependence is affected by the salt content in the swelling solution and is also dependent on the activity coefficient of counterions. The pK₀ values for an undissociated gel approach pK₀ = 5.0 and increase somewhat with degree of neutralization. The swelling equilibria are in qualitative agreement with theoretical assumptions; their quantitative agreement depends on the activity coefficient of counterions.     

On the protonation of partially quaternized poly-4-vinylpyridines in aqueous solution

The ionic equilibria for free pyridine rings in water-soluble macromolecules of poly-4-vinylpyridine (PVP), partially quaternized with either benzylchloride (PP_yBz) or bromoacetone (PP_yAc), were studied by potentiometry and spectrophotometry. Peculiarities of protonation in aqueous solution were observed for the pyridine rings of these polymers compared with the analogue 4-ethylpyridine (EtP_y). The values of intrinsic pK₀ in the absence of protonated pyridine residues for PP_yBz and PP_yAc were found to be 2·4–3·3, and are abnormally low compared with pK_a of EtP_y (6·1). The pK₀ depends insignificantly on the degree of quaternization in PVP but more markedly on ionic strength. However, increase of NaCl concentration (from 0 to 0·1 M) does not cause pK₀ to rise to pK_a of the analogue. The titration data of the base group and keto-enol tautomerism of acetomethylene fragments in PP_yAc allow one to assume the effect of positively charged atoms in α-position of the ring on the proton binding to nitrogen atom of the same ring, thereby decreasing the value of pK₀. The intensification of electrostatic interactions between these charges in the polymer pyridine compared with that of the analogue is likely to be due to partial dehydration of the microregion near to nitrogen atom (microenvironmental effect).     

Salt rejection by polymer membranes in reverse osmosis. II. Ionic polymers

The water permeability K₁ [which is related to water flux J₁ per unit membrane area by J₁ = K₁(Δp ? ΔII)/ΔX, where Δp is the pressure difference, ΔII is the osmotic pressure of feed solution, and ΔX is the membrane thickness] of homogeneous ionic polymer membranes in reverse osmosis and their salt rejection R_s [which is given by R_s ≡ 1 ? (C_2″/C_2′), where C_2′ is the concentration of the salt in feed solution, and C_2″ is the concentration of salt in effluent] were examined with cationic and anionic membranes of block and graft copolymers. For ionic membranes, R_s and K₁ are related by K₁ = A exp { ? BR_s}, where A and B are constants. This equation was found to be independent of the ion charge, the chemical nature of the polymer, and film morphology. The principle of salt rejection by ionic membranes was explained by the difference in the transport volumes (volume elements available for transport) for mobile co-ions and water. The electric repulsive force between a fixed ion and a mobile co-ion decreases the transport volume of the latter, thus creating a transport depletion of salt flux relative to water transport. This transport depletion is governed by the amount of water sorbed by a fixed ionic site, which also determines the water flux. Consequently, R_s and K₁ for ionically charged membranes are related as described above. This relation significantly differs from that found between R_s and K₁ for nonionic polymer membranes, where the size and the solubility of ions in the membrane are mainly responsible for the transport depletion. The decline of R_s with increasing K₁ is much less in ionic membranes than in nonionic ones; however, in the high R_s region, K₁ for both ionic and nonionic membranes become similar as the dominant mode of water transport changes from flow to diffusion.     

Ionization equilibrium in salt‐containing aqueous solutions of synthetic polyampholytes

The peculiarities of ionic equilibrium in salt‐containing aqueous solutions of polyampholytes (acrylic acid–2‐methyl‐5‐vinylpyridine copolymers) of various compositions and molecular weights were studied. The protonation degree of base groups (β_iep), the dissociation degree of acid groups (α_iep), and the ionization constant of methylvinylpyridine groups (pK_b) for the isoelectric points of the studied polyampholytes under various ionic strength values (I) were assessed spectrophotometrically. The dependencies of α_iep and pK_b versus the copolymer composition in the absence of low molecular weight electrolyte are described by the following equations: pK_b = 6.2–0.037z and lg α_iep = 0.27–0.0215z, where z is the molar content of the acrylic acid units. The basicity of methylvinylpyridine groups increases in proportion to the content of acid groups at a constant ionic strength and is independent of the molecular weight and molecular weight distribution of the copolymer. The relationship between pK_b and the ionic strength of the solution for acrylic acid–methylvinylpyridine copolymers was established: pK_b(I) = pK + B · I^1/2, where pK is the thermodynamic ionization constant of base groups and B is 0.21 + 0.0065z. A good agreement between the experimental and theoretical (calculated from the given equation) values of the ionization constant, pK_b, of methylvinylpyridine groups for other polyampholytes (copolymers of methacrylic acid with 2‐methyl‐5‐vinylpyridine) demonstrated that the ionic state of polyampholytes is determined by the basicity of methylvinylpyridine groups, which depends on the copolymer composition and solution ionic strength. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1824–1831, 2000     

Syntheses and crystal structures of two novel Cu(II) and Co(II) complexes with 3-methyl-4-(p-bromophenyl)-5-(2-pyridyl)-1,2,4-triazole

A new ligand, 3-methyl-4-(p-bromophenyl)-5-(2-pyridyl)-1,2,4-triazole (L) and its complexes, trans-[CuL₂(ClO₄)₂] (1) and cis-[CoL₂(H₂O)₂](ClO₄)₂·H₂O·CH₃OH (2), have been synthesized and characterized by UV, IR, electrospray ionization mass spectrum, elemental analyses, and single-crystal X-ray diffraction methods. In the structure, two L ligands are stabilized by intermolecular π···π interactions between the triazole rings. In the complexes, each L ligand adopts a chelating bidentate mode through N atom of pyridyl group and one N atom of the triazole. Both complexes have a similar distorted octahedral [MN₄O₂] core (M = Cu²⁺ and Co²⁺) with two ClO₄ ⁻ ions in the trans position in 1 but two H₂O molecules in the cis arrangement in 2.     

Diffusion of polymer molecules into polymer networks: Effect of stresses and contraints

Fine threads of cis-1, 4-polyisoprene, diameter ca. 50 μm, were prepared by drawing from solution and drying. They were crosslinked by reaction with H₂S and SO₂ and then swollen with linear cis-polyisoprene liquids of varied molecular weight M_s, from 1,000 to 24,000 g/mol. Diffusion coefficients were determined from the initial rate of uptake, for both unrestrained and stretched threads. They were found to be in good agreement for stretches of up to about 300%. On the other hand, the equilibrium uptake increased markedly (> 100%) over this range of strain, similar to the increase in swelling observed with low-molecular-weight liquids. Values of diffusion coefficient were also obtained from the rate of deswelling upon release of swollen threads from tension, and from the rate of uptake of polymer liquids by a thin coating of crosslinked polymer, bonded onto glass fibers. Satisfactory agreement was obtained in all cases. A number of simple experiments thus give similar values for the diffusion coefficient of polymer liquids in lightly crosslinked polymer networks, in the range 10^?13?10^?16 m²/s, depending upon the molecular weight M_s of the polymer liquid approximately weight as M^?2_s. The amount of liquid absorbed was strongly dependent on its molecular weight, M_s, the degree of crosslinking of the host material, and applied stresses or constraints.     

Synthesis,complex formation,and dilute‐solution associative behavior of linear poly(methacrylic acid)‐graft‐poly(2‐ethyl‐2‐oxazoline)*

Poly(methacrylic acid) (PMA) and poly(2‐ethyl‐2‐oxazoline) (PEOZO) are a polyacid/polybase pair capable of forming reversible, pH‐responsive, hydrogen‐bonding complexes stabilized by hydrophobic effects in aqueous media. Linear PMA was modified with long‐chain (number‐average molecular weight: 10,000) PEOZO via statistical coupling reactions in organic media to prepare a series of PMA‐graft‐PEOZO copolymers. Potentiometric titrations revealed that the presence of tethered PEOZO markedly increases the pK_a values for PMA‐g‐PEOZO copolymers as compared with simple PMA/PEOZO mixtures at degrees of ionization, α, between 0.0 and 0.1. The dilute‐solution PMA–PEOZO intramolecular association has been probed by monitoring the PEOZO NMR spin–spin (T₂) relaxation as a function of pH. Covalently attached PEOZO side chains participate in complexation at higher values of α than untethered PEOZO. Surprisingly, most PEOZO side chains did not take part in hydrogen bonding at low α, and the highest level of PEOZO incorporation induced a decrease in the number of PMA/PEOZO hydrogen bonds. The polymer self‐diffusion as a function of α was measured with dynamic light scattering. At low pH, the copolymers had no charge and they were in a collapsed form. At high pH, the expected conformational expansion of the PMA units was enhanced at moderate levels of PEOZO incorporation. However, the highest PEOZO incorporation induced the onset of intramolecular associations between PEOZO units along the copolymer chains. Low shear rheometry and light scattering measurements were used in conjunction with the T₂ NMR measurements to propose a model consistent with the aforementioned behavior. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2520–2533, 2004     

Synthesis and characterization of a photochromic magnesium(II) coordination polymer based on a naphthalene diimide ligand

Naphthalene diimides, which are planar, chemically robust and redox‐active, are an attractive class of electron‐deficient dyes, which can undergo a single reversible one‐electron reduction to form stable radical anions in the presence of electron donors upon irradiation. This makes them excellent candidates for organic linkers in the construction of photochromic coordination polymers. Such a photochromic one‐dimensional linear coordination polymer has been prepared using N ,N ′‐bis(3‐carboxyphenyl)naphthalene‐1,8:4,5‐tetracarboximide (H₂BBNDI). Crystallization of H₂BBNDI with magnesium nitrate in an N ,N ′‐dimethylformamide (DMF)/ethanol/H₂O mixed‐solvent system under solvothermal conditions afforded the one‐dimensional coordination polymer catena‐poly[[bis(dimethylformamide‐κO )magnesium(II)]‐bis[μ‐N‐(3‐carboxylatophenyl)‐N ′‐(3‐carboxylphenyl)naphthalene‐1,8:4,5‐tetracarboximide‐κ²O :O ′]], [Mg(C₂₈H₁₃N₂O₈)₂(C₃H₇NO)₂]_n . The asymmetric unit contains half of a magnesium cation, one HBBNDI⁻ ligand and one DMF molecule. Two partially deprotonated HBBNDI⁻ ligands bridge two magnesium cations to form a one‐dimensional chain. Strong inter‐chain π–π interactions between the naphthalene rings of the HBBNDI⁻ ligand and the imide rings of adjacent chains provide a two‐dimensional structure. The supramolecular three‐dimensional framework is stabilized by π–π interactions between naphthalene rings of neighbouring two‐dimensional supramolecular networks. The complex exhibits a reversible photochromic behaviour, which may originate from the photoinduced electron‐transfer generation of radicals in the HBBNDI⁻ ligand.     

Switching of the helical polymer conformation in a solid polymer film

A chiral photochromic polyisocyanate was incorporated into a solid polymer matrix of poly(methyl methacrylate) (PMMA), yielding an isotropic polymer film. Isomerization of the chiral photochromic azo side groups (cis‐trans) triggers a reversible conformational change of the helical polyisocyanate backbone. Thus the chirooptical properties of the film can be switched photochemically. The isomerization of the helix is much slower than the isomerization of the azo side groups. Below T_g , the photochemically modified helix conformation is thus stable, despite thermal relaxation of the azo chromophores.     

Contribution à l'étude de l'ionisation des films insolubles de polyélectrolytes stéréoréguliers

The surface potentials of monolayers of poly(methacrylic acid) (PMA) have been studied as a function of the pH of the subsolution. The influence of the macromolecular configuration on the properties of the ionizable groups was more particularly investigated. Orientation of molecules in the monolayer may be imposed by the stereoregularity of PMA (isotactic, atactic) and by the nature of the interface (air–water, cyclohexane–water). The results are analyzed in terms of theories of the ionic double layer (Gouy model, Donnan equilibrium) which permit the determination of the degree of ionization α of the film. The value of α gives indications about the orientation of specific groups of the macromolecular chain, i.e., of the ionized monomer units. Indeed, at the air–water interface, the film of isotactic PMA is more acid than that of the atactic sample, and at the cyclohexane–water interface, the degree of ionization is greater than at the air–water interface. These properties are the consequence of a modification of orientation of the hydrophilic groups with respect to the aqueous phase as a function of the stereoregularity of the sample and the nature of the interface. The variation of α with the pH of the substrate phase may be used to calculate an intrinsic dissociation constant K_s° of the molecules in the film. The value of K_s° is not modified by spreading conditions and remains very similar to that obtained in solution.     

Polymerization of α-methylstyrene by living polymer in tetrahydrofuran

Kinetics of polymerization of α-methylstyrene by poly-α-methylstyrylsodium (a “living” polymer) has been studied in tetrahydrofuran at ?78°C. Complex dependences were established: that of the conversion X on reduced time φ and that of the apparent rate constant for the polymer chain propagation on conversion X and on the concentration of living polymers and monomer. The experimental data obtained were explained by assuming a coordination mechanism of anionic polymerization including the following elementary reaction: (a) generation of active polymerization centers (K₁) by interaction of the living polymer with the monomer; (b) propagation of the polymer chains (K₂); (c) monomolecular (K₃₁) and bimolecular (K₃₂) reactions of isomerization of active centers resulting in the formation of high molecular weight living polymers capable of again becoming active centers of polymerization. Approximate derivation of kinetic equation was carried out and the constants of elementary reactions were determined (K₁ = 0.15, K₂ = 24, K₃₂ = 14.1./mole-min and K₃₁ = 0.05 min^?1). The coincidence of the expected dependencies X = F(τ; φ) K_p = F(X; n₀^?1/2); dx/dτ = F(n₀) with the experimental ones was followed with the aid of computers. The expected change in the values of X and K_p depending on the contribution of each elementary reaction to the overall polymerization process was analyzed.     

Bridge‐Splitting Reactions of Platinum(II) Complexes with Parametalated Pyridine Spacer Groups: A Kinetic and Mechanistic Study

Substitution reactions of three dinuclear Pt(II) complexes connected by a pyridine‐bridging ligand of variable length, namely [ cis‐{PtOH₂(NH₃)₂}₂–μ–L]⁴⁺, where L = 4,4′‐bis(pyridine)sulfide ( Pt1 ), 4,4′‐bis(pyridine)disulfide ( Pt2 ), and 1,2‐bis(4‐pyridyl)ethane ( Pt3 ) with S‐donor nucleophiles (thiourea, 1,3‐dimethyl‐2‐thiourea, and 1,1,3,3‐tetramethyl‐2‐thiourea) and anionic nucleophiles (SCN^?, I^?, and Br^?) were investigated. The substitutions were followed under pseudo–first‐order conditions as a function of the nucleophile concentration and temperature, using stopped‐flow and UV–visible spectrophotometric methods. The observed pK_a values were, respectively, Pt1 (pK_a1: 4.86; pK_a2: 5.53), Pt2 (pK_a1: 5.19; pK_a2: 6.42), and Pt3 (pK_a1: 5.04; pK_a2: 5.45). The second‐order rate constants for the lability of aqua ligands in the first step decreased in the order Pt2 > Pt3 > Pt1 , whereas for the second step it is Pt1 > Pt2 > Pt3 . The obtained results indicate that introduction of a spacer atom(s) on the structure of the bridging ligand influences the substitution reactivity as well as acidity of the investigated dinuclear Pt(II) complexes. Also nonplanarity of the bridging ligand of Pt1 complex significantly slows down the rate of substitution due to steric hindrance, whereas release of the strain enhances the dissociation of the bridging ligand. The release of the bridging ligand in the second step was confirmed by the ¹H NMR of Pt1‐Cl with thiourea in DMF‐d₇. The temperature dependence of the second–order rate constants and the negative values of entropies of activation (ΔS^#) support an associative mode of the substitution mechanism.